15.4. Gasification technology
Coal gasification is a mature technology and gasification reactors have been commercially available for decades. Unfortunately, most of the gasification reactors used for conversion of fossil fuels are not suitable for biomass processing. This is due to the different physical and chemical properties of coals and biomass; mainly the high alkali content and the high volatile content, which gives rise to a relatively high amount of tars. Many technical aspects which require detailed solutions are still needed in order to ensure reliable and cost-effective operation of biomass gasification. Consequently, this is not a mature technology and in most markets, it cannot compete against other energy conversion technologies, including biomass combustion (Kirkels and Verbong, 2011).
Biomass gasifiers based on similar basic reactor principles to those existing for coal gasification have been developed: (1) fixe-bed (or moving bed) reactor; (2) fluidized-bed reactor; (3) entrained-flow reactor. Entrained flow technology is the most widely used gasifier type for fossil fuel gasification. However, in the case of biomass gasification, entrained flow gasifiers are not yet available on a commercial scale, and a range of other technologies are applied instead. Around 75% of the commercial biomass gasifiers are fixed bed downdraft, 20% are fluidized bed, 2.5% are fixed bed updraft, and 2.5% corresponded to other designs (Knoef, 2000). The existing gasification technologies and their range of applicability are shown in Fig. 15.3. Small-scale fixed-bed gasifiers are cost-effective and there are a number of them in use worldwide. On the other hand, most of the medium- or large-scale gasifiers are fluidized bed reactors, and they offer higher performance than fixed-bed gasifiers but at higher cost and complexity. Table 15.4 compares biomass requirements, performance parameters, and advantages and disadvantage of the main gasifiers employed for biomass gasification.
15.4.1. Fixed bed or moving bed gasifiers
In fixed bed or moving gasifiers the biomass is supported on a grate, and it slowly moves down the gasifier as a plug. Because of the negligible mixing and heat transfer, fuel, temperature, and gas composition profiles throughout the reactor are not uniform. Average temperature in the gasification zone is usually between 700 and 900°C. Nevertheless peak temperature in the oxidation zone ranges from 1200 to 1800°C (Basu, 2010a,b). Fixed-bed gasifiers can operate at both atmospheric and high pressures.
Table 15.4
Comparison of the main gasifiers applied to biomass gasification (Lettner et al., 2007; Basu, 2010a,b; McKendry, 2002; E4tech, 2009)
| Downdraft | Updraft | Bubbling fluidized bed | Circulating fluidized bed | |
| Operating temperature | Gasification zone = 700–900°C Oxidation zone = 1000–1400°C | Gasification zone = 700–900°C Oxidation zone = 1200–1800°C | 800–900°C | 800–900°C |
| Biomass requirements | Moisture = 10–25 wt% Ash <6 wt% db Ash melting T > 1250°C Particle size = 2–10 cm | Moisture <60 wt% Ash <25 wt% db Ash melting T > 1000°C Particle size = 0.5–10 cm | Moisture = 10–55 wt% Ash <25 wt% db Ash melting T > 900°C Particle size < 5–15 cm | Moisture = 5–60 wt% Ash <25 wt% db Ash melting T > 900°C Particle size <2 cm |
| Performancea | Tar = 0.015–3 g/Nm3 CGE = 60–80% HGE = 85–90% Turn-down ratio = 3–4 Gas exit T ≈ 700°C Gas LHV = 4.5–5 MJ/Nm3 | Tar = 30–150 g/Nm3 CGE = 40–60% HGE = 90–95% Turn-down ratio = 5–10 Gas exit T = 200–400°C Gas LHV = 5–6 MJ/Nm3 | Tar = 1–3 g/Nm3 CGE = 80–90% Turn-down ratio = 3 Gas exit T = 800–900°C Gas LHV = 5 MJ/Nm3 | Tar <5 g/Nm3 CGE = 80–90% HGE = 96–98% Turn-down ratio = 3 Gas exit T = 800–900°C Gas LHV = 5 MJ/Nm3 |
| Advantages | Simple design; Grate is not at high temperature; Small-scale applications; Low tar and particulate formation; Syngas can be directly used in some engines; and, Short start-up period (20–30 min). | Simplest design; Small-scale application; Suitable for biomass with high moisture content, low volatiles (charcoal) and high ash content (MSW); Accepts small particle size and broad size distribution; Slagging problems prevented; High char burnout; High energy efficiency; and, Good turn-down. | Compact construction; Large-scale applications; Direct/indirect heating; Suitable for biomass with high ash content; Flexible on biomass composition (moisture, ash); Broad particle size distribution; Flexible feed rate; High carbon conversion efficiency; Production of high LHV gas and steady gas composition; Easy temperature control; Very good scale-up; and Able to operate at partial load (50–120%). | Compact construction; Large-scale applications; Suitable for biomass with high ash content; Flexible on biomass composition (moisture, ash); Broad particle size distribution; Able to process feedstock with small particle size (<400 μm); High carbon conversion efficiency Production of high LHV gas and steady gas composition; Able to pressurize; Very good scale-up; and Able to operate at partial load (50–120%). |
