2.4 Hydrocarbons from the direct liquefaction of solid fuels

2.4.1 Introduction 1985 once again prevented this technology from becoming established on a commercial scale, and led Coal has always been used prevalently to generate to spending cuts affecting most research activities in power and in the metallurgical industry. At times, the sector. At the beginning of the third millennium, however, the potential for using coal to manufacture interest was reawakened by the construction of various liquid hydrocarbons for vehicle transportation has also new industrial plants for coal liquefaction in Asia been considered. (especially in China), that is economically emerging The production of liquid hydrocarbons from coal areas which possess enormous reserves of this raw (syncrude) can be carried out following two different material. Further refinements of the technology and technological routes: indirect liquefaction, in other the resulting decline in production costs, alongside words, gasification to synthesis gas followed by strategic considerations, could render industrial Fischer-Tropsch synthesis; direct liquefaction, that is, initiatives in this sector attractive, at least on a local the transformation of coal into liquid hydrocarbons in level. a single stage using a hydrocracking process. These technologies were both developed in The role of coal in the international energy Germany before the Second World War to counter scenario the oil embargo to which the country was subject at Coal is the most abundant fossil fuel on our planet. the time, and to produce liquid hydrocarbons using Proven reserves are estimated at about 1012 t, raw materials widely available in its territory. representing two-thirds of all existing fossil fuels. At Starting from the post-war period, however, the current levels of consumption, this source could last availability of large amounts of crude oil made these for over 200 years. Worldwide coal production technologies largely obsolete, except in South Africa (5.4 109t/y in 2003) meets one-fourth of the world’s (Sasol I and II processes), due to the country’s primary energy supply (10.6 Gtoe/y in 2003). isolation from the international community caused by Furthermore, unlike oil and natural gas, coal reserves its apartheid regime (now overcome). From 1967, the are evenly distributed geographically, with almost half year of the first oil shock, these technologies were being located in OECD (Organization for Economic again taken into consideration, and for at least 20 Cooperation and Development) countries. Specifically, years intense R&D (Research and Development) 26.2% of proven reserves are found in North America, work was carried out to identify new solutions able to 12.4% in Europe, 23.5% in ex-USSR countries and as render the production of syncrude from coal much as 29.7% in the Asia-Pacific region, in other competitive with petroleum. In the case of direct words countries such as China and India, with fast liquefaction, for example, important results were growing populations (Fig. 1). obtained, making it possible to significantly improve These factors have contributed to keeping the performance levels to the extent that processes able market price of this fuel sufficiently stable over time, to produce up to 5 bbl of oil per tonne of coal treated even in situations of considerable international became feasible. tension, such as oil shocks, or more recently the Gulf Although various processes were also developed at War and the Iraq War (Fig. 2). Coal is thus the energy the demonstration plant level, falling oil prices from source which is least exposed to risk from the

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Fig. 1. Distribution 258 122 233 of world coal reserves 292 as of 2002 in billions of t.

57

22

standpoint of the vulnerability of supply, and the least the use of these technologies over the coming decades, subject to market disruptions, to the extent that the although, at least on a local level, conditions already long-term scenarios developed by the International seem ripe for their establishment. Energy Agency (IEA, 2001) predict a relatively constant price in real terms. Coal mining and pretreatment technologies As far as consumption is concerned, a mean The technical term used to describe coal mining is growth in demand of 1.4% per year is predicted over exploitation. This may take place at the surface (open the next 20 years, with the most significant growth pit mines) or at depth if the seam is more than 70 m rates in the countries of the Asia-Pacific region. The below ground level. increase in coal consumption is mainly linked to its Open pit exploitation begins with earth moving, potential use for electricity generation in developing and proceeds with the recovery of the coal using countries, especially India and China, but also to mechanical systems that differ depending on the type establishing technologies – such as gasification with of mine. Open pit exploitation is rapid, relatively the sequestration of the carbon dioxide produced – cheap since the need for manpower is reduced without able to improve the social acceptability of coal in presenting excessive risks, and makes it possible to terms of local pollution and its potential impact on extract up to 95% of the coal present in the seam. It is climate change. mainly used in lignite seams – or in any case for low As far as liquefaction is concerned (i.e. the rank coals (brown coal), and is thus preferred when transformation of coal into liquid hydrocarbon blends these are extremely large. for the production of fuels), it is not yet clear whether High rank coal seams, however, are located at the world energy market will be able to benefit from great depth (sometimes above 1,000 m) and are exploited underground by digging shafts until the mineral seam is reached; this may be from a few cm oil to several tens of metres thick. The seam is then 3.0 liquefied natural gas exploited using more or less sophisticated coal 2.5 techniques, which range from conventional mining, that is the direct intervention of man inside the 2.0 mine, to various mechanical techniques including, specifically, so-called room and pillar mining and 1.5 longwall mining. In the first case, special machines known as continuous miners are used; these are able 1.0 to create a series of cavities inside the mine itself. The longwall mining technique, by contrast, price (US cents/1,000 kcal) 0.5 involves digging two parallel tunnels up to 200 m 0 apart; they are then linked by a third tunnel, inside 19651977 1985 1990 1995 2000 which special machines with rotating blades extract year the coal and create a mine face advancing Fig. 2. Mean market prices of major fossil fuels. progressively through the seam. Using these

114 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

techniques, over 50% of the coal present in the Properties and chemical characteristics of coal formation can be recovered (Franklin, 1997). Fossil coal is a sedimentary rock originated by Once extracted, the coal can be sold as it is, or organic substances accompanied by mineral pretreated to remove the mineral component; this also substances and water. The organic component is reduces its contaminant content (for example, present in numerous different varieties, depending on inorganic sulphur). The term pretreatment, or coal the degree of coalification which determines its beneficiation (Mishra and Klimpel, 1987), refers to classification (or rank) according to the parameters operations which are generally carried out in the mine established by the American Society for Testing and area to prepare the coal for final use – treatment to Materials (ASTM D388-99e1). Since coal contains turn it into coke, combustion in thermo-electric power moisture and ash (or more accurately a mineral stations, conversion – to reduce the costs of component), the data deriving from analysis and the transportation and the processing of ashes, in addition conversion yields of direct liquefaction processes must to providing a substrate that is easier to treat in specify which fraction of the sample they refer to downstream plants. (reference basis): ar (as received), in other words on Coal beneficiation operations range from simple the sample as it is; dry, that is on the dry fraction; daf grinding to free the organic component in a crude way or maf (dry-ash-free or mixture-ash-free) or more from some inorganic pollutants (generally accurately dmmf (dry-mineral-matter-free), that is with non-combustible) to sophisticated and expensive respect to the organic component alone. treatments able to significantly reduce the Coals with a calorific value above 14,000 Btu/lb concentration of the mineral component. The extent to (dmmf basis) are classified on the basis of their fixed which the inorganic component can be removed carbon content, whereas below this value they are depends on how it is distributed within the classified on the basis of heating value (Tab. 1). combustible organic component; the higher the degree Coals are usually characterized using two analysis of dispersion, the more severe the grinding must be, protocols – proximate analysis and ultimate analysis – and thus the more expensive the whole beneficiation which are also standardized by the ASTM. The former process. defines properties of applicational type, such as The most common inorganic components are clays, moisture, volatile matter content, fixed carbon, ash carbonates and pyrites, which have a significantly (ASTM D3172-89); the latter defines the chemical higher density (as much as double or triple) than the properties, such as elemental composition (ASTM organic component. By far the most widespread coal D3176-89). beneficiation processes are thus of gravimetric type, in Low rank coals – lignites and sub-bituminous coals – other words processes which exploit precisely these have a very high moisture content and an organic differences in density. The quantity of inorganic structure rich in oxygen (up to 20% in weight). components present in raw coal from the mine may be Bituminous coals – which in turn are further subdivided as high as 40%; using gravimetric beneficiation it can on the basis of their volatile matter content – have a be reduced to 2-5%. Gravimetric treatments can be lower oxygen content (2-10%) and present H/C ratios carried out using simple shaking tables, or fluids of ranging from 0.6 to 0.8. Anthracites must be different density (suspensions of extremely fine considered the final stage of the coalification process, magnetite in water), settling tanks or more modern since they have a low volatile matter content and their hydrocyclones. Almost always, these treatments elemental composition presents a low H/C ratio produce a fine coal dust component in the process (0.6) and low oxygen content (1-2%). waters, which must later be selectively recovered with From a microscopic point of view, the organic ad hoc methods, such as froth flotation. This involves structure of coal is generally classified on the basis of selectively agglomerating the coal particles with a its optical properties (light reflectance) and the froth produced by blowing air into a water bath morphological properties of its various components, containing chemical agents able to facilitate the described as macerals or organic minerals. recompaction of the organic component, thus easing Conventionally, macerals can be grouped into three separation. different primary typologies known as vitrinite, From the standpoint of industrial development, inertinite and liptinite (or exinite), which in turn can gravimetric processes are by far the most widely used, be subdivided into sub-groups (Stach et al., 1982). although other technologies based on Macerals have different chemical properties because chemico-physical treatments do exist. These exploit they derive from the different organic components of the different surface properties of coal with respect to the plants and micro-organisms which generated the the mineral component (for example, the froth coal. Macerals of the vitrinite group are the most flotation described above or oil agglomeration). abundant and representative of the structure of coal;

