2.2.1 Fixed-bed gasifiers

In a fixed-bed process, the feedstock is supported by a grate. Combustion gases (such as steam, air, and oxygen) pass through the supported feedstock where the produced hot gases exit from the top of the reactor. Heat is supplied internally or from an outside source, but some carbonaceous feedstocks (such as caking coal) cannot be used in an unmodified fixed-bed reactor.

The descending-bed-of-solids system is often referred to as a moving or fixed bed or, on occasion, a countercurrent descending-bed reactor. In the gasifier, the feedstock (approximately 1/8-1 in., 3-25 mm, diameter) is laid down at the top of a vessel, while reactant gases are introduced at the bottom of the vessel and flow at relatively low velocity upward through the interstices between the coal lumps. As the feedstock descends, it is reacted first by devolatilization using the sensible heat from the rising gas, then hydrogenated by the hydrogen in the reactant gas, and finally burned to an ash. Therefore, the reactions are carried out in a countercurrent fashion.

Thus, the countercurrent fixed-bed gasifier (updraft gasifier, counterflow gasifier) consists of a fixed bed of carbonaceous fuel through which the gasification agent (steam, oxygen, and/or air) flows in a countercurrent configuration. The ash is either removed dry or as a slag. The slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent. The throughput for this type of gasifier is relatively low but thermal efficiency is high as the gas exit temperatures are relatively low, but as a result, production of methane and tar is significant at typical operation temperatures.

The main advantage of this gasifier is the effective heat exchange in the reactor. Previously, high-temperature syngas led out of the gasifier, drying the biomass material as it moves down the reactor. By that heat exchange that takes place, the raw syngas is cooled significantly on its way through the bulk filling. Syngas temperature at its exit from the reactor is about 250 °C (480 °F); in downdraft gasifiers, it is approximately 800 °C, or 1470 °F. Given that synthesis gas is exploited in order to dry the incoming feedstock, the system sensitivity to feedstock moisture content is less than that in other gasification reactors. On the other hand, the countercurrent flow of feedstock and syngas results in higher tar content (10-20% w/w) in the raw synthesis gas. Other advantages of updraft gasification include (1) simple, low-cost process; (2) able to handle feedstocks (such as biomass) with a high moisture and high inorganic content (such as municipal solid waste, MSW); and (3) proven technology.

The co-current fixed-bed (downdraft) gasifier is similar to the countercurrent gasifier, but the gasification agent gas flows in co-current configuration with the fuel (downward, hence the name downdraft gasifier). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources. The produced gas leaves the gasifier at a high temperature, and most of this heat is often transferred to the gasification agent added in the top of the bed, resulting in energy efficiency almost equivalent to that of the countercurrent gasifier. In this configuration, any produced tar must pass through a hot bed of char, thereby removing much of the tar from the product slate.

Due to the fact that the gaseous products from the pyrolysis step pass through the oxidation zone, the tar compounds concentration in the raw synthesis gas is less than that of updraft gasifiers. These gasifiers are easier to control but are more sensitive to the quality of the feedstock. For example, in the case of biomass feedstocks, updraft gasifiers can process biomass with moisture content up to 50% w/w, but in downdraft gasification a moisture content range between 10% and 25% is required.

The advantages of downdraft gasification are (1) up to 99.9% of the tar formed is consumed, requiring minimal or no tar clean-up; (2) minerals remain with the char/ash, reducing the need for a cyclone; and (3) it is a proven, simple, and low-cost process. However, the disadvantages of downdraft gasification are (1) the feed should be dried to a low moisture content (< 20% w/w moisture); (2) the synthesis gas exiting the reactor is at a high temperature, requiring a secondary heat recovery system; and (3) 4-7% of the carbon remains unconverted.

Cross-draft gasification reactors, which operate well on dry air blast and dry fuel, do have advantages over updraft gasification reactors and downdraft gasifiers. But the disadvantages – such as high exit gas temperature, poor carbon dioxide reduction, and high gas velocity, which are the consequences of the design – outweigh the advantages.

Unlike downdraft and updraft gasifiers, the ash bin, fire, and reduction zone in cross-draft gasifiers are separated. This design characteristic limits the type of fuel for operation to low mineral matter fuels such as wood, charcoal, and coke. The load-following ability of the cross-draft gasifier is quite good due to concentrated partial zones that operate at temperatures up to 2000 °C (3600 °F). The relatively higher temperature in cross-draft gasification reactors has an effect on gas composition, resulting in high carbon monoxide content and low hydrogen and methane content when dry fuel such as charcoal is used.

2.2.2 Fluid-bed gasifiers

In the fluidized bed gasifier (fluid bed gasifier), the fuel is fluidized in oxygen (or air) and steam and the ash is removed dry or as heavy agglomerates. The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive. Feedstock throughput is higher than for the fixed bed, but not as high as for the entrained-flow gasifier. The conversion efficiency is low and a recycle operation or subsequent combustion of solids is necessary to increase conversion. Fluidized-bed gasifiers are most useful for fuels that form highly corrosive ash (such as biomass) that would damage the walls of slagging gasifiers.

The fluidized-bed system uses finely sized feedstock particles and the bed exhibits liquid-like characteristics (in the form of fluid flow) when a gas flows upward through the bed. Gas flowing through the feedstock produces turbulent lifting and separation of particles, which result in an expanded bed having a greater feedstock surface area to promote the chemical reaction.

The fluidized-bed system requires the feedstock to be finely ground, and the reactant gases are introduced through a perforated deck near the bottom of the vessel. The volume rate of gas flow is such that its velocity is sufficient to suspend the solids but not high enough to blow them out of the top of the vessel. The result is an active boiling bed of solids having very intimate contact with the upward-flowing gas, which gives a very uniform temperature distribution. The solid flows rapidly and repeatedly from bottom to top and back again, whereas the gas flows rather uniformly upward. The reactor is said to be completely back-mixed and no countercurrent flow is possible. If a degree of countercurrent flow is desired, two or more fluid-bed stages are placed one above the other. Reaction rates are faster than in the moving bed because of the intimate contact between gas and solids and the increased solids surface area due to the smaller particle size.