| Table Continued | ||||
| Downdraft | Updraft | Bubbling fluidized bed | Circulating fluidized bed | |
| Disadvantages | Sensitive to the quality of the biomass (moisture and ash content, particle size); Feed size limits; Syngas requires cooling if compressed; High amount of ash in the gas product; 4–7 wt% of char unconverted as part of the ash; Limitations in scale-up; Low energy efficiency; Limited turn-down; Work well with internal combustion engine; and, Risk of explosions, fuel blockages, corrosion. | Grate needs protection from high temperature; Unsuitable for high volatile fuels; High tar yield; Extensive cleaning for engine applications; Slow response time and long start-up period; and, Risk of explosions, fuel blockages, corrosion. | Care needed with some agricultural residues; Medium tar yield, and formation of particles and other impurities (nitrogen, sulfur and alkali compounds); Higher particle loading; Operating temperature limited by ash clinkering; High syngas temperatures; Incomplete carbon burnout; Possibility of high carbon content in the fly ash; Ash not molten; Turn-down limited by the gas velocities required; Complex operation and control system; and, Long start-up period (hours). | Medium tar yield, and formation of particles and other impurities (nitrogen, sulfur and alkali compounds); Higher particle loading; High syngas temperatures; Possibility of high carbon content in the fly ash; Ash not molten; Corrosion and attrition problems; Turn-down limited by the gas velocities required; Complex operation and control system; Poor operational control using biomass; and, Long start-up period (hours). |
Fixed-bed reactors present a simple design. However, they produce a synthesis gas with relatively low heating value and significant amount of impurities. The product gas composition (vol%) is typically 15–20% H2, 10–15% CO, 10–15% CO2, 3–5% CH4, and 40–50% N2, although it may change significantly depending on the gasifying agent.
Biomass is fed from the top of the reactor, while the airflow changes depending on the configuration: downdraft, updraft, and crossdraft.
15.4.1.1. Downdraft gasifier
The downdraft gasifier is a cocurrent reactor (see Fig. 15.4). The gasification agent enters the gasifier at a certain height below the top, and it mixes with the pyrolysis gas products while flow downward in parallel with the solids (char and ash) through the oxidation and gasification zones. The drying and pyrolysis zones lie above the oxidation zone and they are maintained at the required temperature by conduction of the heat generated from the combustion of pyrolysis vapors (including tar precursors). The gases leaving the oxidation zone, mainly CO2 and H2O, are reduced into CO and H2 on the glowing char in the gasification area. The gas temperature decreases due to the occurrence of the endothermic gasification up to a level at which no further reaction takes place. The unreacted char and the ash are discharged from the bottom. The synthesis gas is removed from the reactor close to the ash grate.
One of the main advantages of the downdraft configuration is the production of a fuel gas with relatively low quantity of tars (0.015–3 g/Nm3). Since the vapors released from the devolatilization stage pass through the high-temperature oxidation zone, a significant amount of the tar precursors is converted to permanent gases. Downdraft gasifiers can operate at a biomass conversion rate of more than 95 wt% db.
Various designs following the downdraft configuration can be found (Quaak et al., 1999; Basu, 2010a,b; Reed and Das, 1988):
1. Throated (also called constricted, Imbert gasifier, or V-shaped)
This is the most common design among the existing downdraft gasifiers. The throated gasifier consists of a vessel with the oxidation zone located at the narrowest part of a throat, which reduces and then expands the cross-sectional area. The design ensures the pyrolysis vapors pass through the narrow oxidation zone. The gasification agent is introduced above the throat. A further modification of the throated gasifier consists of introducing internal heat exchange by a double-walled vessel. The hot fuel gas leaves the reactor through the space between the walls and the heat is transferred to the biomass in the drying and biomass zone. The exchange of heat is particularly enhanced in gasifiers designed with small diameters.