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Table 1. Classification of coal on the basis of rank according to ASTM D388-99e1

Gross Calorific % Vitrinite ASTM % Fixed % Volatile Rank Value (dmmf), reflectance Code Carbon (dmmf) matter (dmmf) Btu/lb (max)

Anthracite an 92 8 7

Semi-anthracite sa 86-92 8-14 2.83

Low-volatile bituminous lvb 78-86 14-22 1.97

Medium-volatile bituminous mvb 69-78 22-31 14,000 1.58

High-volatile bituminous hvAb (A, B, C) hvBb 69 31 11,500-14,000 1.03 hvCb

Sub-bituminous subA (A, B, C) subB 8,300-11,500 0.63 subC

ligA Lignite 6,300-8,300 0.42 (A, B) ligB

Peat 6,300 0.20

they derive from cellulose and lignin and represent magnesium, phosphorus and sulphur (Al2O3, CaO, 50-90% of the total maceral structure of coal. Na2O, K2O, Fe2O3, TiO2, MgO, P2O3 e SO3). Macerals of the inertinite type (5-40% of the total) Tables 2 and 3 show the properties of some coals of have the same biological origin as vitrinites, but were different rank typically used in direct liquefaction heavily degraded (oxidized) during the first phase of processes. As can be seen, they range from lignites to the coalification process, and therefore have a high volatile bituminous coals. Of these, it should be significantly lower hydrogen content than the other remembered that Illinois n. 6, the most widely studied components. Finally, macerals of the liptinite group coal for this type of application, is often considered (5-15% of the total) derive from the resinous and waxy the reference feedstock in the United States for parts of plants. From a chemical point of view, the evaluating the performance of different processes. main differences concern the H/C ratio (which decreases from liptinite to vitrinite to inertinite), while The chemistry of direct liquefaction the oxygen content of liptinite is significantly lower. The organic structure of coal is usually represented These differences have a considerable impact on the as a three-dimensional macromolecule without reactivity of the macerals in terms of thermal cracking repeated monomer units and consisting mainly of and hydrogenation, in other words the main reactions carbon and hydrogen, alongside significant quantities involved in the direct liquefaction process. Inertinite is of oxygen, nitrogen and sulphur. This macrostructure the least reactive component and is often described in is insoluble in common solvents, but may incorporate liquefaction reactions as IOM (Insoluble Organic smaller hydrocarbon molecules that can be extracted Matter). with polar solvents such as quinoline or The organic component of coal is closely bound tetrahydrofuran (Fig. 3). up with an equally complex mineral component, Since the H/C ratio in coal is significantly lower consisting mainly of clays (illite, montmorillonite, than in oil (0.7-0.9 and 1.4-2.0 respectively), the kaolinite, etc.), carbonates (calcite, dolomite, etc.) transformation of coal into syncrude may involve and iron sulphides (especially pyrite), but also either the removal of carbon or the addition of silicates, sulphides, phosphates and oxides. As such, hydrogen: in addition to the elements characterizing the organic component (carbon, hydrogen, sulphur, H nitrogen and oxygen), the other elements usually coal (H/C 0.8) syncrude (H/C 1.5) sought and expressed as oxides are aluminium, calcium, sodium, potassium, iron, titanium, C

116 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

Table 2. Compositional properties (proximate analysis) of coals of different rank

Sample A Sample B Sample C Sample D Illinois n. 6

ASTM Classification ligA subA hvCb mvb hvCb Proximate analysis (% As Received)

Moisture 34.8 14.9 9.8 1.9 4.7

Ash 5.6 7.6 6.0 11.9 11.0

Volatile matter 30.4 33.9 32.8 25.3 36.0

Fixed carbon 29.2 43.6 51.4 60.9 48.3 Gross Caloric Value (Btu/lb dmmf) 8,100 11,000 10,900 14,920 12,600

Table 3. Chemical properties (ultimate analysis) of selected coal samples used in direct liquefaction processes

Martin Lake Wyodak Pittsburgh Illinois n. 6 (TX) ASTM Classification ligA subB hvAb hvCb Ultimate Analysis (% daf) Carbon 74.4 76.2 84.7 79.8 Hydrogen 5.0 6.2 5.8 5.4 Nitrogen 1.1 1.1 1.7 1.6 Total sulphur 1.4 0.5 1.2 3.7 Oxygen* 18.1 16.0 6.6 9.5 Maceral Composition (% vol) Vitrinite 89.1 Liptinite 5.1 Inertinite 5.9

* The oxygen content is given by the difference between 100 and the sum total of the other elements.

The first and oldest solution involves the syncrude or distillates can be described according pyrolysis of the coal at high temperature to a mechanism involving two successive stages: (T600°C) and allows for the production of highly • Coal-dissolution, in other words transformation of aromatic liquids (tar) and coke, which can be used the coal into soluble organic matter (coal liquids), in the metallurgical industry. The liquid yield as an effect of the rapid release of volatile depends on the properties of the feedstock, but components due to the increase in temperature and does not usually exceed values of about 1.5 bbl/t of the action of the solvent. coal; as a result, this option cannot be seriously • Coal-liquid conversion and upgrading, that is a taken into consideration if the aim is to produce further reduction of the molecular weight until synthetic crude. The classic option for the direct distillates are produced and the quality of the liquefaction of coal is therefore hydrogenation. products improves as an effect of This reaction is usually conducted at high hydrogenation reactions, which lead to an temperature (T400°C) and high hydrogen partial increase in the H/C ratio and a decrease in the pressure, using as a feedstock the coal blended heteroatom content. with a heavy oil, which acts as a reaction solvent. The first phase of the process thus involves the The chemistry of the process to convert coal into homolytic cleavage of the weakest bonds present in the

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Fig. 3. Organic structure of coal O O according to Shinn HO (Shinn, 1984). OH O OH N OH O OH O OH

O O HO OH S OH O O HO HO OH S O O N

N OH HO O OH OH OH O N HO O O OH O OH HO O OH NH O HO HO O O HO O O NH OH S HN O O O N O OH HO O HO O HO O HO HS S O O OH HO O O O O O

HO N O HO N HN HO OH

HO S

coal structure, due to the increase in temperature which may be added using water- or oil-soluble (cracking) and the subsequent stabilization of the precursors of various types (see Section 2.4.5). The in radicals produced by hydrogenation (Fig. 4). Effective situ decomposition of these precursors produces an saturation of the free radicals is extremely important to extremely fine powder consisting of nanometric avoid them recombining (repolymerization) to form particles of molybdenite (MoS2) with a low degree of highly aromatic structures known as char, less reactive aggregation (nanoclusters) highly dispersed within the than the initial feedstock, with a consequent reduction feedstock. Its morphological characteristics and the of the liquid yield and increase in the problems absence of porous supports render molybdenite presented by the downstream separation of the especially suitable as a hydrogenation catalyst in products. systems that are particularly problematic due to the The transfer of hydrogen from the gas phase to the presence of high concentrations of poisons, such as cracking products can be catalyzed by metal sulphides coal. The catalytic activity of molybdenite in a dispersed in the feedstock. A classic catalyst is iron hydrogenating environment appears to be due to the sulphide, which may often be present in the coal itself formation of sulphur vacancies on the profiles of the in the form of pyrite; this, under the typical reaction nanoclusters as an effect of the reaction of hydrogen conditions of liquefaction processes, is transformed with MoS2 and the formation of SH groups which into pyrrhotite, a non-stoichiometric iron sulphide evolve to H2S (Byskov et al., 2000). These vacancies FeS1x with a good catalytic activity for have actually been observed by the STM (Scanning hydrogenation. Other hydrogenation catalysts Tunnelling Microscopy) investigation of triangular – generally used in the dispersed phase in low molybdenum clusters (Derouane, 2000; Fig. 5). concentrations (hundreds of ppm) – are the classic Alongside molecular hydrogen used in the sulphides of transition metals, including molybdenum, presence of suitable catalysts which are generally

118 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

coal liquids

CH3

H2 H S 84 kcal/mol . CH2 ∆ . char S S S 115 kcal/mol

Fig. 4. Simplified diagram of the direct coal liquefaction process.