Compared with the fixed-bed gasifiers, the sequence of reactor processes (drying, pyrolysis, oxidation, and reduction) is not obvious at a certain point of the gasifier because the processes take place in the entire reactor, thus resulting to a more homogeneous type of reaction. This means that more constant and lower temperatures exist inside the reactor, where no hot spots are observed. Due to the lower operating temperatures, ash does not melt and it is more easily removed from the reactor. In addition, sulfur-containing and chlorine-containing constituents of the feedstock can be absorbed in the inert bed material, thus eliminating the fouling hazard and reducing the maintenance costs. Another significant difference is that fluidized-bed gasifiers are much less to biomass quality than fixed-bed systems, and they can even operate with mixed biomass feedstock.

One critical advantage of a fluidized bed gasification system (as opposed to downdraft or fixed-bed system) is the use of multiple feedstocks without experiencing downtime (Capareda, 2011). Another important characteristic of the fluidized-bed system is the ability to operate at various throughputs without having to use a larger diameter unit. This is accomplished by changing the appropriate bed material. By using a larger bed material, a higher air flow rate is required for fluidization and thus more biomass may need to be fed at higher rates to maintain the same fuel-to-air ratio as before. The reactor freeboard must then be high enough so that bed materials are not blown out of the system. Also, a fluidized-bed gasification reactor is designed to be accompanied by a cyclone downstream of the gasifier to capture the larger particles that are entrained out of the reactor as a result of the fluidity of the bed and the velocity of the gas rising though the bed. These particles are recycled back into the reactor but, overall, the residence time of coal particles in a fluidized-bed gasifier is shorter than that of a moving-bed gasifier.

Uniform-bed formation in a fluid-bed reactor is very important for efficient bed utilization and consistent operation during gasification of the feedstock. In order to enhance the mixing and uniformity of a bubbling fluid bed, the feedstock is fed to the bed at multiple feed points around the circumference of the reactor vessel. In addition, the fluidization medium – whether air, oxygen, steam, or some combination of these substances – should be uniform in composition and should be introduced in multiple locations.

Finally, depending on the inflow speed, the fluidized-bed gasifier can be characterized either as a bubbling fluidized-bed system or as a circulating fluidized-bed system. The circulating fluidized-bed system corresponds to higher velocity of the gasification medium.

A bubbling fluid-bed design is generally more sensitive to bed utilization. The size of the feedstock particles greatly affects the rate of gasification and the ability of the biomass to migrate to the center of the bed in a bubbling fluid-bed design. With small particles, the gasification is very quick, and unburned material might not make it to the center of the bed, resulting in oxygen slip and a void center in the bubbling fluid-bed reactor. If all or a majority of the feedstock quickly gasifies, there will be insufficient char to maintain a uniform bed. For this reason, more detail is required in designing the in-feed system with the proper number of in-feed points and controlling and/or monitoring the size particle distribution of the feedstock material. A bubbling fluid bed will generally require additional feed points that must be balanced for larger particle sizes.

The advantages of the bubbling fluidized-bed gasifier are (1) it yields a uniform product gas; (2) it exhibits a nearly uniform temperature distribution throughout the reactor; (3) it is able to accept a wide range of fuel particle sizes, including fines; (4) it provides high rates of heat transfer between inert material, fuel, and gas; and (5) a high conversion is possible with low tar and unconverted carbon. The disadvantages of bubbling fluidized-bed gasification are that a large bubble size may result in gas bypass through the bed.

A circulating fluid-bed design, on the other hand, operates at a higher velocity and incorporates recycling of the char and bed material, resulting in complete mixing regardless of feedstock size. Generally, the circulating fluid-bed designs are more flexible but are still limited by the amount of very fine material that they can process.

The advantages of the circulating fluidized-bed gasifier are (1) it is suitable for rapid reactions, (2) high heat transport rates are possible due to high heat capacity of bed material, and (3) high conversion rates are possible with low tar and unconverted carbon. The disadvantages of the circulating fluidized-bed gasifier are (1) temperature gradients occur in the direction of solid flow, (2) the size of fuel particles determine minimum transport velocity (high velocities may result in equipment erosion, and (3) heat exchange is less efficient than in the bubbling fluidized-bed system.

A novel reactor design that is particularly appropraite for biomass is the indirectly heated gasification technology that utilizes a bed of hot particles (sand), which is fluidized-using steam. Solids (sand and char) are separated from the syngas via a cyclone and then transported to a second fluidized-bed reactor. The second bed is air blown and acts as a char combustor, generating a flue-gas exhaust stream and a stream of hot particles. The hot (sand) particles are separated from the flue gas and recirculated to the gasifier to provide the heat required for pyrolysis. This approach results in a product gas that is practically nitrogen free and has a heating value of approximately 400 Btu/ft³ (Turn, 1999).

Another novel design is the new fluidized bed gasifier with increased gas-solid interaction combining two circulating fluidized-bed reactors (Schmid, Pfeifer, Kitzler, Pröll, & Hofbauer, 2011). The aim of the design is to generate a nitrogen-free product gas with low tar content and low fines (particulate matter) content. The system accomplishes this by division into an air/combustion and a fuel/gasification reactor – the two reactors are interconnected via loop seals to assure the global circulation of bed material.

The fuel/gasification reactor is a circulating fluidized bed but with the special characteristic of almost countercurrent flow conditions for gas phase and solids. The gas velocity and the geometrical properties in the fuel/gasification reactor are chosen in such a way that entrainment of coarse particles is low at the top. Due to the dispersed downward movement of the solids, volatile products are not produced in the upper part of the fuel reactor and the issues related to insufficient gas phase conversion and high tar content are avoided.

Finally, the design of a fluidized-bed gasification reactor is extremely important (for all of the reasons given earlier) because both the axial and radial transport of solids within the bed influence gas-solid contact, the thermal gradient, and the heat transfer coefficient. Segregation in a fluidized bed is affected by the particle density, shape, size, superficial gas velocity, mixture composition, and bed aspect ratio (the ratio of the static bed height divided by the dynamic or expanded bed height). Variations in the size, shape, and density of the fuel particles can cause severe mixing problems that result in changes in temperature gradients within the reactor, increase tar formation and agglomeration, and decrease the conversion efficiency (Bilbao, Lezaun, Menendez, & Abanades, 1988; Cranfield, 1978). Effective mixing of fuels of various sizes is needed to maintain uniform temperature, and a good mix depends on the relative concentrations of the solids in the bed and the velocity of the gas (Bilbao et al., 1988; Ghaly, Al-Taweel, Hamdullahpur, & Ugwu, 1989).

