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 109иt/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 VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 113 HYDROCARBONS FROM NON-CONVENTIONAL AND ALTERNATIVE FOSSIL RESOURCES 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.