mixed into the oil/coal suspension (slurry), a tetrahydrofuran or pyridine), asphaltenes (soluble in particularly effective way of transferring hydrogen is aromatic solvents such as toluene), maltenes (soluble to use hydrogen-donor solvents. These are blends of in paraffinic solvents). As the reaction progresses, and partially hydrogenated aromatic hydrocarbons depending on the operating conditions adopted containing tetralin-type structures which can easily (process severity), these pseudo-components produce transfer hydrogen to the coal, turning into the increasingly small hydrogenated fragments, generating corresponding aromatic hydrocarbons (such as a mixture of hydrocarbon liquids of varying volatility naphthalene; Fig. 6). This method is particularly (distillates, heavy gasoils and residues) or degrading to effective during the coal dissolution stage, in part refractory organic matter (Fig. 7). In second-generation because, given their highly aromatic structure, the processes, this stage of the reaction is usually carried donors have an excellent solvent activity with respect out at low severity to better control the production of to the reactive intermediates produced by cracking. free radicals and ensure an effective transfer of As such, the first phase of the liquefaction process hydrogen from the donor solvent or the gas phase to generates gas (C1-C4 hydrocarbons and the reactive fragment (hydrogen uptake). non-hydrocarbon gases such as H2S, NH3 and H2O) From a kinetic point of view, the coal conversion and a complex blend of hydrocarbons with decreasing reaction can be described as the result of a series of molecular weight and polarity, generally identified on parallel first order reactions of different rates, where the basis of their solubility in organic solvents: char the latter represent the homolytic cleavage of bonds of and/or IOM, preasphaltenes (soluble in varying strength inside the coal matrix (Gorin, 1981):

n kit CT C ΑCie i1

where CT is the maximum percentage (in weight) of convertible coal, C the conversion at time t and Ci the initial percentage (in weight) of maf coal subject to a decomposition process characterized by a rate constant ki. The kinetics of hydrogen transfer can be expressed in an identical way. Using this representation, a good agreement with experimental data can be obtained by dividing coal into two reactivity classes. The resulting values for activation energy depend on the type of coal used: for example, for high volatile bituminous Pittsburgh seam coal, the activation energy calculated for the various reactions ranges from 30 to 45 kcal/mol. After liquefaction, the syncrude is turned into naphtha and gasoil by the progression of cracking and hydrogenation reactions, encouraged by the presence of molecular hydrogen and suitable catalysts able to Fig. 5. Scanning tunnelling microscopy image of a facilitate the removal of the main poisons: sulphur via molybdenite nanocluster exposed to hydrogen at 673 K HDS (HydroDeSulphurization) reactions, nitrogen via (Derouane, 2000). HDN (HydroDeNitrogenation) reactions and oxygen

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Fig. 6. Hydroliquefaction 1/2 of coal using CH hydrogen-donor solvents. 3 S CH3 S 1/2

via HDO (HydroDeOxygenation) reactions. This seems to be gasification, which can be fed directly second phase of the process can be carried out in a with coal in addition to the unreacted organic residues different reactor, often after the separation of the recovered from the liquefaction unit. The production mineral component of the conversion products by of syncrude is thus linked to a single primary fossil filtration, centrifuging or solvent deashing. The coal source (Fig. 8). liquids can thus be subjected to processing on fixed Using the best available technology, one can bed or ebullated bed reactors, using traditional estimate that to produce 1 bbl of syncrude from a hydrotreating catalysts based on NiMo, CoMo, NiW, bituminous coal, it is necessary to process about 250 etc. over alumina. kg of coal (daf basis), of which 20-25% is used for the The above discussion makes it clear that direct production of hydrogen by POx, with an overall liquefaction processes involve considerable hydrogen performance of about 70-75%. consumption; therefore, this represents one of the main items of expenditure for the entire process. Hydrogen for energy purposes is produced from 2.4.2 Liquefaction technology hydrocarbon fuels of various types (solid, liquid or gaseous), using suitable technologies such as steam Bergius process reforming, autothermal reforming and gasification The first attempts at direct liquefaction were (also known as Partial Oxidation, POx), in other words carried out in Germany from 1920 by Friedrich processes that are particularly expensive in energy Bergius. Since then, numerous solutions aimed at terms (Chauvel and Lefebvre, 1989). In the case of improving the performance and economic viability of direct liquefaction, by far the most interesting solution the process have been proposed and developed, in the attempt to render the direct liquefaction of coal a real viable option for the production of synthetic fuels as an alternative to those obtained from oil. gas coal The Bergius process involved the direct liquids hydrogenation of coal at high temperature (430-480°C) and very high pressure (up to 700 bar). To facilitate hydrogenation and avoid problems with

maltene the erosion of materials, the coal was fed to the reactor in the form of a suspension in oil. The reaction was catalyzed by iron-based materials such as iron oxide or red mud (Bergius and Billiwiller, 1919), a by-product of the aluminium industry based on iron, aluminium and titanium oxides.

asphaltene The first industrial plant based on this technology

coal was built by Farbenindustrie in 1927 at Leuna (Germany), and involved two successive hydrogenation stages. The first, later named LPH (Liquid-Phase Hydrogenation), produced a medium distillate which was then further hydrogenated in the vapour phase (second stage) until gasoline and diesel gasoil were obtained. In the following years, several preasphaltene other plants were built, enabling Germany to produce significant amounts of fuel to support its war effort: char towards the end of the Second World War, the 18 direct

IOM liquefaction plants and 9 indirect liquefaction plants 6 Fig. 7. Block flow diagram of the direct coal were able to produce 410 t/y of gasoline, that is 90% liquefaction process. of national consumption.

120 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

hydrogenation of coal, in other words making it solvent syncrude CLUdeashing HDT possible to: a) reduce the severity of the process; b) facilitate the process of hydrogen transfer from the ashes, IOM coal gas phase to the coal; c) improve conversion yields and the liquid selectivity rather than hydrocarbon H gas 2 gases; d) encourage the elimination of poisons from POxtreating WGS the synthetic crude by using catalysts able to facilitate the removal of heteroatoms. Numerous processes were O ashes H S, NH ,... CO proposed and tested on the laboratory or small pilot 2 2 3 2 plant scale, and three of these reached the construction of preindustrial-size units. Fig. 8. Integrated process scheme for direct coal liquefaction and hydrogen production. CLU, Coal Liquefaction Unit; HDT, HyDroTreating SRC I and II processes of coal liquids; POx, gasification; The SRC I (Solvent Refined Coal) and SRC II WGS, Water Gas Shift section. processes were developed by Gulf Oil from the 1960s onwards. The first process was aimed at the production of a clean solid fuel, in other words a type At the end of the war, the allied countries came of coal enhanced by the removal of its mineral into possession of the information concerning the coal components, and with a low sulphur content. Between liquefaction processes, and the US Bureau of Mines 1965 and 1974, two demonstration plants with a financed the Bechtel Corporation with the aim of capacity of 6 and 50 t/d respectively were built at building a 200 bbl/d demonstration plant fed with Wilsonville and Fort Lewis (United States). North Dakota sub-bituminous coal. Immediately The SRC I process involves blending the coal with afterwards, the enormous supply of oil from the an aromatic solvent in ratios of between 1:2 and 1:3, Middle East put a stop to the development of and its dissolution by low severity hydrotreating liquefaction processes until, as noted above, the first conducted at about 440°C and 70 bar for 30-60 oil shock in 1967 led to a renewed interest in the minutes, but without specific catalysts. After potential for producing synthetic crudes from coal. hydrotreating, the liquid product is filtered and the During these years, numerous R&D projects were solvent is recovered and recycled by distillation. started on the direct and indirect liquefaction of coal, The SRC II process, which evolved from SRC leading to the definition of various processes, technology, was developed using the Fort Lewis plant developed over the following decades. to maximize the production of distillates; this still remains in the order of 20-25% with respect to the Post-war processes feed coal. In this case, the coal-recycled solvent During the period between the mid-1960s and suspension is hydrogenated under significantly higher 1985, a series of industrial initiatives were begun, severity conditions than SRC I (460°C and a pressure aimed at developing direct liquefaction technologies of up to 190 bar). Unlike the EDS process (Exxon able to produce syncrude at costs competitive with oil, Donor Solvent; see below), in this case the recycle in order to reduce the dependence of western nations solvent, consisting of a heavy fraction of the on crude oil producing countries. Much of this R&D liquefaction effluents, is not hydrogenated before effort was sustained by the United States, with the reuse. direct intervention of several petroleum companies (especially Exxon and Gulf Oil), and government EDS process funding from the DOE (Department of Energy), with During the 1970s, Exxon undertook an important numerous projects entrusted to public and private industrial research programme to develop a new direct research institutes such as the Pittsburgh Energy and liquefaction technology based on the use of hydrogen Technology Center, Oak Ridge National Laboratories, donor solvents. The programme was partially funded SRI International (formerly Stanford Research by the DOE, and led to the construction of a 250 t/d Institute) and numerous university institutes. demonstration plant at Baytown (Texas). The plant was Although the basic chemical approach remained built in 1980 and remained in operation for two years, identical to that developed by Bergius, the in-depth thus demonstrating the technological feasibility of the investigation of the chemical and chemico-physical process: this involves the hydrogenation of coal in a aspects of the coal hydrogenation process made it medium able to transfer hydrogen from the gas phase possible to define reaction schemes and operating to the coal without the use of hydrogenation catalysts conditions allowing for the more rational (H-donor solvent).