2.2.3 Entrained-bed gasifier

An entrained-bed system (entrained flow system) uses finely sized feedstock particles blown into the gas steam prior to entry into the reactor. Combustion occurs with the feedstock particles suspended in the gas phase.

In the entrained-flow gasifier (entrained-bed gasifier) a dry pulverized solid, an atomized liquid fuel, or a fuel slurry is gasified with oxygen (or, much less frequently, air) in co-current flow and the gasification reactions take place in a dense cloud of very fine particles. The high temperatures and pressures also mean that a higher throughput can be achieved; however, thermal efficiency is somewhat lower because, with existing technology, the gas must be cooled before it can be cleaned. The high temperatures also mean that tar and methane are not present in the product gas; however, the oxygen requirement is higher than for the other types of gasifiers.

The entrained-flow reactor requires a smaller particle size of the feedstock than the fluid-bed gasifier so that the feedstock can be conveyed pneumatically by the reactant gases. Velocity of the mixture must be about 20 ft/s (6.1 m/s) or higher, depending on the fineness of the feedstock. In this case, there is little or no mixing of the solids and gases, except when the gas initially meets the solids. Furthermore, apart from higher temperature, entrained-flow gasification usually takes place at elevated pressure (pressurized entrained-flow gasifiers), reaching operating pressures even up to 40 and 50 bars. The existence of such high temperatures and pressures requires a more sophisticated reactor design and construction materials.

The design of an entrained-flow reactor gives a residence time of the feedstock in the reaction zone to be on the order of seconds, or tens of seconds. This short residence time requires that entrained-flow gasifiers operate at high temperatures to achieve high carbon conversion. Consequently, most entrained-flow gasifiers are designed to use oxygen rather than air, as well as operate above the slagging temperature of the feedstock mineral matter.

All entrained-flow gasifiers are designed to remove the major part of the ash as a slag, because the operating temperature is well above the ash fusion temperature. A smaller fraction of the ash is produced either as a very fine dry fly ash or as black-colored fly ash slurry. Some fuels, in particular certain types of biomass, can form slag that is corrosive for ceramic inner walls that serve to protect the gasifier outer wall. However, some entrained-bed type of gasifiers do not possess a ceramic inner wall but have an inner water- or steam-cooled wall covered with partially solidified slag. For fuel that produces ash with a high ash fusion temperature, limestone can be mixed with the fuel prior to gasification in order to lower the ash fusion temperature. Typically, the fuel particles must be smaller than for other types of gasifier; in fact, the fuel must be pulverized.

2.2.4 Molten salt gasifier

The molten salt gasifier (molten metal gasifier), as the name implies, uses a molten medium of an inorganic salt (or molten metal) to generate the heat to decompose the feedstock into products. There are numerous applications of the molten bath gasification.

A number of different designs have evolved through various stages of development, but the basic concept is that instead of using a formed gasifying chamber where the reactions occur in suspension, the feedstocks are gasified in a molten bath of salt or metal. This type of design allows for more complete processing of the feedstock as well as greater variety of feedstocks to be efficiently processed in the same gasifier.

In molten bath gasifiers, crushed feedstock, steam air, and/or oxygen are injected into a bath of molten salt, iron, or feedstock ash. The feedstock appears to dissolve in the melt where the volatiles crack and are converted into carbon monoxide and hydrogen. The feedstock’s carbon reacts with oxygen and steam to produce carbon monoxide and hydrogen. Unreacted carbon and mineral ash float on the surface from which they are discharged.

High temperatures (approximately 900 °C, 1650 °F and above, depending on the nature of the melt) are required to maintain the bath molten. Such temperature levels favor high reaction rates and throughputs, and low residence times. Consequently, tar and volatile oil products are not produced in any great quantity, if at all. Gasification may be enhanced by the catalytic properties of the melt used. Molten salts, which are generally less corrosive and have lower melting points than molten metals, can strongly catalyze the steam-coal reaction and lead to very high conversion efficiencies.

In the process, the carbonaceous feedstock devolatilizes with some thermal cracking of the volatile constituents, leaving the fixed carbon and sulfur to dissolve in the molten salt (such as an iron salt) whereupon carbon is oxidized to carbon monoxide by oxygen introduced through lances placed at a shallow depth in the bath. The sulfur migrates from the molten salt to the slag layer where it reacts with lime to produce calcium sulfide.

The product gas, which leaves the gasifier at about 1425 °C (2600 °F), is cooled, compressed, and fed to a shift converter where a portion of the carbon monoxide is reacted with steam to attain a carbon monoxide to hydrogen ratio of 1:3. The carbon dioxide, that is, produced is removed and the gas is again cooled and enters a methanator where carbon monoxide and hydrogen react to form methane. Excess water is removed from the methane-rich product and, depending on the type of feedstock used and the extent of purification required, the final gas product may have a heat content of 920 Btu/ft³.

As another example, the Pullman-Kellogg process involves contacting feedstock with a melt of an inorganic salt such as sodium carbonate to convert the feedstock. In the process, air is bubbled into the bottom of the gasifier through multiple inlet nozzles and the feedstock (typically sized to 1/4 in.; 6 mm) is fed beneath the surface of the molten salt bath using a central feed tube whereupon natural circulation and agitation of the melt disperses the material. The main gasification reaction is a partial oxidation reaction and any volatile matter from the feedstock reacts to produce a fuel gas free of oils, tars, and ammonia. A water-gas shift equilibrium exists above the melt and, accordingly, in the reducing environment, carbon dioxide and water concentrations are minimal.

In practice, the molten salt design allows for some of the catalysis process to take place within the gasifier instead of downstream. For example, if the reactor or process design allows the hydrogen and carbon monoxide to be produced in separate distinct streams, the need for post-process separation prior to catalyzing into synthetic fuels will be eliminated.

