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Deliverable Deliverable 5.8
Study on energy carrier use for entrained flow gasification
Workpackage: WP5 Deliverable No: D5.8 Due date of deliverable: 30/09/14 Actual date of delivery: 06/03/15 Version: Final / Vers.1.2 Responsible: KIT Authors: Prof. Dr. Edmund Henrich, Prof. Dr. Nicolaus Dahmen, Andreas Niebel Contact: [email protected] Dissemination level: PU-Public
Publishable Summary For biomass gasification, a variety of technologies exist. For large-scale syngas generation with downstream synthesis of organic chemicals or synfuels, pressurized entrained flow (PEF) gasification has emerged as the preferred technology. The technology is flexible and can accommodate many different feedstocks, but at the expense of more technical effort for gasifier feed preparation. The various feed preparation methods and feed properties as well as the gasifier feeding systems are described in context with the other interacting steps in the process network. Gases and liquids (fluids) either with or without an entrained or suspended pulverized fuel are suitable feed forms. The fluid feed can be continuously transferred with pumps or compressors into a highly pressurized gasification chamber. Immediately at the gasifier inlet the fuels are mixed in special nozzles with pure oxygen (and steam) as the gasification agent; liquid and slurry fuels are atomized simultaneously. PEF gasification proceeds at high temperatures >1000 °C and high pressures up to 100 bar or more in a gasifier flame in the course of a second. The total residence time in the gasification vessel is only few seconds and the gasifier volume is correspondingly small. Solid or liquid fuels must be present as small particles or droplets with a sufficiently large surface area for complete conversion in few seconds. Ash is removed as molten slag and their melting behavior determines the minimum gasification temperature. In biofeedstocks – mainly lignocellulose like wood or straw – the cellulose fibers prevent direct milling to a suitable powder and generate fiber muddles. A suitable PEF gasifier feed can be prepared from biomass pyrolysis products; preferred processes are fast pyrolysis or torrefaction. Biomass pyrolysis destroys cellulose fibrils and the chars are brittle and easily pulverized. The pulverized chars can be transferred to a pressurized on-site gasifier either with an inert gas as a dense char particle stream from a pressurized fluidized bed or as a slurry, after char suspension in the pyrolysis liquids or any other combustible (waste) liquid or even as a water slurry, as it is already practiced with pulverized coal. Slurries, especially bioslurries, are not only a suitable PEF gasifier feed form, but also a storage and transport form with a ca. 10 times higher energy density compared to the initial biomass. Bioslurry transport from many regional pyrolysis facilities to a large and more economic gasification/synthesis plant is a unique feed preparation and handling characteristic of the KIT bioliq® process.
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Table of Content
Publishable Summary ...... 2 Table of Content ...... 3 Report ...... 4 1 A suitable gasifier for downstream synthesis of organic chemicals and synfuels ...... 4 1.1 Fixed-bed gasifiers ...... 5 1.2 Fluidized-bed gasifiers ...... 5 1.3 Entrained flow gasifiers ...... 6 2 Essential operating and design features of PEF gasifiers ...... 8 2.1 Operating conditions of PEF gasifiers ...... 8 2.2 Design characteristics of PEF gasifiers ...... 11 2.3 Interactions in the total PEF gasification/synthesis process network ...... 14 3 Suitable feed preparation, feed properties and feeding systems for PEF gasifiers ...... 17 3.1 Gaseous fuel ...... 17 3.2 Liquid fuel, transfer and atomization ...... 17 3.3 Pulverised fuel, handling and transfer ...... 17 4 Experience from commercial PEF gasifiers ...... 20 4.1 Characteristics of commercial gasifiers ...... 20 4.2 Description of typical fuel feeding techniques ...... 22 4.3 Status of the world gasification technology ...... 26 5 Biomass conversion to PEF gasifier feed ...... 27 5.1 Focus on lignocellulosic feedstocks ...... 27 5.2 Torrefaction ...... 27 5.3 Fast pyrolysis ...... 27 6 Pilot projects for large-scale PEF gasification of biomass ...... 28 6.1 Lulea university, Sweden (formerly Chemrec): Black liquor feed ...... 28 6.2 Feeding concept of the Choren Carbo-V process ...... 29 6.3 KIT “bioliq” pilot facilities ...... 29 7 Recent process developments ...... 30 8 Experience from slurry gasification ...... 31 9 Summary and conclusions ...... 34 10 Sources ...... 36
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Report 1 A suitable gasifier for downstream synthesis of organic chemicals and synfuels Gasification can convert almost all organic feeds with HHV > 10 MJ/kg into syngas, a mix of CO and H2, and consumes only 20-40 % of the O2 required for stoichiometric combustion. Syngas is a very flexible intermediate (platform chemical) and educt for the selective catalytic production of numerous valuable organic chemicals and fuels at certain temperatures and higher pressures [14],[15]. The alternative use as a fuel is syngas combustion in IGCC plants for the generation of electricity or high temperature process heat. All these technical possibilities are already applied commercially. The main gasifier types are depicted in Fig. 1; essential design and operating characteristics are summarized in Tab. 1.
