Biomass gasification

Current status of development

As a kind of thermochemical process technique, biomass gasification is able to convert solid biomass materials into gas fuel. Its basic principle is to heat biomass materials so as to break the chains between organic carbon-hydrogen compounds with high molecular weight and decompose them to hydrocarbons with light molecular weight, CO, hydrogen, etc. This kind of conversion will change the form of biomass material into one that can be used more conveniently. It also has a much bigger improvement on the energy conversion efficiency than direct combustion of solid biomass. From Table 1, we can see the gasification features of various biomass fuels.

Table 1 Gasification features of various fuels

fuel type

fuel features

Gasification intensity

gas production

Caloric value of gas

 

Humidity (%)

ash content (%)

Size (mm)

Caloric value (kJ/m3 )

(kg/ m2 /h

(m3 /kg)

(kJ/m3 )

air-dried wood

25 1.0

80-100

13,598.0

200-250

2.2

4,272.9

wood wastes

23 1.0

sawdust

13,598.0

260

2.3

4,359.7

wheat stalk and rice straw

10 3.5 cracked

14,727.1

180-220

2.3

4,690.

cowpat

16 6.0

50x50

11,715

200-230

2.2

3,907.9

leaves

10 5.0

natural size

13,807.2

200-230

2.0

3,694.5

Source : Biomass energy conversion technology, Northwest University Press (Xi'an: China), 1993.

There are three typical gasification techniques: distillation, rapid pyrolysis, and gasification. The first two are suitable for pyrolysis of wood or wood chips; the last is for gasification of stalks of crops such as corn and cotton.

Assessment for technical features and development level of gasification gas-supply system

a.  Technical features

Gasification set uses a fixed-bed down-draft gasifier. A blower is placed at the downstream of gasifier, which results that the gasifier works at a sub-atmospheric pressure. The top of gasifier is open. Its basic features are illustrated in Table 2

. Table 2 Basic features of biomass pyrolysis gasification set

Model

XFF-1000

XFF-2000

Gas production (m3/h)

200

500

Gasification temperature

Over 1,000oC

Gasification efficiency

72-75%

Caloric value of gas (KJ/m3)

4,000-5,200

Ingredients for gas

N2

CO2

CO

H2

 

49-56%

12-14%

18-23%

9-12%

Adaptability for feedstock

Stalks of corn, sorghum, cotton, soybean, etc. and wood branch, bark, etc.

Source: The Research Institute of Energy, Shangdong Academy of Sciences.

Biomass pyrolysis gasification and centralized gas-supply system is a crop stalk-based biomass energy conversion and cooking gas-supply system in a unit of natural village. Its major equipment is Model XFF-2000 biomass gasification set, which includes crop stalk-based down-draft fixed-bed gasifier, gas cleaning system, and a blower suitable for a distance of 1 km. Its technical features are: using down-draft fixed-bed gasifier, operating at a slightly sub-atmospheric pressure, top-open and consecutive feed. The framework for technical system is illustrated in the figure below.

b.  Technical assessment for gasification gas-supply system

1)  Advancement and Adaptability

Compared with woody feedstocks, crop stalks have the disadvantages such as low density, high ash content, low carbon ingredient. Thus it has much difficulty in gasification technique and gas clarification than previous gasification technologies. Model XFF-2000 pyrolysis gasification system realized the steady gasification of low-grade biomass materials such as crop stalks. Its main technical indices are equivalent to those of the woody fuel-based types in home or aboard, namely: gasification efficiency of 75%, low caloric value of gas of 5,000 kJ/Nm3. For gasification of low-grade biomass materials and centralized supply of gas with low caloric value, it is suitable for the current economic and technical levels in China.

2)  Integrity and reliability

The whole system consists of biomass preparation and handling, gasifier, gas cleaning system, ash removal, gas supply and delivery system and related stoves.

Crop stalks are stored under the situation of natural air-dry. The hay cutter is a general agricultural machine with cheap price and high reliability. The gasifier uses heat-proof cast iron furnace bed, sealed ash bin and machinery ash-removal device. Furnace tank is made up of firebricks, which can withstand a temperature far higher than the furnace temperature. The whole device is composed of steel plates.

To overcome the problems that crop stalks are soft-quality and of high ash content, and that there is high tar content in the gas, the clarification process of ash-remova cooling sedimentation filtering is adopted. The gas clarifier consists of two-stage whirlwind separator, cooler, gravity settling chamber and filter. At the end of the pipeline, a filter is installed inside the userĄŻ s house, to absorb remnant foggy tar using activated charcoal as filtering material.

The gasification remainder include carbonaceous ash-dust (accounting for 10% of feedstock) and tar (accounting for 5% of feedstock). Currently no effective disposal technique is used. Gas supply and delivery system is composed of gas storage tank and pipeline networks.