The molten salt/metal design also allows for a greater variety of co-products to be produced on site. All gasification methods allow for co-production of various chemicals and gases, but the molten metal process adds various metals, such as vanadium and nickel as well as a variety of trace elements, to the mix. Most gasifier feedstocks contain trace metals that can then be extracted in the molten metal process, instead of being disposed of as slag. Also, the design and operation of molten metal reactors is such that the use of a fluxing material, such as lime or limestone, is required. When combined with the silica ash that is generated through normal gasification, the slag produced and removed from the molten metal reactor can be used directly as cement or formed into bricks for construction materials.

2.3.1 Gases

The products from gasification may be of low, medium, or high heat content (high Btu) as dictated by the process as well as by the ultimate use for the gas (Speight, 2008, 2011, 2013).

Product gases from fixed-bed versus fluidized-bed gasifier configurations vary significantly. Fixed-bed gasifiers are relatively easy to design and operate and are best suited for small- to medium-scale applications with thermal requirements of up to several megawatts thermal (MWt). For large-scale applications, fixed-bed gasifiers may encounter problems with bridging of the feedstock (especially in the case of biomass feedstocks) and non-uniform bed temperatures. Bridging leads to uneven gas flow, whereas non-uniform bed temperature may lead to hot spots, ash formation, and slagging. Large-scale applications are also susceptible to temperature variations throughout the gasifier because of poor mixing in the reaction zone.

Pressurized gasification systems lend themselves to economical syngas production; they can also be more flexible in production turndown depending on the reactor design. Typically, this is the case for both a pressurized bubbling reactor and a circulating fluidized-bed reactor, whereas the flexibility of an atmospheric fluidized-bed reactor is typically limited to narrower pressure and production ranges. Both designs are well suited for pressurized syngas production. Pressurized designs require more costly reactors, but the downstream equipment (such as gas clean-up equipment, heat exchangers, synthesis gas reactors) will consist of fewer and less expensive components (Worley & Yale, 2012).

In the process, the feedstock undergoes three processes during the conversation to synthesis gas (syngas) – the first two processes, pyrolysis and combustion, occur very rapidly. In pyrolysis, char is produced as the feedstock heats up and volatile products are released. In the combustion process, the volatile products and some of the char reacts with oxygen to produce secondary products (primarily carbon dioxide and carbon monoxide) and the heat required for subsequent gasification reactions. Finally, the char reacts with steam to produce hydrogen (H₂) and carbon monoxide (CO).

Combustion:

$2{\mathrm{C}}_{\mathrm{feedstock}}+{\mathrm{O}}_2\to 2\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$

Gasification:

${\mathrm{C}}_{\mathrm{feedstock}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{CO}$

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\mathrm{CO}}_2$

At the gasifier temperature, the ash and other feedstock mineral matter liquefies and exits at the bottom of the gasifier as slag, a sand-like inert material that can be sold as a co-product to other industries (e.g., road building). The synthesis gas exits the gasifier at pressure and high temperature and must be cooled prior to the cleaning stage. Full-quench cooling, by which the synthesis gas is cooled by the direct injection of water, is more appropriate for hydrogen production. The procedure provides the steam necessary to facilitate the water-gas shift reaction, in which carbon monoxide is converted to hydrogen and carbon dioxide in the presence of a catalyst:

Water-gas shift reaction:

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2$

This reaction maximizes the hydrogen content of the synthesis gas, which consists primarily of hydrogen and carbon dioxide at this stage. The synthesis gas is then scrubbed of particulate matter, and sulfur is removed via physical absorption (Chadeesingh, 2011; Speight, 2008, 2013). The carbon dioxide is captured by physical absorption or a membrane and either vented or sequestered.

Given that the synthesis gas is at high pressure and has a high concentration of carbon dioxide, a physical solvent can be used to capture carbon dioxide (Speight, 2008, 2013). The carbon dioxide is desorbed from the solvent by pressure reduction, and the solvent is recycled into the system.

2.3.2 Other gaseous products

There is a series of products that are called by older (even archaic) names that should also be mentioned here as clarification.

Producer gas is a low Btu gas typically obtained from a coal gasifier (fixed-bed) when air is introduced into the fuel bed instead of oxygen. The composition of the producer gas is approximately 28% v/v carbon monoxide, 55% v/v nitrogen, 12% v/v hydrogen, and 5% v/v methane with some carbon dioxide.

Water gas is a medium Btu gas that is produced by the introduction of steam into the hot fuel bed of the gasifier. The composition of the gas is approximately 50% v/v hydrogen and 40% v/v carbon monoxide, with small amounts of nitrogen and carbon dioxide.

Town gas is a medium Btu gas that is produced in the coke ovens and has the approximate composition of 55% v/v hydrogen, 27% v/v methane, 6% v/v carbon monoxide, 10% v/v nitrogen, and 2% v/v carbon dioxide. Carbon monoxide can be removed from the gas by catalytic treatment with steam to produce carbon dioxide and hydrogen.

Synthetic natural gas (SNG) is methane obtained from the reaction of carbon monoxide or carbon with hydrogen. Depending on the methane concentration, the heating value can be in the range of high-Btu gases.

2.3.3 Tar

Another key contribution to an efficient gasifier operation is the need for a tar reformer. Tar reforming occurs when water vapor in the incoming synthesis gas is heated to a sufficient temperature to cause steam reforming in the gas conditioning reactor, converting condensable hydrocarbons (tars) to non-condensable lower molecular weight molecules. The residence time in the conditioning reactor is sufficient to also allow a water-gas shift reaction to occur and generate increased amounts of hydrogen in the synthesis gas.

Thus, tar reforming technologies, which can be thermally driven and/or catalytically driven, are utilized to break down or decompose tar products and high-boiling hydrocarbon products into hydrogen and carbon monoxide. This reaction increases the hydrogen/carbon monoxide ratio of the syngas and reduces or eliminates tar condensation in downstream process equipment. Thermal tar reformer designs are typically fluid-bed or fixed-bed type. Catalytic tar reformers are filled with heated loose catalyst material or catalyst block material and can be fixed- or fluid-bed designs.

