Gasification and pyrolysis of biomass (summary)

Summary of TAB working report No. 49

"Gasification and pyrolysis of biomass"

The gasification of biomass is a developing energy technology among various systems for the energetic utilisation of biomass which has the following main advantages compared to conventional combustion technologies:

The combined heat and power generation via biomass gasification techniques connected to gas-fired engines or gas turbines can achieve significantly higher electrical efficiencies between 22 % and 37 % compared to biomass combustion technologies with steam generation and steam turbine (15 % to 18 %). If the produced gas is used in fuel cells for power generation, an even higher overall electrical efficiency can be attained in the range between 25 % and 50 %, even in small scale biomass gasification plants and under partial load operation.
Due to the improved electrical efficiency of the energy conversion via gasification, the potential reduction in CO₂ is greater than with combustion. The formation of NO_x compounds can also be largely prevented and the removal of pollutants is easier for various substances. The NO_x advantage, however, may be partly lost if the gas is subsequently used in gas-fired engines or gas turbines. Significantly lower emissions of NO_x, CO and hydrocarbons can be expected when the produced gas is used in fuel cells instead of using it in gas-fired engines or gas turbines.

Pyrolysis of biomass generates three different energy products in different quantities: coke, oils and gases. Flash pyrolysis gives high oil yields, but because of the technical efforts needed to process pyrolytic oils this energy generating system does not seem to be very promising at the present stage of development. However, pyrolysis as a first stage in a two-stage gasification plant for straw and other agricultural feedstocks posing technical difficulties in gasification does deserve consideration.

In most biomass gasification processes, air is used as gasifying agent with the result, that a low calorific value gas (3-5 MJ/m³) is generated, which can be used after cleaning in gas-fired engines or gas turbines. For gas turbines connected to a steam turbine, medium calorific value gas (12-15 MJ/m³) is more favourable than low calorific gas. Steam injection into the gas turbine combustion chamber (Cheng process) requires at least medium calorific value gas. The production of methanol or hydrogen via biomass gasification or the use of producer gas in low-temperature fuel cells also require either gasifiers operating with highly-enriched oxygen and steam or indirectly heated (allothermic) gasifiers must be used with steam as a gasification medium to generate the necessary medium calorific value raw gas with high hydrogen content.

Gasification of wood and wood-type residues and waste in fixed bed or fluidised bed gasifiers with subsequent burning of the gas for heat production is state of the art. The wood gasifiers employed primarily in the Scandinavian countries are used almost entirely for heat generation. Significantly greater technical problems are posed by gasification of straw and other solid agricultural feedstocks, which mostly have higher concentrations of nitrogen, sulphur, chlorine and alkalines. The gasification of herbaceous biomass is still at an early stage of research and development. Intensified development efforts on gasification technologies for herbaceous biomass feedstocks are desirable as the potential supply of this group of fuels is comparatively large.

Thorough gas cleaning and perfect adaptation of the gas from biomass gasification to the specific requirements of the gas utilisation systems are the prerequisites for gas use in gas-fired engines, gas turbines and fuel cells. Tar compounds can be removed effectively by increasing the gas temperature or by catalytic tar cracking with dolomite or nickel. However, even for wood gasifiers there is still no economically viable solution of the tar problem. None of the gasifier types currently on the market have been successfully tested in connection to gas-fired engines in long term operation under practical conditions in combined heat and power stations.

Under EU demonstration projects, integrated biomass fluidised bed gasifiers with combined cycles (gas and steam turbines) and with an electrical capacity of 5 MW and more are planned, being under construction or in operation. Both gasifiers operating under atmospheric conditions and under pressure (up to 20 bar) and with cold or hot gas cleanup systems are involved in the EU demonstration programme. With pressurised gasification higher overall electrical efficiencies can be achieved, but greater technical and financial resources are required to feed the biomass into the gasifier, and problems with gas cleaning may occur.

For power plants with integrated biomass gasification in the range 3 to 20 MW_el, biomass fluidised bed gasification under atmospheric pressure connected to gas turbines, Cheng cycles or gas and steam turbines (IGCC) appear to be the most promising technology at present in technical and economic terms. For combined heat and power stations with capacities up to about 2 MW_el, gas use in gas-fired engines is currently more interesting than gas use in gas turbines. Because of problems with fuel supply and logistics, biomass gasification plants with capacities above approximately 30 MW_el are not a suitable size for biomass gasification plants in Germany and most other European countries.

The joint combustion of biomass in existing large coal-fired power stations (< 100 MW_el) is currently being investigated in different countries. The integration of biomass-fuelled gasifiers in coal-fired power stations would have different advantages over stand alone biomass gasification plants. Of importance are the greater flexibility in response to annual and seasonal fluctuations in biomass availability and the lower investment costs for the biomass gasification unit.

In case the cleaned and upgraded producer gas is used in fuel cells for power production, the low-temperature proton exchange membrane fuel cell (PEMFC) and the high-temperature molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) are more attractive in the longer term because of their higher overall electrical efficiency than the medium-temperature phosphoric acid fuel cell (PAFC). Power generation in high-temperature MCFC or SOFC with integrated biomass gasification also has the advantages that no separate unit is needed for CO-shift reaction prior to gas injection into the fuel cell, and that in addition to electricity, process heat is provided by the fuel cell at a high temperature level. For the PEMFC and MCFC, which are on the threshold of the demonstration phase, several companies are aiming to move into small series production in the next few years.

Although each of the fuel cell types listed above has made substantial advances in technological development in recent years, all three types still have several major technical problems to overcome. It remains to be seen which type will overcome its problems most successfully. The start of series production should in any case significantly reduce the present cost disadvantage compared to other gas utilisation techniques. To reach the point of technological maturity for fuel cell systems with integrated biomass gasification (IGFC), extensive R&D work going beyond fuel cell technology is necessary.

Under the frame conditions currently reigning on the energy market there is little motivation for plant manufacturers and potential operators to fund the bulk of R&D work themselves. In this situation, encouraging R&D activities requires not only promoting application related demonstration projects but also R&D in the field of gasification, gas cleaning and gas utilisation. Promotion of further research, development and demonstration projects is recommended, with priority for the following areas:

Demonstrating low-outage plant operation with an integrated biomass gasification plant (initially wood), a gas cleaning system and gas use in gas motors and gas turbines in regular operation in technical test installations and subsequently in demonstration units.
Development and technical demonstration of gasifiers for straw and other straw-type biomass and associated gas cleaning processes.
Integration of units for gasifying or pyrolysing biomass into existing large coal-fired power stations.
Experimental testing of combining processes for biomass gasification, gas cleaning and gas use in fuel cells.

At present, biomass gasification is starting from an even less favourable economical position than energy use of biomass through combustion, as the technically attractive gasification systems are in an earlier phase of development and demonstration. Assessments on the cost-effectiveness of energy generating systems with integrated biomass gasification have to be proven by practical experience in regular operation. There are, nevertheless, indications that technical advances in developing reliable systems for biomass gasification and efficient gas utilisation can lead to economical advantages over combustion. However, they can only raise heat and power generation from biomass above the break-even point if there is also a significant change in the frame conditions, for example through greater financial reward for the environmental benefits associated with biomass utilisation for energy production. Otherwise the market for biomass gasifiers in Germany and Europe will be limited for the foreseeable future to the treatment of organic wastes (e. g. process residues from the cellulose, paper and sugar industries).

Status: April 1997 - Questions or comments buero@tab.fzk.de