Fixed bed gasifier Fluidized bed gasifier Entrained flow gasifier
Counter current Stationary Downdraft
Solidsolid fuel fuel Rawraw syngas syngas O2 (air) Solid fuel Raw syngas Solid or steam slurry fuel Fuel spray Bed (liquid, slurry, material Bubblingsand bed bed Fuel pulverized..) ( fuel) Ash fluidising gas Ash (slag) ash Fluidization medium Air (O2), steam syngas recycle Molten slag Raw syngas
Co-current Circulating Updraft
Air (O2) Raw syngas - Raw syngas Solid fuel steam
Bed material recirculation
Ash Fuel, air, Fuel, air, Fuel O2, steam O2, steam Ash (slag) Raw syngas Fluidization medium Slag
Fig. 1: Main gasifier types [8]
Tab. 1: Design and operating characteristics of the main gasifier types [14],[22] fixed bed fluid bed entrained flow (EF) gasification conditions: fuel type solid solid powder, liquid, gas fuel size 10-1-10-2 10-2-10-3m ca. ≤ 10-4m pressure 1-30 bar 1-30 bar 1-100 (+) bar residence time 103-104s ca. 102 s 1-few s raw syngas purity low (tar, CH4) low (tar, CH4) high fuel/gas flow countercurrent mixed cocurrent design characteristics: reactor geometry cylinder cylinder cylinder reactor wall refractory refractory membrane, refractory bed material - sand (olivine) - carbon conversion >90% >95% >99% ash dry (solid) dry (solid) molten slag
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1.1 Fixed‐bed gasifiers In fixed-bed reactors, the feedstock is exposed to the gasifying agent in a packed bed that slowly moves from the top of the gasifier to the bottom, where the ash or slag is discharged. By moving through the reactor, the biomass passes through distinct zones of drying, pyrolysis, oxidation, and reduction. Usually the different types of fixed-bed reactors are characterized by the direction of the gas flow through the reactor and consequently are denoted as updraft, downdraft and horizontal (crossdraft) gasifiers. Depending on fuel and product gas application, a multitude of fixed-bed gasifier designs exist. On a small scale, fixed-bed reactors are used for district heat and power production up to fuel input capacities of 20 MWth. On a large scale, the updraft pressurized type has been successfully used since decades. Among all gasification technologies, the Lurgi pressurized fixed-bed gasifier is the economically most successful one. The gas of high calorific value generated by autothermal counter-current pressurized gasification, with a water steam/technical oxygen mixture being the gasification agent, is used for town gas, SNG and synthesis gas production for ammonia and FT-syntheses as well as being integrated in IGCC power plants. To study the influence of the operating pressure on the gas yield and composition, specific performance and thermal efficiency, a 10 MPa coal fired pilot plant (Ruhr100) was constructed and operated between 1979 and 1983. By increasing the pressure from 3 to 9 MPa, the methane yield was improved from 10.3 to 15.5%. At the same time, the oxygen demand in this plant was reduced by ca. 12%, due to the increased exothermic methane formation. The cold gas efficiency was raised from 70 to 80%. Based on the results, a detailed comparative study for SNG production for a plant design for 3 and for 9 MPa was performed. It turned out that lower investment costs and thus capital-dependent and maintenance costs are required for the high pressure alternative.