Typically, the tar reformer is a refractory-lined steel vessel equipped with catalyst blocks, which may contain a noble metal or a nickel-enhanced material. Synthesis gas is routed to the top of the vessel and flows down through the catalyst blocks. Oxygen and steam are added to the tar reformer at several locations along the flow path to enhance the syngas composition and achieve optimum performance in the reformer. The tar reformer utilizes a catalyst to decompose tars and heavy hydrocarbons into hydrogen and carbon monoxide. Without this decomposition, the tars and heavy hydrocarbons in the synthesis gas will condense as the synthesis gas is cooled in the downstream process equipment. In addition, the tar reformer increases the hydrogen/carbon monoxide ratio for optimal conversion. The syngas is routed from the tar reformer to downstream heat recovery and gas clean-up unit operations.

2.4.1 Feedstock devolatilization

In a gasifier, the carbonaceous feedstock is exposed to high temperatures generated from the partial oxidation of the carbon. The devolatilization (or pyrolysis) process commences at approximately 200-300 °C (390-570 °F), depending on the nature and properties of the feedstock. Volatiles are released, and a carbonaceous residue (char) is produced, resulting in up to 70% weight loss for many feedstocks. The process determines the structure and composition of the char, which will then undergo gasification reactions.

More specifically, as the feedstock particle is heated, any residual moisture (assuming that the feedstock has been pre-dried) is driven off. After all the moisture contained in the feedstock particle(s) has evaporated, the particles undergo devolatilization. The devolatilization and discharge of volatiles generates a range of products varying from carbon monoxide and methane to high molecular weight hydrocarbons comprising paraffin/olefin hydrocarbons, aromatic hydrocarbons, heavy oil, and tar, which are also feedstock dependent. As these products pass from the devolatilization (pyrolsis) zone, further thermal reactions will occur, and gasification of the volatile products will commence.

At temperatures above 500 °C (930 °F). the conversion of the feedstock to char and mineral matter ash is completed. The gasification of char particles occurs after the devolatilization process has finished (Silaen & Wang, 2008). For gas generation, the char provides the necessary energy to promote further heating. Typically, the char is contacted with air or oxygen and steam to generate the product gases.

For some feedstocks, carbon conversion is believed to be independent of the devolatilization rate and less sensible to feedstock particle size. However, it is sensitive to the heterogeneous char-oxygen, char-CO², and char-steam reaction kinetics (Chen, Horio, & Kojima, 2000).

2.4.2 Char gasification

The gasification process occurs as the char reacts with gases such as carbon dioxide and steam to produce carbon monoxide and hydrogen. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing the high-temperature combustion of the gas from otherwise problematic feedstocks.

Although the initial gasification stage is completed in seconds, or even less at elevated temperature, the subsequent gasification of the char produced at the initial gasification stage is much slower, requiring minutes or hours to obtain significant conversion under practical conditions. Reactor designs for commercial gasifiers are largely dependent on the reactivity of the char, which in turn depends on nature of feedstock. The reactivity of char also depends on parameters of the thermal process required to produce the char from the original feedstock. The rate of gasification of the char decreases as the process temperature increases due to the decrease in active surface area of char. Therefore, a change of char preparation temperature may change the chemical nature of char, which in turn may change the gasification. The reactivity of char may be influenced by catalytic effect of mineral matter in the char.

Heat and mass transfer processes in fixed- or moving-bed gasifiers are affected by complex solids flow and chemical reactions. Moving-bed gasifiers are countercurrent flow reactors in which the feedstock enters at the top of the reactor, and oxygen (air) enters at the bottom of the reactor. Because of the countercurrent flow arrangement of the reactor, the heat of reaction from the gasification reactions serves to pre-heat the coal before it enters the gasification reaction zone. Consequently, the temperature of the synthesis gas exiting the gasifier is significantly lower than the temperature needed for complete conversion of the feedstock. However, coarsely crushed feedstock may settle while undergoing (1) thermal drying, (2) pyrolysis-devolatilization, (3) gasification, and (4) reduction. In addition, the particles change in diameter, shape, and porosity – non-ideal behavior may result from bridges, gas bubbles, and channeling, and a variable void fraction may also change heat and mass transfer characteristics.

Although there is a considerable overlap of the processes, each can be assumed to occupy a separate zone where fundamentally different chemical and thermal reactions take place. The gasification technology package consists of a fuel and ash handling system, gasification system – reactor, gas cooling, and cleaning system. There are also auxiliary systems – namely, the water treatment plant to meet the requirements of industry and pollution control board. The prime mover for power generation consists of either a diesel engine or a spark-ignited engine coupled to an alternator. In the case of thermal system, the end-use device is a standard industrial burner.

2.4.3 Chemistry

The major difference between combustion and gasification from the point of view of the chemistry involved is that combustion takes place under oxidizing conditions, whereas gasification occurs under reducing conditions. In the gasification process, the feedstock (in the presence of steam and oxygen at high temperature and moderate pressure) is converted to a mixture of product gases. The chemistry of the gasification of various feedstocks can be conveniently (and simply) represented by the following reactions:

${\mathrm{C}}_{\mathrm{feedstock}}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$

${\mathrm{C}}_{\mathrm{feedstock}}+\mathrm{½}{\mathrm{O}}_2\to \mathrm{CO}$

${\mathrm{C}}_{\mathrm{feedstock}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{CO}$

${\mathrm{C}}_{\mathrm{feedstock}}+{\mathrm{CO}}_2\leftrightarrow 2\mathrm{CO}$

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_2+{\mathrm{CO}}_2$

${\mathrm{C}}_{\mathrm{feedstock}}+2{\mathrm{H}}_2\to {\mathrm{CH}}_4$

Reactions (1) and (2) are exothermic oxidation reactions and provide most of the energy required by the endothermic gasification reactions (3) and (4). The oxidation reactions occur very rapidly, completely consuming all the oxygen present in the gasifier, so that most of the gasifier operates under reducing conditions. Reaction (5) is the water-gas shift reaction, when water (steam) is converted to hydrogen. This reaction is used to alter the hydrogen/carbon monoxide ratio when synthesis gas is the desired product, such as for use in Fischer-Tropsch processes. Reaction (6) is favored by high pressure and low temperature and is mainly important in low-temperature gasification systems. Methane formation is an exothermic reaction that does not consume oxygen and, therefore, increases the efficiency of the gasification process and the final heat content of the product gas. Overall, approximately 70% of the heating value of the product gas is associated with the carbon monoxide and hydrogen, but this can be higher depending on the gasifier type (Chadeesingh, 2011).

