1.7.Gasification

Gasification is the process of biomass conversion into a combustible gas mixture by the partial oxidation of biomass at high temperature (800–900 ^oC). A typical gasification process generally follows the sequence of steps illustrated schematically in Figure 15.

Figure 15. Potential paths for gasification of biomass [4]

After brief stages of initial heating and drying biomass undergoing pyrolysis process, described by the overall reaction (1), and devolatilization. Carbonaceous char residue and gaseous products, including water vapour, then undergo the following heterogeneous and homogeneous reactions:

Tar compounds thermally and catalytically (e.g., in presence of char) crack to produce permanent gases

The reaction of hydrogen formation from hydrocarbon gasification

Water gas reaction

Water-gas shift to produce hydrogen

Methane formation

Steam and CO₂ methane conversion reactions (similar reactions can be written for hydrocarbons and tars)

Boudouard reaction

In steam and air gasification, all these reactions occur so that the endothermic water-gas and Boudouard reactions are coupled to the exothermic methane formation reaction. Coupling of endothermic and exothermic reactions is very attractive in order to achieve a high thermodynamic efficiency [1].

Several types of gasifiers have been developed; an overview is shown in Figure 16. These gasifiers have different hydrodynamics (especially the way in which the solid fuel and the gasification agent are contacted), gasification agents (air, oxygen and/or steam) and operating conditions such as temperature and pressure. The most important types are fixed-bed gasifiers, operated in counter-current, co-current or cross-current mode, fluidized bed gasifiers and entrained flow gasifiers.

The majority of gasifiers are partial oxidation reactors, in which just sufficient air or oxygen is introduced to burn part of the input biomass to provide the heat for pyrolysis and gasification. The heating value of the fuel gas derived from air driven partial oxidation gasifiers ranges from 4–6 MJ/ Nm³ on a dry basis. The value can be higher if the feedstock is very dry, thus minimizing the heat demand for the process and the amount of oxidant required.

Figure 16. Overview of gasifier types [1]

Updraft fixed or moving bed gasifier. Biomass is fed from the top and a gasifying agent, typically air (with steam), is fed from the bottom (counter-current flow). The product gas leaves from the top while solids leave from the bottom. The temperature is highest close to the grate, where oxygen meets and burns the char. The hot gas travels up, providing heat to the endothermic gasification reactions, and meets pyrolyzing biomass at a low temperature (200–500 °C). Primary tar is produced in this temperature range. The tar travels upward through cooler regions and therefore has no opportunity for conversion into gases and secondary tar. For this reason, updraft gasifiers generate the highest amount of tar – typically 10 to 20% by weight of the feed [4]. For engine applications the gas has to be cleaned up and cooled to ambient temperatures. Typically this is done with the scrubbing system.

Downdraft gasifier. Here, both gas and feed travel downward (co-current flow). The temperature is highest in the downstream combustion zone. The tar is produced after drying at lower temperatures (200–500 °C) close to the feed point. The oxygen in the air, along with the tar, travels downward to the hotter zone. Owing to the availability of oxygen and high temperature, the tar readily burns in a flame, raising the gas temperature to 1000 to 1400 °C. The flame occurs in the interstices between feed particles, which remain at 500 to 700 °C (Milne et al., 1998, p. 14). This phenomenon is called flaming pyrolysis. Since the pyrolysis product, tar, contacts oxygen while passing through the highest-temperature zone, it has the greatest opportunity to be converted into permanent (non-condensable) gases. For this reason, a downdraft gasifier has the lowest tar production (<1 g/Nm³) [4].

The low calorific value (LCV) gases produced from air gasification of biomass can be used to fire boilers (directly) as well as kilns and power generation in internal combustion machines. The high calorific value (HCV) gas from oxygen-blown (14-18 MJ/Nm³) and steam (10-18 MJ/Nm³) gasification has a wider range of applications: power generation in gas turbines, production of chemicals, city bio-gas (methane), liquid fuels, and hydrogen. After MSV purification, these are produced from the syngas via catalytic conversion or methanation.

Despite the long experience with biomass fixed-bed gasifiers, reliable and large-scale operation continues to be suffering from several problems. As it is difficult to scale-up fixed-bed units to a large capacity, most large scale biomass gasifiers are based on bubbling fluidized bed (BFB) or circulating fluidized bed (CFB) technology (see Table 6). Sand is used as a bed material and heat carrier. The operating temperature is limited to 900-950°C to prevent ash sintering problems. At these relatively low gasification temperatures, tarry components are formed. Upon cooling of product gas, condensation of these tars can lead to choking of equipment and piping. Although many technologies have been developed to tackle the tar problem, such as physical separation, thermal and catalytic (e.g. using dolomite as bed material with/or instead of sand) cracking, these methods add to the complexity and the cost [1].

Table 6. Thermal capacity of different gasifier designs [17]

Gasifier Design	Thermal Capacity
Downdraft fixed bed	1 kW – 1 MW
Updraft fixed bed	1 – 12 MW
BFB	1 – 50 MW
CFB	10 – 200 MW

Fluidized beds offer the best solution for the gasification of biomass, and municipal solid waste (MSW), and refuse-derived fuels (RDF). In a typical BFB or CFB air enters from the bottom, but there is also a fuel fed from the side or top. In other case, the fuel is immediately mixed throughout the bed owing to its exceptionally high degree of mixing. Thus, the fresh oxygen (in air) entering the grid comes into immediate contact with fresh biomass particles undergoing pyrolysis as well as with spent char particles from the biomass, which has been in the bed for some time. Oxygen’s contact with the fresh biomass burns the tar released, while its contact with the spent char particles causes the char to burn. Though the solids are back-mixed, the gases flow upward in plug-flow mode. This means that further up in the bed neither older char particles nor fresh pyrolyzing biomass particles come in contact with the oxygen. Any tar released moves up in the bed and leaves along with the product gas. For this reason, tar generation in a fluidized-bed gasifier is between the two extremes represented by updraft and downdraft gasifiers, averaging about 10 mg/Nm³ [4, 17].

In entrained flow or flame gasifiers gas clean-up problems are largely avoided because biomass is gasified at elevated temperatures from 1000 up to 1700 °C. However, this requires the use of pure oxygen and the removal and handling of molten ashes. Although this technology has been demonstrated in the coal gasifier in Buggenum (the Netherlands), the properties of biomass impose specific problems. Firstly, the low calorific value and the hygroscopic nature of biomass (which leads to a high moisture content, even after drying) have a negative effect on the efficiency of gasification. Secondly, the fibrous structure of wood makes it difficult to grind the material to the desired size. For co-feeding of biomass with coal in thermal processing plants, a separate feed route is needed because the biomass cannot be grinded fine enough [1, 4].

A dual fluidized bed gasification technology (DFB) has been developed to provide the indirect heat supply for the gasification BBF reactor by circulating hot bed material that is heated in a separate CFB reactor by combustion of residual biomass char and any auxiliary fuel. Unlike the previously discussed gasifiers with direct heat supply by means of partial biomass oxidation inside the gasifier chamber, DFB system produces high caloric value (HCV) gas free of N₂ dilution, despite air is used to generate the gasification-required heat via in-situ combustion [18].

Figure 17 shows the principle of the DFB steam gasification processes. The biomass enters a fluidized bed gasification zone where drying, thermal devolatilization, and partially heterogeneous char gasification takes place at bed temperatures of about 850 ^oC. Residual biomass char leaves the gasification zone together with the bed material towards the combustion zone which is operated at about 920 ^oC. The combustion zone is used for heating up the bed material which afterwards flows back to the gasifier. Air is used as fluidization agent in the combustion zone. The temperature difference between the combustion and the gasification zone is determined by the energy needed for gasification as well as the bed material circulation rate. The gasification temperature can be influenced by addition of fuel into the combustion reactor. The pressure in both zones, gasification and combustion, is close to atmospheric conditions. The process results in two separate gas streams, a high quality producer gas and a conventional flue gas, at high temperatures. The producer gas is generally characterized by a relatively low content of condensable higher hydrocarbons (2–10 g/m³ of tar), low N₂ ( < 1 vol%, dry basis), and a high H₂ content of 35–40 vol %, dry basis. The process is used commercially in the demonstration 8 MW (fuel power) combined heat and power plant in Guessing, Austria.

Figure 17. DFB steam gasification with indirect heat supply [18]

AER concept. By using a bed material that not only acts as a heat carrier, but also influences the gasification process and especially the product gas composition a hydrogen rich gas can be produced. Addition of carbonates (limestone, dolomite) to the bed material and adequate adjustment of operation temperatures in the reactors allow selective in-situ adsorption of CO₂ from the producer gas, thus providing absorption enhanced reforming (AER). As a result, hydrogen contents of up to 75 vol%, dry basis can be achieved [18].

Figure 18. AER-DFB steam gasification with selective CO₂ transport [18]

The AER process (Figure 18) is more efficient than conventional gasification with downstream gas cleaning due to (i) in-situ integration of the reaction heat of CO₂ absorption and water–gas shift reaction heat (both exothermic) into the gasification zone and (ii) the internal reforming of primary and secondary tars, which avoids the formation of higher tars. Thus, the chemical energy of tars remains in the product gas. The product gas after dust removal can directly be used in a gas engine for electric energy generation without further tar separation. Due to the low operation temperature (up to 700 ^oC) and due to CaO-containing bed materials, the AER process allows the use of problematic feedstocks such as biomass with high mineral and moisture contents, e.g. straw, sewage sludge, etc., leading to an increased market potential for biomass gasification processes.

Figure 19 - Simplified flow sheet of the 100 kW process development unit at Vienna University of Technology [18]

The AER process is being experimentally proved on the 100 kW (fuel power) DFB gasifier at the Vienna University of Technology (Figure 19). Table 7 gives a summary of the main product gas components achieved by using either the AER-DFB or the conventional DFB process.

Table 7. Comparison of the producer gas concentrations (mol%) of the two processes [18]

Process type	DFB	AER-DFB
Gasification temperature(◦C)	850	645
H2	37.7	73.9
CO	29.1	6.1
CO2	19.6	6.0
CH4	10.4	11.7
C2H4	2.8	0.7
C2H6	0.3	1.3
Hydrocarbons C3–C5	0.1	1.1