1.2 Fluidized‐bed gasifiers For the production of synthesis gas for chemical syntheses fluidized-bed gasifiers with high carbon conversion efficiencies and low yields of hydrocarbons have been developed. In fluidized-bed gasifiers, fine fuel particles are rapidly mixed and heated e.g. by hot fluidized sand. Due to the intense mixing, the gasification reactions cannot be divided into local zones as in the case of fixed-bed reactors, but occur throughout the whole bed, leading to a uniform temperature distribution. The degree of fluidization can be small (bubbling fluidized bed, BFB) or high (circulating fluidized bed, CFB). The former reactors, which have a well- defined interface between the reaction zone of the fluidized bed and the freeboard above the bed surface, are commonly used because of their robust operation. In a CFB gasifier, there is no distinct interface between the fluidized sand bed and the freeboard; the entrained media and char are recycled back to the gasifier via a cyclone. The carbon conversion is considerably better than in BFB gasifiers, but operation is more complex und less robust.
High temperature Winkler generators (HTW) exhibit high carbon conversion efficiencies and a low hydrocarbon formation. This has been achieved by two reaction zones in the reactor. In the BFB, the fuel is contacted with the main quantity of gasification agent, a mixture of steam and oxygen or air. Above the fluidized bed, an additional gasification agent is added to increase the temperature in the zone downstream of the gasification. These syngas generators have been commercially operated between 1956 and 1997 with feedstock capacities of up to 30 t/h and at pressures of up to 2.5 MPa. Efficient hydrogenating gasification to produce SNG was demonstrated with coal in a stationary fluidized-bed gasifier on pilot (7.5 t/h, 8 MPa) and semi-technical scales (300 kg/h of lignite, 10 MPa) within the prototype nuclear process heat project (PNP) in Germany. Helium of high temperatures of about 900 °C, generated in a high-temperature nuclear reactor (HTR), should be used for steam reforming to provide hydrogen in the subsequent coal gasification unit. Different types of coal were converted at around 920 °C to a methane-rich
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raw gas with a carbon conversion degree of 65%. Within the same project, steam gasification of coal in an allothermal BFB reactor was studied on a semi-scale plant (230 kg/h). The required process heat should be generated in the HTR reactor, transferred to the gasification unit by 1000 °C hot helium, leading to an increased carbon efficiency of the overall process. In the test facility, electrically heated He was supplied to a tube bundle heat exchanger installed in the fluidized bed of the gasification reactor. Gasification temperature and pressure were 800-850 °C and 4 MPa, respectively. The first pressurized CFB pilot gasifier for biomass is operated in Värnamo, Sweden. The plant was run between 1996 and 1999 and has been shut down in 2000 after highly successful operation as a Biomass Integrated Gasification Combined Cycle (BIGCC) demonstration plant and a CHP gasifier plant. The feedstocks used successfully in the plant were different wood fuels, bark, straw and waste-derived fuels. Of the total fuel input of 18 MW, 6 MWel were fed into the public network, and 9 MWth were supplied to the district heating network of Värnamo during the EU funded Chrisgas project. In this plant, dried, comminuted wood fuel (e.g. wood chips) was fed by a lock-hopper and a screw into the air-blown CFB gasifier. Hot syngas carried the bed material up into a cyclone; solids returned to the bottom of the gasifier. Average gasification temperatures were slightly below 1,000°C at 1.8 MPa of operating pressure. Re-organized to the Växjö Värnamo biomass gasification centre (VVBGC) the plant has been rebuilt to produce clean synthesis gas for chemical syntheses starting in 2012.
1.3 Entrained flow gasifiers In fixed bed gasifiers the fuel particle size must be large enough to allow a free co- or countercurrent gas flow with little pressure drop through the gaps of the bed. The temperature in fixed and fluidized beds must be below the ash softening point somewhere below 1000 °C, because a sticky ash or molten slag would plug the gas flow channels. At lower gasification temperatures the raw syngas is contaminated with substantial amounts of unconverted tar vapours with high molar masses and boiling points and alkanes, especially methane and requires much effort for gas cleaning. In order not to poison the sensitive synthesis catalysts, the syngas must be of high purity and free of dust and tar. Unlike most fixed-bed and fluidized-bed gasifiers, entrained-flow gasifiers (EF) are able to generate a gas practically free of tar with only little methane. Entrained flow gasifiers operate above 1000 °C, usually above 1200 °C, and the ash is removed as a molten slag. In a small gasifier volume with one or few s residence time, small solid fuel particles and little liquid droplets below ca. 10-4m size can be rapidly and completely gasified with > 99% carbon conversion. At higher pressures and partial pressures the gasification rate of solid particles accelerates with about the square root of the operating pressure (in the film diffusion regime!) and feed conversion can become as high as 99,9 %. At higher temperatures above about 1200 °C, tar vapours are efficiently gasified and an almost tar-free (e.g. benzene ≤ 100 ppmv) and low methane (≤ 0.5 % CH4) syngas is obtained for thermodynamic reasons. This simplifies the downstream cleaning and conditioning steps for raw syngas prior to the selectively catalysed syngas reactions for the production of organic chemicals and fuels like methanol, DME, Fischer-Tropsch-diesel etc. [21],[30]. Modern catalysts are sensitive to a number of poisons down to the trace level in the ppb range. Thus, a low-poison gasifier feed is desirable. Soon after the first application of gasifiers to coal conversion operated at atmospheric pressure, large scale operation of pressurized gasifiers at pressures between 2.5 to 4.0 MPa became state of the art.