Many other reactions, besides those presented here also occur. In the initial stages of gasification, the rising temperature of the feedstock initiates devolatilization of the feedstock and the breaking of weaker chemical bonds to yield tar, oil, volatile species, and hydrocarbon gases. These products generally react further to form hydrogen, carbon monoxide, and carbon dioxide. The fixed carbon that remains after devolatilization reacts with oxygen, steam, carbon dioxide, and hydrogen.

Depending on the gasifier technology employed and the operating conditions, significant quantities of water, carbon dioxide, and methane can be present in the product gas, as well as a number of minor and trace components. Under the reducing conditions in the gasifier, most of the sulfur in the fuel sulfur is converted to hydrogen sulfide (H₂S) as well as to smaller yields of carbonyl sulfide (COS). Organically bound nitrogen in the feedstock is generally (but not always) converted to gaseous nitrogen (N₂) – some ammonia (NH₃) and a small amount of hydrogen cyanide (HCN) are also formed. Any chlorine in the feedstock (such as coal) is converted to hydrogen chloride (HCl) with some chlorine present in the particulate matter (fly ash). Trace elements, such as mercury and arsenic, are released during gasification and partition among the different phases, such as fly ash, bottom ash, slag, and product gas.

2.5.1 Influence of feedstock quality

There is an influence of physical process parameters and the effect of feedstock type on gasification. For example, the reactivity of coal generally decreases with increase in rank (from lignite to subbituminous coal to bituminous coal anthracite). Furthermore, the smaller the particle size, the more contact area between the coal and the reaction gases, leading to a more rapid reaction. For medium-rank coal and a low-rank coal, reactivity increases with an increase in pore volume and surface area, but for coal having a carbon content greater than 85% w/w, these factors have no effect on reactivity. In fact, in high-rank coal, pore sizes are so small that the reaction is diffusion controlled.

Other feedstocks (such as petroleum residual and biomass) are so variable that gasification behavior and products vary over a wide range. The volatile matter produced during the thermal reactions varies widely and the ease with which tar products are formed as part of the gaseous products makes gas clean-up more difficult.

The mineral matter content of the feedstock also has an impact on the composition of the produced syngas. Gasifiers may be designed to remove the produced ash in solid or liquid (slag) form. In fluidized- or fixed-bed gasifiers, the ash is typically removed as a solid, which limits operational temperatures in the gasifier to well below the ash melting point. In other designs, particularly slagging gasifiers, the operational temperatures are designed to be above the ash melting temperature. The selection of the most appropriate gasifier is often dependent on the melting temperature and/or the softening temperature of the ash and the feedstock that is to be used at the facility.

High-moisture content of the feedstock lowers internal gasifier temperatures through evaporation and the endothermic reaction of steam and char. Usually, a limit is set on the moisture content of feedstock supplied to the gasifier, which can be met by drying operations if necessary. For a typical fixed-bed gasifier and moderate carbon content and mineral matter content of the feedstock, the moisture limit may be on the order of 35% w/w. Fluidized-bed and entrained-bed gasifiers have a lower tolerance for moisture, limiting the moisture content to approximately 5-10% w/w of the feedstock. Oxygen supplied to the gasifiers must be increased with added mineral matter content (ash production) or moisture content in the feedstock.

Depending on the type of feedstock being processed and the analysis of the gas product desired, pressure also plays a role in product definition (Speight, 2011, 2013). In fact, some (or all) of the following processing steps will be required: (1) pre-treatment of the feedstock; (2) primary gasification; (3) secondary gasification of the carbonaceous residue – char – from the primary gasifier; (4) removal of carbon dioxide, hydrogen sulfide, and other acid gases; (5) shift conversion for adjustment of the carbon monoxide/hydrogen ratio to the desired ratio; and (6) catalytic methanation of the carbon monoxide/hydrogen mixture to form methane. If high- heat content (high-Btu) gas is desired, all of these processing steps are required because gasifiers do not yield methane in the concentrations required (Speight, 2008, 2011, 2013).

Thus, the reactivity of the feedstock is an important factor in determining the design of the reactor because feedstock reactivity, which determines the rate of reduction of carbon dioxide to carbon monoxide in the reactor, influences reactor design insofar as it dictates the height needed in the reduction zone.

In addition, certain operational design characteristics of the reactor system (load following response, restarting after temporary shutdown) are affected by the reactivity of the char produced in the reactor. There is also a relationship between feedstock reactivity and the number of active places on the char surface, these being influenced by the morphological characteristics as well as the geological age of the fuel. The grain size and the porosity of the char produced in the reduction zone influence the surface available for reduction as well as the rate of the reduction reactions that are facilitated by reactor design.

2.5.2 Mixed feedstocks

Both fixed-bed and fluidized-bed gasifiers have been used in co-gasification of coal and biomass – and these include a downdraft fixed-bed gasifier (Kumabe, Hanaoka, Fujimoto, Minowa, & Sakanishi, 2007; Speight, 2011). However, operational problems when a fluidized-bed gasifier was employed included (1) defluidization of the fluidized-bed gasifier due to agglomeration of low melting point ash present in the biomass and (2) clogging of the downstream pipes due to excessive tar accumulation (Pan, Velo, Roca, Manyà, & Puigjaner, 2000; Vélez et al., 2009). In addition, co-gasification and co-pyrolysis of birch wood and coal in an updraft fixed-bed gasifier as well as in a fluidized-bed gasifier has yielded overhead products with 4.0-6.0% w/w tar content, whereas the fixed-bed reactor gave tar yields on the order of 25-26% w/w for co-gasification of coal and silver birch wood mixtures (1:1 w/w ratio) at 1000 °C (1830 °F) (Collot, Zhuo, Dugwell, & Kandiyoti, 1999).

From the perspective of the efficient operation of the reactor, the presence of mineral matter has a deleterious effect on fluidized-bed reactors. The low melting point of ash formed from the mineral matter present in woody biomass can lead to agglomeration. Such agglomeration influences the efficiency of the fluidization – the ash can cause sintering, deposition, and corrosion of the gasifier construction metal. In addition, biomass containing alkali oxides and salts can cause clinkering/slagging problems (McKendry, 2002).

2.5.3 Mineral matter content and ash production

Finally, gasification reactors are very susceptible to ash production and properties. Ash can cause a variety of problems, particularly in up or downdraft gasifiers. Slagging or clinker formation in the reactor, caused by melting and agglomeration of ashes, at best will greatly add to the difficulty of gasifier operation. If no special measures are taken, slagging can lead to excessive tar formation and/or complete blocking of the reactor. A worst-case scenario is the possibility of air channeling, which can lead to a risk of explosion, especially in updraft gasifiers.

Whether slagging does or does not occur depends on the ash content of the fuel, the melting characteristics of the ash, and the temperature pattern allowed by gasifier design. In the fuel bed, local high temperatures in voids in the oxidation zone, caused by bridging in the bed, may cause slagging even when using fuels with a high ash melting temperature.

Generally, slagging is not observed with fuels having mineral matter ash contents less than below 5-6% w/w. Severe slagging can be expected for fuels having mineral matter contents in excess of 12% w/w. For fuels with mineral matter contents between 6% and 12%, the slagging behavior depends to a large extent on the mineral matter composition – reflected in the ash melting temperature, which is influenced by the presence of trace elements giving rise to the formation of low melting point eutectic mixtures.

Updraft and downdraft gasification reactors are able to operate with slagging fuels if they are specially modified (continuously moving grates and/or external pyrolysis gas combustion). Cross-draft gasification reactors, which work at temperatures on the order of 1500 °C (2700 °F) and higher, need special safeguards with respect to the mineral matter content of the fuel. Fluidized bed reactors, because of their inherent capacity to control the operating temperature, suffer less from ash melting and fusion problems.

2.5.4 Heat release

The gasification reactor must be configured to accommodate the energy balance of the chemical reactions. During the gasification process, most of the energy bound up in the fuel is not released as heat. In fact, the fraction of the feedstock’s chemical energy, or heating value, that remains in the product gases (especially the synthesis gas) is an important measure of the efficiency of a gasification process (which is dependent on the reactor configuration); it is known as the cold gas efficiency. Most commercial-scale gasification reactors have a cold gas efficiency on the order of 65-80%, or even higher.

Thus, it is important for the reactor to limit the amount of heat that is transferred out of the zone where the gasification reactions are occurring. If not, the temperature within the gasification zone could be too low to allow the reactions to proceed. As an example, a minimum temperature on the order of 1000 °C (1830 °F) is typically needed to gasify coal. As a result, a gasification reactor is typically refractory lined with no water cooling to ensure as little heat loss as possible. Gasification reactors also typically operate at elevated pressure (often as high as 900 psia), which allows them to have very compact construction with minimum surface area and minimal heat loss.

2.5.5 Other design options

In addition to being designed and selected for feedstock type, another design option for the gasification reactor involves the method for cooling the synthesis gas produced by the gasifier.

Regardless of the type of gasifier, the exiting synthesis gas must be cooled down to approximately 100 °C (212 °F) in order to utilize conventional acid gas removal technology. This can be accomplished either by passing the syngas through a series of heat exchangers that recover the sensible heat for use (for example, in the stem cycle of an IGCC unit) or by directly contacting the synthesis gas with relatively cool water (a quench operation). The quench operation results in some of the quench water being vaporized and mixed with the synthesis gas. The quenched syngas is saturated with water and must pass through a series of condensing heat exchanges that remove the moisture from the synthesis gas (so it can be recycled to the quench zone).

Quench designs have a negative impact on the heating rate of related equipment (such as the IGCC unit) because the sensible heat of the high temperature synthesis gas is converted to low-level process heat rather than high-pressure steam. However, quench designs have much lower capital costs and can be justified when low-cost feedstock (such as biomass or waste) is available. Quench designs also have an advantage if carbon dioxide capture is desired. The saturated synthesis gas exiting a quench section has near the optimum water/carbon monoxide ratio as the feedstock to a water-gas shift reactor that will convert the carbon monoxide to carbon dioxide. Non-quench designs that require carbon dioxide capture need to add steam to the syngas before it is sent to a water-gas shift reactor.

2.6.1 Primary gasification

Primary gasification involves thermal decomposition of the feedstock by way of various chemical processes (Table 2.3) and many schemes involve pressures ranging from atmospheric to 1000 psi. Air or oxygen may be admitted to support combustion to provide the necessary heat. The product is usually a low-heat content gas (low-Btu gas) ranging from a carbon monoxide/hydrogen mixture to mixtures containing varying amounts of carbon monoxide, carbon dioxide, hydrogen, water, methane, hydrogen sulfide, nitrogen, and typical products of thermal decomposition such as tar, hydrocarbon species, and other chemical species.

A solid char product may also be produced, and may represent the bulk of the weight of the original feedstock. This type of feedstock being processed determines (to a large extent) the amount of char produced and the analysis of the gas product.

2.6.2 Secondary gasification

Secondary gasification usually involves the gasification of char from the primary gasifier. This is usually done by reaction of the hot char with water vapor to produce carbon monoxide and hydrogen:

${\mathrm{C}}_{\mathrm{char}}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CO}+{\mathrm{H}}_2$

2.6.3 Shift conversion

The gaseous product from a gasifier generally contains large amounts of carbon monoxide and hydrogen, plus lesser amounts of other gases. Carbon monoxide and hydrogen (if they are present in the mole ratio of 1:3) can be reacted in the presence of a catalyst to produce methane. However, some adjustment to the ideal (1:3) is usually required. To accomplish this, all or part of the stream is treated according to the water-gas shift (shift conversion) reaction. This involves reacting carbon monoxide with steam to produce a carbon dioxide and hydrogen whereby the desired 1:3 mole ratio of carbon monoxide to hydrogen may be obtained.