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The process scheme involves feeding the coal to hydrogenated at a temperature of 425-450°C and a the reactor in the form of a suspension, with a suitable pressure of 200 bar in an ebullated bed reactor, partially hydrogenated aromatic solvent obtained by using pellet catalysts based on NiMo or CoMo over the catalytic hydrogenation of an ad hoc fraction of the alumina, with a size of 0.8-1.5 mm. The technology product of coal conversion on a fixed bed reactor used makes it possible to constantly replace the (Fig. 9). The solvent hydrogenation is managed in such catalyst poisoned by the contaminants present in the a way as to encourage the production of naphthenic coal with fresh catalyst, so as to maintain constant and aromatic structures of the tetrahydronaphthalene performance over time in terms of conversion and type (tetralin); these then act as hydrogen donors upgrading. The conversion products exiting the plant during liquefaction. The coal conversion reactor are fractionated, and then further hydrogenated using operates at a temperature of 420-460°C and a traditional fixed bed technologies; the distillation hydrogen pressure of 100-140 bar, depending on the residue can be recycled and used for the initial type of coal used. The reaction products are then sent preparation of the coal-oil suspension. As for other to a fractionation unit to recover the distillates and liquefaction processes, in this case, too, yields recycle solvent; the distillation residue, containing the depend on the type of feed; for bituminous coals, unreacted organic component and mineral matter, can they are in the order of 50%. be sent to a coking unit to recover an additional A 200 t/d pilot plant for this process was built at quantity of distillates, and then used as feedstock for Catlettsburgh (Kentucky), which remained in gasification to produce the hydrogen needed for the operation until 1983. The H-Coal technology was then process. taken up again and further developed over at least ten The distillate yield obtainable with through EDS years (up to 1992) in a smaller plant built at technology depends on the type of coal with which it Wilsonville, directly funded by the DOE. is fed, but generally ranges from 35-38% for lignites and low rank coals, up to over 50% for some TSL processes (Two-Stage Liquefaction) particularly reactive bituminous coals (unless The processes described above were considered a otherwise specified, all the process yields refer to a daf success from the standpoint of their technical basis). feasibility, but were not yet able to produce syncrude at costs competitive with oil. The main problems were H-Coal process the low distillate yields and the high consumption of The H-Coal process is a variant of the H-Oil hydrogen due to excess production of gas. A process for the conversion of petroleum residues, contribution of fundamental importance to overcoming and was developed by Hydrocarbon Research (now these problems was made by studies on the chemism Hydrocarbon Technology) in the early 1980s. The of liquefaction carried out at various American state coal, suspended in a recycle solvent, is catalytically and university research centres, under the supervision

Fig. 9. Block flow recycle hydrogen diagram of the EDS gases naphtha process (Harwell fresh hydrogen vacuum Laboratory, 1999). tubular distillation reactor middle separation distillate slurry mixing

coal

flexicoker

fixed bed hydrotreater preheating coke to recycle solvent gasification

122 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

of the DOE. These clarified numerous aspects of the Wilsonville plant, built in 1972 by Southern molecular structure of coal and its reactivity during the Company, but from the following year fell under the various phases of the process of transformation into technical and financial control of the Electric Power liquids and distillates. Research Institute (EPRI). In 1976, the DOE In 1976, Richard Neavel demonstrated that it is became the project’s major financial backer. In possible to almost completely solubilize coal under 1978, with the completion of a new reactor coupled relatively mild conditions without consuming with the hydrotreating reactor, the integrated ITSL hydrogen (Neavel, 1976). He proved that, by heating scheme was developed. The thermal treatment coal to 400°C in the presence of suitable polynuclear reactor is fed with a slurry of coal-recycle solvent aromatic solvents, even after a few minutes yields and with hydrogen; the reaction pressure and above 90% of products soluble in polar solvents could temperature are 90-150 bar and 400-450°C be obtained. People also began to consider the respectively. The product exiting the reactor (solid importance of the solvent not only as a hydrogen at ambient temperature) is fractionated into gas, donor but also as an agent able to promote the distillates and residue, and in turn sent to a unit that dissolution of the coal and encourage the transfer of removes the mineral component (Kerr-McGee hydrogen from the gas to the liquid phase. Solvents of Critical Solvent Deashing). It then passes into the this type thus had to consist of aromatic and catalytic hydrogenation reactor (ebullated bed hydroaromatic hydrocarbons, but also of compounds plant) operating at a temperature of between containing heteroatoms such as oxygen and nitrogen 390-400°C in the presence of traditional supported with phenol and pyridine functionality, preferably with catalysts used to treat extremely heavy feedstocks. a high molecular weight. The same group of Exxon Using this type of configuration, the Wilsonville researchers who had followed the development of the plant managed to significantly reduce hydrogen EDS process – and who had already identified consumption, halving the production of gas and standard methods for evaluating the properties of maintaining yields of distillable products donor solvents (solvent quality index) – discovered (C5-350°C) above 60%. that recycling heavy hydrocarbon residues led to a In around 1990, a further development of the ITSL substantial increase in conversion yields (Schlosberg, process was created, using two H-Oil reactors in a 1985). series before the deashing unit. This configuration These considerations made it possible to (Close-Coupled ITSL) made it possible to couple the develop a new two-stage process scheme, in which thermal and catalytic treatment stages without an coal dissolution and upgrading were separated, intermediate reduction of pressure (Fig. 10). reducing the disadvantages entailed by a single Furthermore, it was particularly attractive because it high-severity stage. The first stage had to be made it possible to further limit the negative effects of optimized to encourage the complete dissolution of repolymerization reactions, improving yields and the the coal, minimizing the consumption of hydrogen. quality of products. This was obtained by reducing the severity These ideas formed the basis for the various conditions and, in particular, by lowering residence process options developed during the 1990s by HTI times to a few minutes (short contact time). (Hydrocarbon Technologies Inc.), involving the use Subsequently, the solubilized coal could be freed of two or more slurry and/or ebullated bed reactors, from mineral substances and sent to the second and therefore described with the name CTLS catalytic hydrogenation stage, optimized to (Catalytic Two-Stage Liquefaction) and CMSL maximize conversion and upgrading. (Catalytic Multi-Stage Liquefaction; see Section One of the first plants based on this new 2.4.3). The distillate yields obtainable with the latter scheme was built by Lummus Crest in early 1980. configurations can reach values of over 70%, with a The 0.2 t/d plant consisted of a first thermal hydrogen consumption of 6-7.5 weight % on feed. dissolution reactor operating at 430-450°C under Of the total hydrogen consumed, about 70% is used hydrogen pressure, followed by an LC-Fining to produce liquids, 20% to remove heteroatoms, and ebullated bed unit for the catalytic hydrogenation only 10% to produce gaseous hydrocarbons. In most of the liquid products. This process configuration, of the single-stage processes developed earlier, only described with the acronym ITSL (Integrated Two- 50% of the hydrogen consumed ended up in the Stage Liquefaction) was taken up and studied on liquids. The research carried out at Wilsonville various scales by numerous companies (Amoco, continued until 1992, concentrating on the study of Chevron, HRI and others). coprocessing, in other words the combined Much of the work to develop this technology upgrading of coal and petroleum residues or heavy was carried out over almost a decade at the 6t/d crudes (see below).

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Fig. 10. Block gases recycle hydrogen flow diagram of the fresh hydrogen light Close-Coupled ITSL distillate process (Harwell Laboratory, 1999). middle distillate first second slurry stage stage heavy mixing ebullating ebullating distillate reactor reactor

coal

solid separation

residues preheating recycle solvent

Other processes In the same years in Germany, Saarbergwerke Outside the United States, R&D activities in the developed a second liquefaction process named direct liquefaction sector were undertaken mainly Pyrosol. Unlike the Kohleoel technology, the latter by Germany, Japan and the United Kingdom. In the involves a low-severity coal dissolution stage, period between the 1970s and 1980s, the German followed by the pyrolysis of the non-distillable company Veba Oel developed the process known as residues in the presence of hydrogen. The process was Kohleoel, an evolution of the Farben-Bergius developed with a 6 t/d demonstration plant unit. technology. In 1982, a 200 t/d demonstration plant In the United Kingdom, during the period between was built at Bottrop (Rhineland-Westfalia), later 1973 and 1995, the British Coal Corporation worked used to develop the technology for the deep to develop the technology known as LSE (Liquid conversion of residues and heavy crudes known as Solvent Extraction), to the point of building a 2.5t/d Veba Combicracking. The process involves a single demonstration plant at Point of Ayr in north Wales. liquefaction stage at 470°C and over 300 bar of Like the EDS technology, this process is also based hydrogen pressure, in which the coal is converted on the idea of using a hydrogen donor solvent to into distillates in the presence of iron-based encourage the coal dissolution step, operating under catalysts, used without recovery (once-through low severity conditions. The coal is blended with a mode; Fig. 11). The cracking products exiting the heavy oil in a ratio of 1:2 and sent to the so-called reactor are separated from the mineral component digestion unit, where it is treated under hydrogen and unreacted residue while hot, and sent directly to pressure (10-15 bar) at a temperature of 420-450°C, a fixed bed reactor, where they are further for residence times of about 1 hr. The reaction hydrogenated to remove sulphur, nitrogen and products are separated with an appropriate filtration oxygen. The residue from the first stage is sent to system, kept operational with constant counter-washes vacuum fractionation to recover heavy gasoil, some using some of the reaction products. The liquids are of which is recycled to prepare the feed coal-oil upgraded using traditional ebullated bed slurry. hydrogenation reactors, which produce the donor The Kohleoel process is able to convert bituminous solvent used in the first reactor. The process is able to coals with yields of over 80%, but the distillates convert coal with distillate yields – mainly naphtha selectivity rarely exceeds 50%. The high process and light gasoil – of about 60%. severity, required to convert the coal into distillates in Another technology that reached the demonstration a single stage, leads to a significant production of plant stage was developed in Japan by the industrial hydrocarbon gases (up to 20%), therefore consuming group NEDO (New Energy and Industrial Technology large amounts of hydrogen. Development Organization), which built a 50 t/d plant

124 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

Fig. 11. Block flow hydrogen recycle hydrogen diagram of the Kohleoel process catalyst (Harwell Laboratory, coal 1999). separation gases LPG hydrotreating primary reaction flash separator naphtha

slurry mixing vacuum distillation distillation atmospheric middle recycle solvent oil vacuum bottoms

at Morwell in Australia in 1985. This technology, Often, studies on coprocessing have attempted to known as BCL (Brown Coal Liquefaction) is specific identify the conditions under which the combined for low rank coals, which may contain significant treatment of coal and oil produces synergic effects that quantities of water, and involves the liquefaction of the may lead to improved yields and product quality as coal aided by a heavy oil and a limonite-based compared with the separate treatment of the two catalyst, used once-through. As for other processes, in feedstocks. According to SRI International researchers this case as well, the liquefaction step (150 bar, (McMillen et al., 1987), one possible advantage of 430-450°C) is integrated with the hydrogenation of the coprocessing may be due to the fact that the products. simultaneous presence in the feedstock of hydrogen acceptor compounds (such as the PolyNuclear Coprocessing technologies Aromatic hydrocarbons in coal, PNA) and hydrogen Coprocessing involves the simultaneous upgrading donors (such as the naphthenes in oil) may lead to the of coal and petroleum residues or heavy crudes, which production of cyclohexadienyl radicals which are co-fed to the hydrogenation reactor in ratios encourage cracking reactions through free between 1:1 and 1:2. radical-based hydrogen transfer reactions (H-transfer The idea of coprocessing coal and oil dates to the reaction; Fig. 12). It also appears that the combined 1930s, with the first attempts in Canada using the treatment of oil and coal provides benefits in terms of bitumen extracted from the Athabasca tar sands and product quality. For example, the presence of coal coal. In any case, most studies relating to the tends to favour the precipitation of the metals present development of coprocessing technologies began in in the oil, leading to a lowering of the nickel and the 1970s in connection with those on direct coal vanadium content in the conversion products. By liquefaction. Liquefaction plants worked by contrast, the properties of oils of petroleum origin as recycling some of the products (distillates or coal dissolution solvents are significantly worse than distillation residues) used as a carrier and/or reactive those of the liquids produced by the coal itself, since fluid for the coal to be liquefied. Using low-cost they contain significant quantities of paraffinic and oils, such as distillation residues or heavy crudes, to naphthenic hydrocarbons. be processed alongside the coal has the practical Coprocessing has long been studied by various advantage of eliminating the solvent recycling stage, North American companies (HRI, UOP, Lummus, thus simplifying the process and reducing specific Mobil, Chevron, Ohio Ontario, Canada Centre for investment costs per unit of product. From the Mineral and Energy Technology – CANMET, Alberta refiner’s point of view, replacing some of the oil Research Council) and Japanese companies (Ministry with coal in residue conversion plants may entail of International Trade and Industry – MITI, Osaka benefits linked to the reduction of upgrading costs Gas, Mitsubishi Heavy Industries), often reaching the per barrel of product. experimental phase at the laboratory or pilot plant

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 125 HYDROCARBONS FROM NON-CONVENTIONAL AND ALTERNATIVE FOSSIL RESOURCES

Fig. 12. Cleavage H H coal H H of bonds by H-transfer coal . . coal . reaction (McMillen et al., 1987).

scale (various t/d), but hardly ever on the countries possess enormous reserves of coal (but not demonstration plant scale. Some major experiments on of oil) and thus, given the forecasts of strong economic coprocessing technologies have been carried out in growth, deem it strategically important to invest in this Canada, which possesses enormous quantities of technology. During recent years, various industrial heavy crudes and tar sands, and which has developed initiatives have begun which should lead to the specific experience in the treatment of these construction of the first industrial-scale plants for the feedstocks over the past 30 years. The CANMET production of syncrude from coal since the post-War coprocessing technology is a variant of that developed period. to the 5,000 bbl/d plant scale at the Montreal refinery for the hydrocracking of petroleum residues and heavy HTI-Shenhua project crudes. Using the same process solutions (multiphase The main industrial initiative, which should single-stage reactor operating at 440-460°C and a become concrete in 2010, concerns the HTI pressure of up to 150 bar) and the same type of (Hydrocarbon Technologies Inc.) and Shenhua Group catalyst (iron sulphate) the potential for cofeeding Corporation. During the 1990s, Hydrocarbon 30-40% of coal with the oil was assessed. The Technologies (a spin-off of Hydrocarbon Research, experiment was carried out on a plant of reduced size, now controlled by Headwaters) worked on the obtaining good levels of coal to liquids conversion. development of numerous technologies for the The solution adopted by the ARC (Alberta upgrading of petroleum residues, heavy crudes, the Research Council), on the other hand, is specific for direct liquefaction of coal and coprocessing, centred low rank coals and Canadian bitumens. The process on the use of dispersed catalysts obtained from involves a first stage of coal liquefaction in a blend different precursors based on iron and/or with oil, conducted in a carbon monoxide atmosphere molybdenum. The portfolio of processes and catalysts so as to use the water present in the coal as a source of proposed by HTI is extremely broad: hydrogen through the Water-Gas-Shift Reaction • HTI GelCat is a dispersed catalyst based on iron (WGSR). The reaction takes place in a reactor into oxide, used in the form of a gel in concentrations of which the feedstock and reactive atmosphere are fed in up to 5,000 ppm and potentially containing counter-flow (CFR, Counter-Flow Reactor), with the promoters such as Molyvan (an oil-soluble gas sent in from below to desorb the lighter reaction compound based on molybdenum, added up to products. The reaction conditions are relatively mild 100-200 ppm of molybdenum) or other transition (temperature below 400°C and pressure below 100 metals such as cobalt, nickel, palladium and bar), but make it possible to convert the coal into platinum. The application of choice appears to be liquids with yields above 90%, removing most of the coal-oil coprocessing. The advantage of using the oxygen present in the feedstock. Subsequently, the catalyst in gel form results from the fact that upon liquid is subjected to upgrading in a second entering the reactor, as an effect of the high multiphase hydrogenation reactor, obtaining distillate temperature, there is an ‘explosion’ caused by the yields of nearly 70%. The first stage of this process rapid evaporation of the water. This produces has been tested up to the 0.25 t/d scale. extremely fine particles with a very high surface area, allowing them to be used at low concentrations (a few thousand ppm of iron). Operating costs are 2.4.3 New-generation processes thus lowered, since these catalysts cannot be recycled directly, and are therefore lost with the Though at rates and attracting investments unconverted residue and ashes. significantly lower than those lavished on it during the • HTI (HC)3, technology for the upgrading of heavy 1970-1980s, research in the field of direct liquefaction crudes, uses a dispersed molybdenum-based processes continued during the last decade of the catalyst originally developed by the ARC. twentieth century, with special attention to specific • HTI Resid-Cat entails the hydrocracking of heavy applications in countries of the Asia-Pacific region, feedstocks of various types into ultradesulphurized and particularly China, Japan and Australia. These distillates.

126 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

• HTI Co-Pro and HTI Co-ProPlus, upgrades the 1999). The companies involved in the project have not combined coal-residue feedstocks derived from provided information on the economic aspects of the (HC)3. initiative, but claim that the availability of enormous • HTI Coal is the direct liquefaction of coal based on coal reserves and low production costs will render it CTSL, that is a process based on the distinction profitable when crude oil prices are above 30 $/bbl. between the stage of conversion into liquids (liquefaction of the organic component) and that of NEDOL process conversion into distillates and upgrading. As a A second initiative, announced in 2001, concerns a matter of fact, the process involves a stage of plant to be built in Sumatra (Indonesia) by 2011, the hydrogenation of the feedstock, previously result of collaboration between the Indonesian state impregnated with a dispersed iron-based catalyst company BPPT (Badan Pengkajian dan Penerapan (GelCat) and blended with some of the heavy oils Teknologi) and the Japanese group NEDO (New produced by the process itself at a temperature of Energy and industrial technology Development between 400 and 420°C and a hydrogen pressure of Organisation). The reference technology for the 170 bar. The low temperatures make it possible to liquefaction section is the NEDOL process, developed maintain a high concentration of in Japan from the early 1980s and perfected over naphthenic-aromatic structures in the reaction various years of experimentation carried out as part of system to better control repolymerization a joint programme with the Chinese ministry for processes. The liquid produced can then be sent industry, later tested on a 7 t/d pilot plant, which is still into a second hydroconversion unit, at a higher in operation (Wasaka et al., 2003). temperature to maximize conversion into The process has four macrosections: a) preparation distillates. These are recovered and further treated of the coal feed, in other words dewatering, reduction through a series of flashing operations, vacuum to an extremely fine powder, blending with the solvent distillation, final residue extraction with toluene and the catalyst (natural or synthetic pyrite); and hydrotreating. To better control the initial coal b) liquefaction in three slurry reactors in series at dissolution process, it is possible to operate with a 450-465°C and 180-190 bar, with residence times for system of several stages conducted at different the liquid phase of 1.5-3 hr; c) distillation severities. The conversion yields claimed for this (atmospheric and vacuum) with the separation of the process are extremely high (90%) with a fractions (gas, naphtha, residue and solvent); distillates selectivity of around 75%. d) recovery and hydrogenation of part of the gasoil The latter technology has been developed with a 50 used as a coal liquefaction solvent, with recycling to kg/d pilot plant unit; recently, Headwaters announced the feed preparation section. the formalization of an agreement with the Chinese The peculiarities of this process are thus the coal company Shenhua Group (with an annual combined use of low-cost donor solvents and production of 60 million t of coal) to build a plant for iron-based catalysts, and the replacement of the the direct conversion of coal into hydrocarbon distillates. The plant will be built on an HTI licence as far as the liquefaction stage is concerned, whereas Table 4. Elemental composition of Shenhua coal liquid upgrading will be carried out using Axens and the syncrude produced with the HTI technology technology. The industrial complex is due to be (Comolli et al., 1999) completed by 2008 at Baotou in Inner Mongolia (China), and will have a production capacity of 50,000 Coal Syncrude bpd (barrels per day) of distillates (mainly gasoline Elemental composition (% daf) and gasoil) obtained by processing about 12,800 t/d of Carbon 75.9 87.0 sub-bituminous Shenhua coal on six production lines. The level of conversion of coal into liquids has been Hydrogen 4.2 12.7 estimated at around 91-93%, with a distillate yield of Sulphur 0.42 0.10 63-68 weight % on feed, of which naphtha and Nitrogen 0.98 0.14 medium distillates represent about 20 and 50% respectively. Hydrogen consumption is in the order of Oxygen 12.3 0,06* 6.5 weight % on feed. The gasoline and gasoil Ash 6.2 – produced will have very good quality characteristics, API Gravity 29.7 at least as far as the sulphur content is concerned (15 and 140 ppm respectively); the properties of the * The oxygen content is given by the difference between 100 and the syncrude are summarized in Table 4 (Comolli et al., sum total of the other elements.