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The advantages motivating their development were: Increase in the reaction rate, (ii) Higher specific throughput, (iii) Increased methane yield at low temperature operation (for SNG production), (iv) Reduction of the gas volume to be treated, (v) Saving the work of compression for the subsequent use of the gas produced (gas turbine, methanol, ammonia, Fischer-Tropsch synthesis)
Most syngas reactions are conducted at higher pressures – up to 100 bar or even more – for thermodynamic reasons. If the gasifier is already operating slightly above the downstream synthesis pressure, expensive intermediate syngas compression can be avoided. High pressure gasification is easily realized in pressurized entrained flow (PEF) gasifiers because of the short residence times and the corresponding small reactor dimensions, but at the expense of special feed properties, especially the use of pulverized solids. In order to guarantee a smooth integration into the total process chain, the choice of a suitable feed and feeding system must be based on a detailed knowledge of design and operating characteristics of PEF gasification and the downstream gas cleaning and synthesis steps. PEF gasifiers are the preferred type for processes with downstream synthesis. The following script is therefore confined to production and handling of suitable feed forms for this gasifier type. The information in the script should enable the reader to propose a reliable sequence of operations and to select suitable equipment types for the conversion of a certain feedstock into a suitable PEF gasifier feed. The main advantages and disadvantages of PEF gasifiers are briefly summarized as follows: Advantages: 1. Short conversion time of one or few seconds due to high temperature and pressure and small solid fuel particles ≤ 0.1 mm. This results in a small reactor volume. 2. Complete carbon conversion > 99% 3. Ash is removed as liquid slag 4. Clean and almost tar-free and low methane raw syngas; this simplifies downstream gas cleaning 5. Membrane type PEF gasifiers allow rapid start-up and digest immediate shut-down without damage. 6. No expensive intermediate syngas compression if p(gasifier) > p(synthesis) 7. High feedstock flexibility 8. High state of development, large 0.5 GW PEF-gasifiers are already in operation for coal (Siemens EGT activities in China), 0.85 GW gasifiers are in development
Disadvantages: 1. At high syngas temperatures a significant percentage of the feed energy is converted into less valuable sensible heat of raw syngas and lowers syngas efficiency. 2. PEF gasifiers require more effort for feed preparation, but are therefore flexible “omnivores”. (Feed preparation efforts are usually overcompensated by process simplifications later on) 3. Membrane wall gasifiers (Shell, Siemens) have a higher heat loss through the membrane wall; only in large gasifiers (> 100 MW) losses are reduced to the percent range because of the lower surface-to-volume ratio in the gasifier chamber. 4. Poor heat recovery from the hot syngas with a simple and reliable water quench. With very expensive syngas coolers economic benefits are doubtful.
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It should be clear that the preferred gasifier type will depend on the type of feed used as well as the application/use of the product gas. 2 Essential operating and design features of PEF gasifiers Further reading: [14],[15],[27],[26]
2.1 Operating conditions of PEF gasifiers
2.1.1 Gasification temperature Almost all PEF gasifiers operate at temperatures above 1200 °C to get a molten slag with a sufficiently low viscosity in order of ca. 1 Pa·s for a sufficiently fast draining down the reactor wall by gravity. At such high temperatures and supported with efficient feed atomization, the thermodynamic equilibrium is quickly attained and easily estimated with the pressure-independent homogeneous shift reaction as the only key reaction:
CO + H2O ⇄ CO + H2
The disadvantage of a high gasification temperature is a higher O2-consumption and a reduction of the cold-gas or syngas efficiency. The lowest gasification temperature is usually given by the ash melting range, not by kinetic or thermodynamic restrictions. Black liquor gasification (see Lulea university, Sweden, section 6.1) is conducted at 950-1000 °C due to the much lower fusion range of the recovered mix of Na-salts of the cooking chemicals; this low temperature is an exception.