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2$

2.6.4 Hydrogasification

Not all high-heat content (high-Btu) gasification technologies depend entirely on catalytic methanation. In fact, a number of gasification processes use hydrogasification – that is, the direct addition of hydrogen to the feedstock (in most cases, coal) under pressure to form methane (Anthony & Howard, 1976).

${\mathrm{C}}_{\mathrm{char}}+2{\mathrm{H}}_2\to {\mathrm{CH}}_4$

The hydrogen-rich gas for hydrogasification can be manufactured from steam by using the char that leaves the hydrogasifier. Appreciable quantities of methane are formed directly in the primary gasifier and the heat released by methane formation is at a sufficiently high temperature to be used in the steam-carbon reaction to produce hydrogen so that less oxygen is used to produce heat. Hence, less heat is lost in the low-temperature methanation step, thereby leading to higher overall process efficiency.

2.6.5 Catalytic gasification

Catalysts are commonly used in the chemical and petroleum industries to increase reaction rates, sometimes making certain previously unachievable products possible (Hsu & Robinson, 2006; Speight, 2002, 2014). Acids, through donated protons (H⁺), are common reaction catalysts, especially in the organic chemical industries. It is not surprising that catalysts can be used to enhance the reactions involved in gasification, as the use of appropriate catalysts not only reduces reaction temperature but also improves the gasification rates.

In addition, thermodynamic constraints of the gasification process that limit the thermal efficiency are not inherent; rather, they are the result of design decisions based on available technology, as well as the kinetic properties of available catalysts. The latter limits the yield of methane to that obtainable at global equilibrium over carbon in the presence of carbon monoxide and hydrogen. The equilibrium composition is shown to be independent of the thermodynamic properties of the char or feedstock. These limitations give non-isothermal two-stage processes significant thermodynamic advantages. The results of the analysis suggest directions for modifying present processes to obtain higher thermal efficiencies. The two-stage process scheme would have significant advantages over present technologies and should be applicable to a wide range of catalytic and non-catalytic processes (McKee, 1981; Shinnar, Fortuna, & Shapira, 1982).

Alkali metal salts of weak acids – such as potassium carbonate (K₂CO₃), sodium carbonate (Na₂CO₃), potassium sulfide (K₂S), and sodium sulfide (Na₂S) – can catalyze the carbon-steam gasification reaction. Catalyst amounts on the order of 10-20% w/w K₂CO₃ will lower the temperature required for gasification of bituminous coal from approximately 925 °C (1695 °F) to 700 °C (1090 °F) and then the catalyst can be introduced to the gasifier impregnated on coal or char.

Ruthenium-containing catalysts are used primarily in the production of ammonia. It has been shown that ruthenium catalysts provide 5-10 times higher reactivity rates than other catalysts. However, ruthenium quickly becomes inactive due to its necessary supporting material, such as activated carbon, which is used to achieve effective reactivity. But during the process, the carbon is consumed, thereby reducing the effect of the ruthenium catalyst.

Catalysts can also be used to favor or suppress the formation of certain components in the gaseous product. For example, in the production of synthesis gas (mixtures of hydrogen and carbon monoxide), methane is also produced in small amounts. Catalytic gasification can be used to either promote methane formation or suppress it.

Disadvantages of catalytic gasification include increased cost of materials for the catalyst itself (often rare metals), as well as diminishing catalyst performance over time. Catalysts can be recycled, but their performance tends to diminish with age or by poisoning. The relative difficulty in reclaiming and recycling the catalyst can also be a disadvantage. For example, the potassium carbonate catalyst can be recovered from spent char with a simple water wash, but some catalysts may not be so accommodating. In addition to age, catalysts can also be diminished by poisoning. On the other hand, many catalysts are sensitive to particular chemical species that bond with the catalyst or alter it in such a way that it no longer functions. Sulfur, for example, can poison several types of catalysts, including palladium and platinum.

2.6.6 Plasma gasification

Plasma is a high-temperature, highly ionized (electrically charged) gas capable of conducting electrical current. Plasma technology has a long history of development and has evolved into a valuable tool for engineers and scientists who need to use very high temperatures for new process applications (Messerle & Ustimenko, 2007). Human-made plasma is formed by passing an electrical discharge through a gas such as air or oxygen (O₂). The interaction of the gas with the electric arc dissociates the gas into electrons and ions, and causes its temperature to increase significantly, often (in theory) exceeding 6000 °C (10,830°F).

Serious efforts have been made, with some success, to apply plasma gasification technology and to treat industrial and MSW over the last two decades. It is believed that the technology can be used as a gasification reactor, thereby allowing (1) greater feedstock flexibility enabling a variety of fuels such as coal, biomass, and MSW to be used as fuel without the need for pulverizing; (2) air blowing and thus not requiring an oxygen plant; (3) high conversion (> 99%) of carbonaceous matter to synthesis gas; (4) the absence of tar in the synthesis; (5) production of high heating value synthesis gas suitable use in a combustion turbine operation; (6) production of little or no char, ash, or residual carbon; (7) production of a glassy slag with beneficial value; (8) high thermal efficiency; and (9) low carbon dioxide emissions.

In the process, the gasifier is heated by a plasma torch system located near the bottom of the reactor vessel. In the gasifier, the feedstock is charged into a vertical reactor vessel (refractory-lined or water-cooled) at atmospheric pressure. A superheated blast of air, which may be enriched with oxygen, is provided to the bottom of the gasifier, at the stoichiometric amount required for gasification. The amount of air fed is such that the superficial velocity of the upward flowing gas is low, and that the pulverized feedstock can be fed directly into the reactor. Additional air and/or steam can be provided at different levels of the gasifier to assist with pyrolysis and gasification. The temperature of the syngas leaving the top of the gasifier is maintained above 1000 °C (1,830 °F). At this temperature, tar formation is eliminated.

Gasification takes place at very high temperatures, driven by the plasma torch system, which is located at the bottom of the gasifier vessel. The high operating temperatures break down the feedstock and/or all hazardous and toxic components into their respective elemental constituents. They then dramatically increase the kinetics of the various reactions occurring in the gasification zone, converting all organic materials into hydrogen (H₂) and carbon monoxide (CO). Any residual materials from inorganic constituents of the feedstock (including heavy metals) will be melted and produced as a vitrified slag that is highly resistant to leaching.