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deashing stage (the critical stage in all direct coal derivatives of furan. Heteroatoms are found liquefaction processes) with vacuum distillation, from throughout the distillation curve of the syncrude, with which the unreacted organic residue is discharged a tendency to concentrate in the highest boiling together with other ashes and fed to a gasification unit fractions, such as vacuum gasoils and residues; this is for the production of hydrogen. Liquefaction yields are particularly true for nitrogen and sulphur. in the order of 65% with a selectivity for naphtha and Another property differentiating coal liquids from gasoil of 85%. crude oil derivatives – in this case in a positive way – is the absence of metals, such as nickel and vanadium that are normally present as oil-soluble compounds of 2.4.4 Properties of coal liquids porphyrinic type in petroleum residues. The saturated hydrocarbon component is characterized by the fact Conversion yields and chemical and chemical-physical that it contains mainly isoparaffins and naphthenes, properties of the liquids produced from coal are while the concentration of n-paraffins is significantly strongly influenced by the type of feedstock used and lower than in oil. As far as aromatics are concerned, the type of process adopted (Sullivan, 1981). Over the polycondensate structures with alkyl substituents and a years, technological advances have made it possible to low-medium length chain prevail. improve the performance of the processes, increasing These compositional properties make coal liquids conversion levels and the selectivity to distillates, and unsuited for direct use as fuels, in part because – in above all to improve the quality of conversion addition to not meeting commercial standards for products. The data summarized in Tables 5 and 6 gasoline, jet fuel and gasoil – they are potentially toxic provide a fairly clear picture of the route taken in and carcinogenic. passing from first generation processes to integrated two-stage catalytic technologies. Upgrading of coal liquids and their use in refineries In general, however, compared to crude oils, the However they are produced, coal liquids are highly syncrudes produced by the direct liquefaction of coal aromatic hydrocarbon blends. For this reason, the present significantly different distillation curves, various cuts must be subjected to hydrogenating especially as concerns the content of high boiling treatments (hydrotreating and hydrocracking), usually fractions and residues. In liquefaction processes, these carried out in fixed bed plants using traditional fractions are generally used to prepare the coal/oil feed catalysts developed for the petroleum industry, in other blend, and thus tend to become lighter during words those based on CoMo, NiMo, and NiW on subsequent passes through the liquefaction reactor. alumina-type supports. Process conditions depend on The various cuts are characterized by the fact that they the distillation cut to be treated (temperature between are highly aromatic and contain significant 300 and 500°C, space velocity between 0.5 and 5 h1 concentrations of heteroatom such as sulphur and pressures of up to 190 bar); generally, given an (0.1-2.5% in weight), nitrogen (0.2-2% in weight) and identical distillation cut, they are more severe than oxygen (1.5-7% in weight). Sulphur is present mainly those used for petroleum products. The aromatic as aromatic sulphur (thiophenes and polycondensate nature of coal liquids makes it necessary to operate at derivatives of thiophene), whereas nitrogen is found in high pressure to limit the deposition of coke on the amine and pyridine-type structures and as condensate catalysts. As a consequence, hydrogen consumption is derivatives of pyrrole (indoles, carbazoles etc.). higher than that for derivatives of petroleum origin, Oxygen, almost absent in products of petroleum due in part to the need to remove the high origin, is usually present in the form of phenols and concentrations of sulphur, nitrogen and oxygen. The

Table 5. Conversion yields (weight %, daf basis) of direct liquefaction processes using Illinois n. 6 coal (Burke et al., 2001)

SRC II EDS H-Coal ITSL CMSL Non-hydrocarbon gas 12.9 17.4 11.3 15.2 15.2 (H2S, NH3, H2O)

HC gas (C1-C3) 14.5 16.0 12.8 5.4 11.4 Distillates (naphtha + gasoil) 47.3 47.2 50.5 65.8 72.3 Hydrogen consumption (% p) 5 5.9 6 6 7.5

128 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

Table 6. Liquid products’ quality from direct coal liquefaction processes using Illinois n. 6 coal (Burke et al., 2001)

SRC II EDS H-Coal ITSL CMSL Naphtha

Yield (weight %) 19.3 22.8 22.9 14.5 20.7 API Gravity 39 31 35 50 53 Sulphur (weight %) 0.2 0.5 0.2 0.04 0.02 Nitrogen (weight %) 0.4 0.2 0.3 0.02 0.002 Oxygen (weight %) 3.9 2.8 3.0 0.3 0.1 Syncrude

API Gravity 27 22 38 Sulphur (weight %) 0.2 0.1 0.1 Nitrogen (weight %) 0.5 0.5 50 ppm Oxygen (weight %) 2.1 2.2 0.5

life of catalysts is heavily conditioned by the tendency diesel fraction is concerned, it is also necessary to of feedstocks to form coke and the more or less hydrotreat middle distillates, but in this case the low significant presence of inorganic particles due to the concentration of n-paraffins may make it difficult to mineral component of coal, which is not always easy reach density and cetane number specifications unless to remove. there is specific intervention on the naphthene All these factors entail high additional costs; component (ring opening). From this point of view, the however, these can be partially lowered by suitably gasoil from direct liquefaction is similar to the LCO integrating direct liquefaction plants with the (Light Cycle Oil) from catalytic cracking, for which refinery. Although there are no sufficiently detailed suitable upgrading processes are being developed. and up-to-date studies on this subject, it is Finally, heavy distillates and residues can be reasonable to assume that it will be possible to suitably treated in conversion units alongside products identify synergies able to reduce the production of petroleum origin, since they ‘dilute’ the heavy metal costs for finished fuels by intervening appropriately content characteristic of residues derived from oil; in on the units devoted to reaching commercial addition, their highly aromatic nature may contribute specifications and blending methods. For example, to improving the stability of conversion products with hydrotreated naphtha from liquefaction processes is respect to asphaltene precipitation phenomena, making an excellent feedstock for reforming units since it it possible to increase conversion into distillates contains high concentrations of cyclic hydrocarbons, without affecting the stability of the fuel oil produced. which are easy to convert into aromatic hydrocarbons with a high octane content. As such, this naphtha can produce a component for gasoline 2.4.5 Further developments of excellent quality, in addition to benzene, toluene of the technology and xylenes for the petrochemical industry. In order to be turned into jet fuel, the kerosene One of the main objectives of research in the field of fraction must be strongly hydrogenated to reach the direct liquefaction is to improve knowledge of process smoke point values required by current specifications. chemistry. This makes it possible to identify solutions This generates hydrocarbon blends containing high capable of lowering investment costs by reducing concentrations of naphthenes with two rings (decalins), severity conditions (temperature and pressure) and which have high heat of combustion in volumetric containing operating costs, determined mainly by the terms, excellent stability and very low freezing points: consumption of hydrogen and catalysts. Other efforts all properties which make them particularly suited as to improve the economic feasibility of the process components of high quality jet fuels. As far as the concern specific technological aspects such as the

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separation of liquids from the mineral component and A good hydrogenation catalysts’ dispersion can be unreacted organic materials. obtained by using water or oil-soluble precursors. This Numerous attempts have also been made to is the case for sulphides of iron, molybdenum or other evaluate the potential offered by so-called transition metals, which are generated in situ by the ‘non-conventional approaches’, that is solutions using decomposition and sulphidation by endogenous (or a chemistry different from that of hydrocracking. suitably added) sulphur of water-soluble precursors, such as iron sulphate, ammonium molybdate etc., or Pretreatment oil-soluble precursors, like some organic carboxylates It has already been noted that, in general, such as molybdenum-naphthenate or other liquefaction processes involve heating the coal rapidly metallorganic derivatives such as Molyvan A (N, to a temperature usually between 400 and 450°C; N-dibutyldithiocarbamate of oxythiomolybdenum). under these conditions this causes the instantaneous Studies of the efficacy of molybdenum-based production of reactive fragments (free radicals catalytic systems at low concentrations (hundreds of produced by thermal cracking), which, if not ppm) were started by Clyde Aldridge and Roby immediately saturated by hydrogenating reactions, Bearden at Exxon in the late 1970s (Aldridge and tend to recombine forming char (see again Fig. 4). Bearden, 1978) and were taken up by numerous other The reducing agent present in the system is not companies and research institutes, both for always able to compete effectively with applications in the field of direct liquefaction and for repolymerization reactions; for this reason, it may be the development of upgrading technologies for heavy preferable to control the production of free radicals by crudes (Montanari et al., 2003). As already noted, using increasing temperature stages or low microcrystalline molybdite, which is generated in situ temperature pretreatment stages (Temperature-Staged from oil-soluble precursors, presents as a very fine Liquefaction). Numerous studies have confirmed that, powder, consisting of nanometric particles of by operating with stages of increasing temperature molybdenum sulphide (MoS2) with a low degree of from 200-350 up to 450°C, it is possible to increase aggregation, highly dispersed within the feedstock. Its conversion yields with respect to the adoption of morphology and the absence of porous supports render isothermal conditions at high temperature (Derbyshire molybdenite particularly suitable for working et al., 1986). effectively as a hydrogenation catalyst in systems which are particularly difficult to upgrade due to the New catalytic systems presence of high concentrations of poisons, such as The development potential of direct liquefaction coal ashes or the heavy metals present in processes is closely linked to the improvement of concentrations of up to 700-800 ppm in some heavy catalytic systems. Catalysts for liquefaction processes crudes (such as the Venezuelan extra heavy crudes must guarantee an increase in the reaction velocity of produced in the Orinoco belt). hydrogen transfer processes, limiting repolymerization In the attempt to further increase the ratio between reactions to allow a high level of upgrading to be the degree of dispersion as well as the specific activity obtained even at low severity. – and thus reduce the concentration of the active phase In this context, if a distinction is made between the while maintaining an identical catalytic effect –, two main phases involved in liquefaction – coal numerous other systems have been proposed and dissolution and upgrading –, it is reasonable to studied, able to produce micrometric or suppose that it is better to use dispersed catalysts in submicrometric particles of transition metal sulphides the first stage, whereas it is preferable to use and nitrites. Examples are the use of specific supported catalysts in the second. precursors containing heteroatoms or bimetallic As far as the former are concerned, although pairings, laser-pyrolysis, plasma techniques, numerous basic studies have been carried out, microemulsions etc. (Delbianco et al., 1995). None of experiments at the level of pilot plant have mainly these attempts, however, seem able to significantly been limited to the oxides and sulphides of metals improve the performances obtainable with such as iron, molybdenum and a few others. For all molybdenite generated in situ, as described above. these, the most important property is the capacity for Another critical aspect of the use of dispersed adequate dispersal throughout the feedstock. Good catalysts concerns their recovery or, more accurately, dispersion makes the catalyst available on a local level, the impossibility of recovering them from the limiting the production of char; this also reduces the unconverted product at an acceptable cost. For this consumption of catalyst, which cannot be easily reason, economic considerations have limited recovered in this stage of the process since it mixes experiments to low cost materials, and especially iron, with the mineral component of the coal. sometimes promoted with the addition of small

130 ENCYCLOPAEDIA OF HYDROCARBONS HYDROCARBONS FROM THE DIRECT LIQUEFACTION OF SOLID FUELS

quantities of molybdenum and other metals, such as component completely, and in any case entail wolfram, ruthenium, etc. additional costs. For this reason, it is necessary to Supported catalysts are generally proposed and examine all the implications of a pretreatment process used in the second phase of the process to produce upstream of liquefaction on a case by case basis, distillates, that which transforms the syncrude from evaluating the cost/benefit ratio of this operation. coal, still highly aromatic and rich in heteroatoms, into naphtha and gasoil with a low sulphur, nitrogen and New conversion systems oxygen content. In general, the catalysts used are Over the years, research on new coal conversion classic hydrotreating catalysts developed for the routes has suggested several solutions that are upgrading of petroleum products. However, since coal certainly of interest from the standpoint of knowledge, liquids have fairly different properties from those of but which for various reasons cannot currently be comparable distillation cuts or residues deriving from considered equally valid from an industrial point of petroleum, the performance of these catalysts are often view (cost, process complexity, performance, etc.). lower, both in terms of efficacy and life. Among these are the use of water-based systems, acid To prepare suitably tailored catalysts, supports catalysts and bioconversion. which are less sensitive to the deposition of coke are used (stabilized aluminas, active carbons with a high Water-based systems surface area, etc.) alongside active phases specially The first studies of the use of water and carbon suited to removing nitrogen compounds, in particular monoxide to liquefy low rank coal were carried out by the pairing Ni/W, Ru, etc. (Derbyshire, 1988). Franz Fischer in around 1920. More recently, this However, the lack of genuine industrial interest has not solution has been taken up again and widely studied at encouraged research on tailor-making these catalysts. various research centres (Pittsburgh Energy Research Center, SRI International, Eni and others); it is Product separation technologies generally known by the name of Costeam or CO-steam One of the major critical aspects of technological liquefaction (Ross, 1984). type inherent in liquefaction processes is the In Costeam, the hydrogen is produced in situ by the separation of the liquids produced in the first stage of WGS reaction, suitably catalyzed by alkalis such as the process from the mineral component of the sodium carbonate. According to various authors, the original coal. Among the proposed solutions, in reaction to convert coal into liquids may in this case, addition to classic filtration and centrifuging, solvent rather than following a non-radical mechanism, be extraction seems to provide the best performance. A promoted by the formiate ion, that is the intermediate particularly interesting solution has been developed by of the WGS reaction. Additionally, under the reaction Kerr McGee along the lines of deasphalting processes, conditions adopted (T 400°C and a pressure of over known as solvent deashing. This involves a multi-stage 200 bar), the water is in almost supercritical conditions extraction of the products of coal conversion with and thus constitutes an excellent reaction medium, toluene-type solvents, used in a ratio of 2:1 with able to solvate the organic fragments deriving from respect to the feedstock to be treated, at a temperature cracking processes. close to 200°C. This system makes it possible to easily Costeam seems particularly suited to liquefying low separate the insoluble fraction (ashes and unreacted rank coals, which usually contain large amounts of coal and IOM which can be gasified to produce the water (up to 60%). The experimental evidence obtained hydrogen needed for the liquefaction reaction) from shows that this process is able to guarantee high the extract, from which the solvent is then recovered conversions of coal to liquids, and seems particularly by suitably varying the temperature and pressure. This effective at removing heteroatoms, especially oxygen. technology seems to be fairly consolidated, although Other potential advantages concern the non-use of there are optimization margins in the choice of organic solvents and the direct use of syngas rather than solvents and process conditions. pure hydrogen. However, it requires extremely severe The simplest way of overcoming the deashing reaction conditions, especially as concerns pressure (a problem involves the prior removal of the mineral direct consequence of the amount of water needed) and component with coal beneficiation processes. poses problems of separation during the product Using coals treated in this way may be extremely recovery phase. For this reason, the development of this advantageous from an operational point of view, in line of research was effectively halted in the late 1980s. part due to the potential for using the most suitable The idea of carrying out the coal dissolution phase catalytic systems from the very first stage. However, using reactives able to generate hydride ions was beneficiation processes are not particularly selective, relaunched in the late 1990s by CONSOL Energy in the sense that they are unable to remove the mineral R&D, which – in collaboration with the University of

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Kentucky and with the sponsorship of the DOE – is catalysts. Nonetheless, over time there has been an researching a process based on the use of sodium increase in research activities on biosolubilization formiate produced at 340°C from carbon monoxide phenomena in coal, especially following the brilliant and sodium hydrate, or on the use of methyl formiate. results obtained by René Fakoussa and later by Martin Cohen in the early 1980s (Fakoussa, 1981; Cohen and Acid catalysts Gabriele, 1982), showing that solid particles of low Acid catalysts, such as zinc and tin chlorides etc, rank coal, placed in contact with fungus type have the ability to promote cracking under micro-organisms (micorganisms), were turned into sub-pyrolysis conditions (300-330°C) through a droplets of liquid. However, the biological process is mechanism of ionic type. These Lewis acids not equivalent to the direct liquefaction of coal by the encourage the rupture of bonds of the ether type with chemical route, since the reduction in molecular the formation of carbon ions, which then trigger weight progresses as an effect of oxidative degradation cracking processes giving rise to light hydrocarbons and the liquid produced thus maintains a low H/C ratio (Fig. 13). The reaction takes place at high hydrogen and a high content of oxygen and other heteroatoms pressure (up to 350 bar). present in the feedstock. These catalysts were originally studied by the Bioconversion involves the preparation of a cell Consolidation Coal Company around the 1960s, and culture, which is placed in contact with the substrate to proved effective in promoting coal conversion, be treated, finely milled (less than 1 mm, but more showing a high selectivity for the production of light frequently around 100 mm) and dispersed in water. The distillates (Alpert and Wolk, 1981). Subsequently, the conditions under which the bioreactor operates Conoco company developed a process based on the (temperature, pH, etc.) are closely linked to the type of use of molten zinc chloride, which reached the micro-organism used, but generally are extremely mild construction of a 1 t/d demonstration plant; however, if compared with those used for hydroliquefaction this only remained in operation for a short time, since processes (which is the main potential advantage of enormous problems emerged due to the corrosion of this approach). the metal parts of the plant. Other negative aspects The types of coal most easily attacked are the most included the difficulty of recovering these catalysts oxidized ones, such as lignites and sub-bituminous (part of which were also consumed by the formation of coals, while the most active micro-organisms in these insoluble oxides and sulphides) and by the fact that, transformations are also those which are best suited to not possessing hydrogenating properties, they were the biodegradation of the lignin present in plants, unable to control repolymerization processes, so that which has a complex natural polymeric structure that the production of distillates was accompanied by that is difficult to degrade. The following is a list, albeit of large amounts of char; finally, the degree of liquid incomplete, of the most studied micro-organisms: upgrading was extremely low. Coriolus versicolor, Phenerochaete chrysosporium, However, this approach also continues to be Penicillium waksmanii, Streptomyces viridosporus, studied on a basic level, and various studies have been Pleorotus ostreatus, Pycnoporus cinnabarinus, undertaken on the use of superacids (especially Streptomyces setonii, Trametes versicolor. trifluoromethanesulphonic acid, also known as triflic The living cells, placed in contact with the coal acid) under low severity conditions, in other words particles, secrete highly oxidative enzymes of various temperatures below 300°C (Fraenkel et al., 1991). types (peroxydases) into the culture medium (or the immediately surrounding atmosphere). These Bioconversion enzymes, with the help of cofactors which have not yet The extremely complex molecular structure of coal been fully identified, depolymerize the complex makes this substrate difficult to use for biological structures of coal in a non-specific way. The

Fig. 13. Cracking of the molecular O ZnCl2 CH2 CH2 structure of coal using O acid catalysts.

ZnCl2 H OH

ZnCl2

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development of large-scale processes, involving the synthesis gas, in other words a mixture of hydrogen activity of living cells on coal in batch, is hindered by and carbon monoxide, is favoured; this can the activity of the enzymes released by the cells, which subsequently be turned into hydrogen through the kill the cells themselves under the concentrations WGS reaction. The syngas mixture, however, can also capable of ensuring interesting conversion rates. In any be transformed directly into hydrocarbon liquids case, attempts to use cell-free systems, in other words (waxes) by Fischer-Tropsch synthesis (indirect more or less purified enzymes, have shown that these liquefaction, see Chapter 2.6) or monetized as a fuel systems are less efficient than those based on living for power generation, for example with combined cells and are in any case too expensive. cycles in the so-called IGCC configuration The optimization and scale-up of the solubilization (Integrated Gasification Combined Cycle) (see Vol. reaction, then, present technical problems that are II, Chapter 7.3). difficult to resolve, to the extent that the only As far as the process is concerned, the gasification commercial application currently reported is the of coal or other solid fuels (pet-coke, etc.) can be production of fertilizers from lignite. On a laboratory carried out using various technological solutions; scale, however, it has been shown that it is possible to among these, by far the most widespread are the use the products of coal biosolubilization for the Chevron-Texaco, Shell and Lurgi technologies production, via the biotechnological route, of (Higman and van der Burgt, 2003). polyhydroxyalkanoates (bioplastics); the use of The Chevron-Texaco technology, recently acquired biosurfactants able to ease the transportation of stable by General Electric (GE-Chevron Texaco), uses a coal suspensions in water is also being researched single reactor fed from above with a dispersion of coal (Fakoussa and Hofrichter, 1999). in water (60-70% coal) and 95% oxygen. The gasification reactor operates at about 1,200-1,500°C and a pressure of 20-50 bar depending on the purpose 2.4.6 Integration with hydrogen of the syngas (chemical or energy use). In the production technologies configuration with heat recovery (radiant cooler), the gases produced are cooled by heat exchangers from Apart from bioconversion technologies, which which high-pressure steam is recovered, while the currently do not seem to offer interesting development molten ashes are ‘plunged’ into water, producing potential, direct liquefaction processes involve a vitrified ashes and particulate matter (slag). significant hydrogen consumption; this represents one Alternatively, the GE-Chevron Texaco process also of the main items of expenditure for the entire process. possesses the so-called quench technology, in which It has already been noted that the hydrogen for energy the gas is cooled by rapid direct contact with water. purposes is produced from hydrocarbon fuels of In the Shell technology, by contrast, the gasifier is various nature; in the case of direct liquefaction, by far fed with powdered coal, which is forced into the the most interesting solution seems to be gasification. reactor by pressurized gas (nitrogen or the syngas The latter is an endothermal reaction between coal and produced). Inside the reactor, the coal is gasified with steam, supported by the simultaneous partial oxygen and steam in a temperature interval generally combustion of the feedstock, generally using pure between 1,500 and 1,600°C, and a pressure range of oxygen as a combustion agent. 27 to 50 bar. Under these conditions, a gas mixture The main elemental reactions that occur during this consisting mainly of syngas is produced; this is cooled process (under standard conditions) are shown by the heat exchange, which occurs through radiation schematically below: and convection, producing high pressure steam. Under • Water gas reaction the reaction conditions adopted, the mineral ᭤ C+H2O CO + H2 131 kJ/mol component fed in alongside the feedstock melts and • Combustion reactions runs down the walls of the reactor; it is then ᭤ C+1/2O2 CO 111 kJ/mol discharged into water to recover the ashes. ᭤ CO + 1/2O2 CO2 283 kJ/mol Finally, the Lurgi Dry Ash process operates with a ᭤ H2 +1/2O2 H2O 242 kJ/mol moving bed reactor fed in a counter-flow: the anhydrous • Boudouard’s reaction coal enters from the top, whereas the blend of reagent ᭤ C+CO2 2CO 172 kJ/mol gases (oxygen and steam) are sent in from below. As a • Methanization reaction result, the temperature of the reactor increases from top ᭤ C+2H2 CH4 75 kJ/mol to bottom (from 500-600°C up to over 1,000°C), so that The composition of the gases exiting the reactor there are significant quantities of condensed organic depends on the nature of the feedstock and the products among the reaction products; as such, this operating conditions. Generally, the formation of process, though consolidated, reliable and less

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expensive in terms of oxygen consumption, is not Despite this, the production of synfuel from coal particularly suited to the production of hydrogen. remains a complex technology with extremely high specific investment costs (above 60,000 $/bpsd), especially if it is desired to guarantee acceptable 2.4.7 Process economics environmental standards. However, industrial and development potential initiatives (see Section 2.4.2) show that using the of the technology best available technologies it may be possible, at least on a local level, to invest in these processes, The enormous efforts made, from the post-war period which present the enormous advantage of onwards, in over 50 years of R&D, have enabled dissociating a country’s economic development from significant improvements to the direct coal the uncertainties associated with oil supply. liquefaction processes, leading to a gradual decrease in Considering that the R/P (Reserves/Production) ratio the costs of producing syncrude. DOE data show that, for conventional crude oil and natural gas is using the same evaluation bases (Coal Illinois n. 6, generally estimated to be 40 and 60 years 50,000 bbl/d capacity plants, identical economic respectively, it is reasonable to predict that recourse hypotheses), today’s direct coal liquefaction plants may to alternative fossil fuels will play an increasingly be able to produce synthetic crudes at a cost less than important role, due in part to technological advances half that which could be obtained with first generation which have made it possible to significantly reduce EDS-type plants (Burke et al., 2001; Fig. 14). the costs of transformation into syncrude. Among This achievement can be attributed largely to these sources, alongside heavy oils, tar sands etc., it research on the chemistry of the reaction, which has is reasonable to suppose that also coal (i.e. the most made it possible to propose more effective process abundant fossil fuel with an R/P of over 200 years) schemes, able to increase overall conversion yields by may contribute to supplying the international energy more than 30% (extremely important for capital market with additional quantities of liquid intensive technologies such as those under discussion), hydrocarbons through direct or indirect liquefaction in addition to improving the distillates selectivity as technologies. opposed to hydrocarbon gases, with a resulting It is important to remember that the first of reduction in hydrogen consumption (another factor these two approaches presents advantages from an with a significant impact on the costs of the process). environmental point of view, linked to improved Furthermore, the enormous amount of work carried energy efficiency (70% as opposed to the 55% of out on numerous pilot and demonstration plants has the indirect approach), in other words lower CO2 made it possible to improve many other aspects of the emissions per barrel of syncrude produced. technology, especially as concerns the solid-liquid Furthermore, coal syncrudes can be transported separation of the products, and to overcome specific using existing infrastructure, mainly controlled by problems presented by the process, such as the erosion the petroleum industry, thus simplifying of valves or corrosion in the distillation columns due marketing. Rather than being refined at the to the presence of high concentrations of acid production site, the synthetic crude could be sent compounds. directly to the refinery and treated in blends with traditional crudes without compromising the 100 reliability of classic refinery unit operations. There are also a series of potential synergies between liquefaction plants and the refinery, 80 linked to the production of hydrogen, the management of by-products etc., which may 60 contribute to lowering the cost of producing distillates from coal. 40

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