2.1.2 Ash melting behavior For slagging gasifiers the choice of ash melting ranges are of eminent importance for practicable gasification temperatures and efficiencies. For feedstocks with little ash and a high ash melting point, the addition of a flux could be reasonable to lower ash melting and gasification temperature. Prohibitive amounts of flux would be needed for high-ash fuels. Fig. 2 shows an oversimplified picture of the slag melting behavior, suited for rough estimates. A better representation is obtained from triangle diagrams with three basic constituents (see Fig. 3). Conclusion: The ash melting behavior is of eminent importance for PEF gasifiers and must be considered already during feed preparation. Flux addition is possible as powder or solution and also in a separate gasifier feed line. Partial evaporation of constituents which reduce the slag melting point like common for potassium, must be taken into account. [26]
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Fig. 2: Melting behavior of CaO/SiO2 slags (from [14])
Fig. 3: Slag melting behavior in the K2O-CaO-SiO2- triangle diagram (from [28])
2.1.3 Gasification pressure Pressurised gasification has been developed from previous versions at atmospheric pressure; e.g. the Koppers-Totzek gasifier [14] developed during World War II is the precursor of the SCGP type. Most IGCC versions operate at pressures below 30 bar e.g. in the Wabash river, Buggenum and Puertollano plants (see chapter 4); PEF gasifiers for chemical synthesis operate usually at higher pressures of up to 80 bar or more. At higher pressures the gasification rate increases with about the square root of pressure with a desirable further reduction of the equipment size. A synthesis of larger molecules from the small syngas molecules CO and H2 is favoured thermodynamically at higher pressures. Pressurizing already the gasifier at the beginning of the process is not only favourable for
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accelerating the gasification, but also avoids more technical effort later on for intermediate gas compression prior to synthesis. Rather high pressures up to 80 bar have already been realized in PEF gasifiers for chemical synthesis applications in the Texaco gasifier (now GEE) with pulverized coal/water slurries and organic waste liquids [31]. Another feeding technique is a dense stream of entrained coal powder in compressed inert gases like N2, CO2 or recycled syngas. This “dry” feed mode is typical for IGCC plants up to 30 bar gasification pressure, but causes much dilution at higher pressures and needs technically more complex lock hopper systems. Slurries or pastes can easily be pumped into pressure vessels up to 100 bar or more with various pump types like piston pumps or continuous screw, lobe and gear pumps or others which can digest corrosive or abrasive particles. A liquid must be efficiently atomized for fast gasification. The energy for efficient pneumatic atomization of liquids or slurries is supplied by the kinetic energy of the concurrent O2 gas stream with a relative velocity of > 100m/s. Volumes and velocity of the gasification agents O2 and eventually some steam are reduced in proportion with increasing pressure. In the same atomizing nozzle, the kinetic energy v² of the O2- injection for slurry atomization becomes too small, because the energy is reduced with the square of the velocity. Since the O2-to-feed mass ratio is in the range between ½ to 1, the design and safe operation of the very narrow O2-nozzles for high pressure applications is a critical task.
2.1.4 Elementary feed composition and oxygen consumption
The feed is represented by the moisture and ash-free (maf) organic composition CxHyOz plus inert ash and moisture. The small amount of volatile heteroatoms N, S, P can be neglected to a first approximation, as well as trace impurities like heavy metals as potential catalyst poisons. The formula representation of lignocellulose (LC) is helpful for use in empirical chemical equations for mass and energy balances. A feed with the formula [CxHyOz + u (H2O) + v ash] requires (x+y/4-z/z) O2 for a complete stoichiometric combustion to CO2 and H2O, and (x-z- u) O2 for an allothermal gasification, an endothermal conversion reaction to CO and H2. The O2 needed for autothermal gasification at a given temperature in a PEF gasifier, can be found by trial and error in few iterative steps by variation of the O2 consumption, which is usually in the 30 ± 10 % range of stoichiometric combustion. The reaction enthalpy rH for autothermal gasification can be calculated from the reaction equation and the known, tabulated thermodynamic data. The reaction enthalpy rH increases with increasing oxygen consumption and must be just sufficient to heat the gasification products to a preselected temperature e.g. to 1200 °C and to cover some thermal losses.
The trial and error procedure starts by assuming a zero CO2 formation in the shift reaction and continues by iteration for finding a suitable x value, which reproduces the known equilibrium constant K: