Gasification

Gasification is the conversion by partial oxidation at elevated temperature of a carbonaceous feedstock into a gaseous energy carrier consisting of permanent, noncon-densable gases. Development of gasification technology dates back to the end of the 18 th century, when hot gases from coal and coke furnaces were used in boiler and lighting applications [27]. Gasification of coal is now well established, and biomass gasification has benefited from that sector [28]. However, the two technologies are not directly comparable due to differences between the feedstocks (e.g., char reactivity, proximate composition, ash composition, moisture content, density). Gasifiers have been designed in various configurations (Figure 9.3), but only the fluid-bed configurations are being considered in applications that generate over 1 MWe [4, 5]. Fluid-bed gasifiers are available from a number of manufacturers in thermal capacities ranging from 2.5 to 150 MWth for operations at atmospheric or elevated pressures, using air or oxygen as a gasifying agent. Ideally, the process produces only a noncon-densable gas and an ash residue. However, incomplete gasification of char and the pyrolysis tars produce a gas containing several contaminants such as particulate, tars, alkali metals, fuel-bound nitrogen compounds, and an ash residue containing some char. The composition of the gas and the level of contamination vary with the feedstock, reactor type, and operating parameters (Table 9.1).

Since the mid-1980s, interest has grown on the subject of catalysis for biomass gasification [1]. The advances in this area have been driven by the need to produce a tar-free product gas from the gasification of biomass, since the removal of tars and the reduction of the methane content increase the economic viability of the

Updraft Downdraft

Biomass Product gas Biomass

Biomass Product gas

Circulating Fluid Bed product gas Product gas

Ash Oxidant Product gas and ash

Circulating Fluid Bed product gas Product gas

Bubbling Fluid Bed

Product gas Product gas

Freeboard

iomass iomass

ยป.Ash

Fluid bed

Bioneer (Finland), General Ventec (UK), TPS (Sweden), Lurgi JWP (USA), VUB Electric (USA), Hitachi Southern (Germany), Aerimpianti (Belgium), U.

(Japan), Manzano (Italy) California Edison (Italy) Sherbrooke (Canada)

Fluidyne (New Zealand)

FIGURE 9.3 The main gasifier configurations. (Adapted from Bridgewater, A.V., Toft, A.J., and Brammer, J.G., Renewable and Sustainable Energy Reviews, 6, 181, 2002.)

TABLE 9.1

Gasifier Product Gas Characteristics

Gas Composition (% v/v) HHV

TABLE 9.1

Gasifier Product Gas Characteristics

Gas Composition (% v/v) HHV

Gasifier Configuration

H2

CO

CO2

CH4

N2

(MJ/Nm3)

Fluid bed (air-blown)

9

14

20

7

50

5.4

Updraft (air-blown)

11

24

9

3

53

5.5

Downdraft (air-blown)

17

21

13

1

48

5.7

Downdraft (oxygen-blown)

32

48

15

2

3

10.4

Multisolid fluid bed

15

47

15

23

0

16.1

Twin fluid bed

31

48

0

24

0

17.4

biomass gasification process. The criteria for the catalysts are fundamentally the same and can be summarized as follows:

1. The catalysts must be effective in the removal of tars.

2. If the desired product is syngas, the catalysts must be capable of reforming methane, also providing a suitable syngas ratio for the intended process.

3. The catalysts should be resistant to deactivation as a result of carbon fouling and sintering.

4. The catalysts should be easily regenerated.

5. The catalysts should be resistant to abrasion and attrition.

6. The catalysts should be cheap.

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