Bioresource Technology 83 (2002) 55–63

Review paper

Energy production from biomass (part 3): gasification technologies Peter McKendry

1,2

Applied Environmental Research Centre Ltd, Tey Grove, Elm Lane, Feering, Colchester CO5 9ES, UK Accepted 6 July 2001

Abstract The conversion of biomass by gasification into a fuel suitable for use in a gas engine increases greatly the potential usefulness of biomass as a renewable resource. Gasification is a robust proven technology that can be operated either as a simple, low technology system based on a fixed-bed gasifier, or as a more sophisticated system using fluidized-bed technology. The properties of the biomass feedstock and its preparation are key design parameters when selecting the gasifier system. Electricity generation using a gas engine operating on gas produced by the gasification of biomass is applicable equally to both the developed world (as a means of reducing greenhouse gas emissions by replacing fossil fuel) and to the developing world (by providing electricity in rural areas derived from traditional biomass).
2002 Elsevier Science Ltd. All rights reserved. Keywords: Gasification; Renewable energy; Biomass

1. Introduction Gasification is the conversion of biomass to a gaseous fuel by heating in a gasification medium such as air, oxygen or steam. Unlike combustion where oxidation is substantially complete in one process, gasification converts the intrinsic chemical energy of the carbon in the biomass into a combustible gas in two stages. The gas produced can be standardised in its quality and is easier and more versatile to use than the original biomass e.g. it be used to power gas engines and gas turbines, or used as a chemical feedstock to produce liquid fuels. Strictly, gasification includes both biochemical and thermochemical processes, the former involving microorganisms at ambient temperature under anaerobic conditions i.e. anaerobic digestion, while the latter uses air, oxygen or steam at temperatures >800 C. In accordance with common practice, the term ‘‘gasification’’ in this study will refer only to the thermochemical conversion of biomass. 2. Basic chemistry The reactions taking place in the gasifier can be summarised as indicated below: 1 Present address: MSE Ltd, Arle Crt, Hatherley Lane, Cheltenham GL51 6PN, UK. 2 Correspondence address: Green Acre, Dark Lane, Bristol BS40 8QD, UK. Tel.: +44-1242-269685.

Partial oxidation Complete oxidation Water gas reaction

C þ 1=2O2 $ CO C þ O2 $ CO2 C þ H2 O $ CO þ H2

dH ¼ 268 MJ=kg mole dH ¼ 406 MJ=kg mole dH ¼ þ118 MJ=kg mole

The heats of reaction 3 for the three processes show that the greatest energy release is derived from the complete oxidation of carbon to carbon dioxide i.e. combustion, while the partial oxidation of carbon to carbon monoxide accounts for only about 65% of the energy released during complete oxidation. Unlike combustion that produces only a hot gas product, carbon monoxide, hydrogen and steam can undergo further reactions during gasification as follows: Water gas shift reaction Methane formation

CO þ H2 O $ CO2 þ H2 CO þ 3H2 $ CH4 þ H2 O

dH ¼ 42 MJ=kg mole dH ¼ 88 MJ=kg mole

The arrows indicate that the reactions are in equilibrium and can proceed in either direction, depending on the temperature, pressure and concentration of the reacting species. It follows that the product gas from gasification

3 Heats of reaction are þ for endothermic reactions and  for exothermic reactions.

0960-8524/02/$ - see front matter
2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 1 2 0 - 1

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P. McKendry / Bioresource Technology 83 (2002) 55–63

consists of a mixture of carbon monoxide, carbon dioxide, methane, hydrogen and water vapour. Three product gas qualities can be produced from gasification by varying the gasifying agent, the method of operation and the process operating conditions. The main gasifying agent is usually air but oxygen/steam gasification and hydrogenation are also used. Catalytic steam gasification is another mode of operation that influences both the overall performance and efficiency. The three types of product gas have different calorific values (CV): 3

Low CV

4–6 MJ=Nm

Medium CV

12–18 MJ=Nm

High CV

40 MJ=Nm3

3

Using air and steam/air Using oxygen and steam Using hydrogen and hydrogenation

Low CV gas is used directly in combustion or as an engine fuel, while medium/high CV gases can be utilized as feedstock for subsequent conversion into basic chemicals, principally methane and methanol. As the use of oxygen for gasification is expensive, air is normally used for processes up to about 50 MWth . The disadvantage is that the nitrogen introduced with the air dilutes the product gas, giving gas with a net CV of 4–6 MJ/Nm3 (compared with natural gas at 36 MJ/ Nm3 ). Gasification with oxygen gives a gas with a net CV of 10–15 MJ/Nm3 and with steam, 13–20 MJ/Nm3 . It can be seen that while a range of product gas qualities can be produced, economic factors are a primary consideration. Unlike the reaction with air/oxygen, the reaction of carbon with steam (the water gas reaction) is endothermic, requiring heat to be transferred at temperatures around 700 C, which is difficult to achieve. Gasifiers self-sufficient in heat are termed auto-thermal and if they require heat, allothermal: auto-thermal processes are the most common. The overall efficiency of conversion of biomass to energy using gasification and pyrolysis is estimated as 75–80%.

3. Feedstock pre-treatment The degree of pre-treatment of the biomass feedstock is dependent on the gasification technology used. The main problem areas are: Drying. The biomass moisture content should be below 10–15% before gasification. Particle size. In most gasifiers, gas has to pass through the biomass and the feed has to have sufficient compressive strength to withstand the weight of the feed above. Feed particle sizes in the range 20–80 mm are typical.

Fractionation. The nitrogen and alkali contents of the biomass are critical, as they are partially carried over into the gas-stream. Small particles tend to contain less nitrogen and alkalis, so fractionation into fine and coarse particles helps to produce a gas with fewer impurities. Leaching. The nitrogen and alkali contents of the biomass can be reduced by prior leaching with water. Drying wood from 50% to 60% (as-felled), or using air-dried wood with a moisture content of 20%, to the required level of 10–15% moisture requires the use of driers. The driers can be directly heated rotary driers using the flue gas or indirectly heated fluidised bed driers using steam to heat the feed material. The vapours emitted during drying contain a number of volatile organic compounds (VOCs), mainly terpenes, which require appropriate air pollution control systems.

4. Feedstock properties The characteristics of the biomass feedstock have a significant effect on the performance of the gasifier, especially the following characteristics. 4.1. Moisture content Fuel with moisture content above about 30% makes ignition difficult and reduces the CV of the product gas due to the need to evaporate the additional moisture before combustion/gasification can occur. A high moisture content reduces the temperature achieved in the oxidation zone, resulting in the incomplete cracking of the hydrocarbons released from the pyrolysis zone. Increased levels of moisture and the presence of CO produces H2 by the water gas shift reaction and in turn the increased H2 content of the gas produces more CH4 by direct hydrogenation. The gain in H2 and CH4 of the product gas does not however compensate for the loss of energy due to the reduced CO content of the gas and therefore gives a product gas with a lower CV. Calculations based on a 2:7 MWðeÞ gas engine generator using a dryer with a retention time of one hour, indicate that sufficient waste heat is available from the engine oil and water cooling systems and the exhaust to dry 7.7 t/h of SRC willow feedstock at 35% moisture down to 15% for use in a gasifier (Stamford Consulting Gp., 1994). 4.2. Ash content High mineral matter can make gasification impossible. The oxidation temperature is often above the melting point of the biomass ash, leading to clinkering/ slagging problems in the hearth and subsequent feed blockages. Clinker is a problem for ash contents above 5%, especially if the ash is high in alkali oxides and salts

P. McKendry / Bioresource Technology 83 (2002) 55–63

which produces eutectic mixtures with low melting points. 4.3. Volatile compounds The gasifier must be designed to destruct tars and the heavy hydrocarbons released during the pyrolysis stage of the gasification process.

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1993; Rampling and Gill, 1993). A third type, the entrained suspension gasifier, has been developed for coal gasification but the need for a finely divided feed material (