The Delacotte gasifier includes a system for collection of pyrolysis vapors at the top of the reactor. Vapors are combusted in a secondary reactor and the flue gases are then introduced into the hearth zone along with the gasifying agent. The synthesis gas obtained from this particular design presents significantly low content of tars.
2. Throatless (also called open core or stratified)
The throatless gasifier consists of an open-top nonconstricted cylindrical vessel with the hearth on the bottom. Both the biomass and the gasifying agent are supplied from the top and flow downward through the four zones. It is specially designed to gasify biomass with low bulk densities (eg, agricultural residues such as rice husk).
15.4.1.2. Updraft gasifier
The updraft gasifier is a countercurrent reactor and it exhibits one of the simplest designs (see Fig. 15.5). The gasifying agent is introduced from the bottom through a grate, and it flows upward while the solid bed moves downward. When it enters the gasifier, the gasifying agent meets the hot ash and the unconverted char in the oxidation zone. As a result a very hot flue gas consisting of CO2 and H2O is produced, and it flows upward to the gasification zone where CO and H2 are produced by endothermic reduction reactions. Devolatilization takes place in the area above the gasification zone aided by the residual heat in the flowing gas. Volatiles (including tar precursors) are swept by the gas stream upward. The biomass is dried in the top zone where the syngas is cooled down to approximately 200–400°C. This temperature is too low for many of the cracking and reforming reactions of the tar precursors to occur and consequently the gas is produced with a significant amount of tar impurities (30–150 g/Nm3).
The major advantage of the updraft gasifier is its simplicity and the efficient internal heat exchange which leads to high-energy efficiencies. This configuration is able to cope with high-moisture-content biomass (up to 60 wt%) since the steam released in the drying zone is removed with the syngas. It can also admit biomass with high ash content (up to 25 wt% db).
There are two main designs following the updraft configuration, which mainly differ on the operating temperature in the oxidation zone (Basu, 2010a,b): dry-ash gasifier and slagging gasifier. In the dry-ash mode, the ash is not melted (temperature in the oxidation zone around 1200°C), and it is removed as a solid from the bottom of the reactor. In the slagging gasifier, the ash is melted in the oxidation zone (1500–1800°C). This design requires lower steam/biomass ratio.
15.4.1.3. Crossdraft gasifier
In a crossflow gasifier the feed moves downwards, the gasifying agent is introduced from the side and the gas product is removed from the opposite side (see Fig. 15.6). The gasifying agent is introduced at high velocity into the oxidation zone where it reacts with part of the char giving rise to very high temperatures (>1500°C). The flue gas is reduced to CO and H2 downstream in the gasification zone. The pyrolysis and drying zones are above the high-temperature area and the heat is transferred from the oxidation zone by conduction.
The crossdraft gasifier is not designed to operate with fuels with high volatiles and ash content. Therefore, it is normally used with biomass chars as feedstock. It can handle fuels with high moisture content if the top is open. This gasifier is suitable for small-scale applications (<10 kWe) because it presents a short start-up period (5–10 min) and good response to load changes, and it is built with less expensive materials as the solid surrounding the oxidation zone acts as insulation for the walls. The main drawback of this configuration is that the gas product leaves the unit at around 900–1250°C which results in the low energy efficiency of the process. In addition, slagging issues and high levels of carbon in the ash have been reported.
15.4.2. Fluidized bed gasifiers
In fluidized bed gasifiers, the biomass is maintained in fluidized state by the gasifying agent flowing upward at an appropriate rate. Biomass is mixed with inert (or catalytic) bed material such as quartz sand, alumina, or dolomite in order to improve heat transfer. The bed of solids is perfectly mixed which gives rise to a uniform temperature in the conversion zone and makes this gasifier configuration able to process fuels with different qualities. Drying, devolatilization, oxidation, and gasification occur simultaneously and homogeneously in the conversion zone. Average operating temperature is usually between 800 and 900°C; consequently tar conversion rates are not very high.
Fluidized bed gasifiers present a complex operation; however, they exhibit higher performance than moving-bed gasifiers and are able to produce a synthesis gas with relatively high heating value. The product gas composition (vol. %) depends on the gasifying agent (air, pure O2, steam, O2/steam) and the reactor design, but it typically ranges (E4tech, 2009): 6–35% H2, 15–40% CO, 15–30% CO2, 3–8% CH4, and 3–60% N2.
Two main configurations of fluidized bed reactors can be identified depending on the flow rate of the fluidization/gasification medium: bubbling fluidized bed (BFB) and circulating fluidized bed (CFB). Fluidized bed can be designed to operate at atmospheric and high pressures. High-pressure fluidized bed gasification is normally carried out for power production with relatively large-scale gas turbines (≥5 MWe). Operating pressure depends on the operating pressure of the gas turbine, and it normally ranges from 10 to 25 bars. The major advantages of high-pressure gasifiers are the compact design and the decrease in ash sintering. On the other hand, the installation is difficult and the feeding and gas cleaning systems become more complex.
15.4.2.1. Bubbling fluidized bed (BFB) gasifier
BFB gasifier is one of the most popular designs for biomass gasification, mainly due to its applicability to medium-scale processes (<25 MWth). It consists of a vessel in which the gasifying agent is introduced upward at a velocity fast enough (0.5–1.0 m/s) to agitate the bed material which sits at the bottom part of the gasifier and to maintain the required temperature (see Fig. 15.7). Biomass is fed from the side into the hot bed, where devolatilization occurs. Char particles and volatiles (tar precursors) are gasified and cracked by contact with the hot fluidized bed. Additional gasifying agent can be supplied in a second zone located above the bed with the aim of converting entrained unconverted volatiles and char particles into fuel gas. A final flue gas with low to medium tar content is produced. Ash is separated from the syngas in gas–solid separation units downstream.
The gasifying agent can be supplied in two different zones. The first zone is within the fludized bed in order to maintain the required temperature. The second zone is located above the bed and it aims to convert entrained unconverted volatiles and char particles into fuel gas.
Two of the major disadvantages of the bubbling fluidized bed gasifier are that high conversion of solids is not achieved due to back mixing issues, and the formation of oxidation spots due to the slow oxygen diffusion.
15.4.2.2. Circulating fluidized bed (CFB) gasifier
The CFB gasifier is arising as a major technology for medium- (a few MWth) to large-scale (100 MWth) biomass gasification processes, mainly due to its long residence time which makes it applicable to biomass with high volatile contents. In the circulating fluidized bed reactor, the gasifying agent is introduced upward at a velocity fast enough (3.5–5.5 m/s) to move the bed material throughout a circulating loop, full mixing and long residence times being achieved (see Fig. 15.8). Biomass is fed from the side and mixed with the hot bed material which is dispersed along the tall vessel. The solids leaving the riser are separated in a gas–solid separator (cyclone) and returned to the bottom of the gasifier.
The CFB design overcomes some of the disadvantages exhibit by the BFB design. The gas–solid contact is better due to the absence of bubbles and prevents the gas from bypassing the bed. In addition, biomass is heated at a higher rate due to the recirculation of the solids. Consequently gasification efficiency and carbon burnout are increased compared to BFB, and tar production during the heating up of the feedstock decreases. CFB can process feedstock with small particle size (<400 μm) and a wider particle size distribution without the penalty of the entrainment loss, and they are reliable over a wider range of feedstock.
15.4.3. Entrained flow gasifiers
Entrained flow reactors (Fig. 15.9) are not usually applied to biomass gasification because of their remarkable limitations when processing this feedstock. The residence time in the entrained flow reactor is very short and consequently the fuel particles need to be finely ground (<75 μm) for the reactions to proceed completely. Given their low friability, most of the biomass feedstocks cannot be pulverized easily and the high amount of energy required is detrimental to the overall efficiency of the process. Biomass usually has large amounts of ashes with high alkali content. The ash has a relatively low melting point and it is highly aggressive to the refractory material and the metal lining of the gasifier.
Despite these drawbacks, some examples of operating entrained flow biomass gasifiers can be found (Basu, 2010a,b; E4tech, 2009). This configuration exhibits significant advantages such as its ability for tar destruction, low methane content, and carbon conversion rate close to 100%.
15.4.4. New developments in gasification technology
As it will be detailed in Section 15.6, most of the existing commercial, demonstration and pilot plants for biomass gasification are used for combined production of heat and power. The fuel gas produced from these gasification processes usually contains a significant amount of N2 from the air present in the gasifying agent, which reduces significantly the heating value. This fuel gas is acceptable for the generation of heat and power; however, it cannot be used for more efficient energy generation (fuel cells), or for the production of H2 and other chemicals. Consequently, current research and development is focused on obtaining syngas from biomass gasification valid for hydrogen, biofuels, and high-value chemical production (IEA, b). The key challenge is to enhance the production of syngas with high content in H2 and to minimize impurities (mainly tars) while reducing capital and operating costs. New designs have arisen as alternatives to the current technology based on the separation of gasification and combustion stages, and plasma gasification. From the current available gasifiers, BFB seems to be the most promising technology for hydrogen, biofuels, and chemical production since it has been demonstrated over a wide range of conditions (Ciferno and Marano, 2002).
15.4.4.1. Indirect gasifiers
Indirect gasification consists of the physical separation of the gasification and combustion stages with the heat being transferred between them through the inert bed material (see Fig. 15.10). This configuration is suitable for downstream synthesis applications because it is by avoided the dilution of the fuel gas by the N2 contained in the air used for the exothermic combustion required for the gasification reaction to proceed. Apart from the production of high heating value N2-free gas product, a syngas with relatively low tar content is obtained since most of the tars released during devolatilization are burnt in the combustion stage.
Indirect gasifiers actually consist of dual fluidized bed reactors. Biomass is fed into the BFB gasification chamber and it is gasified to syngas and char using steam. The bed material (char and inert particles) is transferred to the CFB combustion chamber, where the char is burnt with air heating the inert bed material. The hot inert bed is then transferred back to the gasification stage providing indirect heat for the reaction to proceed. Solids are separated from flue gas by means of cyclones.
Indirect gasifiers give rise to complete conversion of the fuel without the need for high temperatures (<900°C to avoid ash melting) and can operate at high pressures. However, some limitations still need to be addressed such as the potential need for external heat in the gasification reactor and the excess of steam fed to this chamber.
15.4.4.2. Plasma gasifiers
As explained in Section 15.3.4.3, in plasma gasification, extremely high-temperature plasma (1200–5000°C) aids biomass devolatilization and gasification of the resulting char and volatiles (tar precursors) in an atmosphere with limited oxygen. Char and tar are completely converted into permanent gases such as H2 and CO, and inorganic materials are fused as inert molten slag. Consequently, a very high-quality syngas suitable for hydrogen production and synthesis of chemicals is produced.
In a plasma gasifier, biomass is dropped from the top coming into contact with the plasma generated by a plasma gun at the bottom of a vessel (see Fig. 15.11). The plasma reactor is designed for relatively long residence time of the gas, which enhances tar cracking. In addition, plasma gasification allows harmful products to be destroyed, which is particularly promising for waste gasification. Plasma gasification is barely sensitive to the quality of the fuel and can process a wide range of biomass, including untreated biomass. Current limitations of plasma gasifiers are the high-power consumption and the corrosion of the reactor liner due to the high temperature and the presence of chlorine in the feedstock (Basu, 2010a,b).
Plasma treatment is a mature technology that has been applied in the chemical and metal industry for many years (GSTC, 2015). This has encouraged the use of plasma for gasification, since all the subsystems are well established in several industries (Dodge, 2015). Nowadays there are plasma treatment plants (for waste treatment and/or gasification) operating in Japan, Canada, the US, Australia, and Mexico (Heberlein and Murphy, 2008; GSTC, 2015; Shareefdeen et al., 2015).
15.4.4.3. Other gasifier designs
Other gasifier designs have been proposed for enhancing the production hydrogen through gasification of biomass. That is the case of the UNIQUE gasifier (Heidenreich and Foscolo, 2015). The UNIQUE reactor consists of catalytic filter elements for particle and tar removal located in the freeboard zone of an FB steam gasifier. Therefore, UNIQUE integrates gasification, and gas cleaning and conditioning in one reactor. This concept reduces thermal losses, equipment cost, and plant space.
The UNIQUE gasifier has been used in the UNIfHY process. UNIfHY has been developed in the frame of an FP7 project funded by the European Commission (UNIfHY, 2015). This scheme produces pure hydrogen from biomass steam gasification coupled to syngas purification. The UNIfHY process involves the UNIQUE gasifier (Heidenreich and Foscolo, 2015), a water gas shift reactor, and a pressure swing operation unit. The main product from UNIfHY is high-purity hydrogen with conversion efficiencies higher than 66%.
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