AIR EUR 17788: Biomass Gasification and Pyrolysis, State of the Art and Future Prospects (AIR3-CT94-2284 and AIR3-CT94-1857)

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Publications : AIR Cluster II - Bioenergy Conversion : Liquid Biofuels and Biogas : Solid Biofuels : Thermochemical Conversion

Achievements:
The consensus was that both technologies have made significant progress over the last three years. Although neither technology is yet truly economic in the absence of fiscal incentives, the scientific and technical knowledge base has grown considerably. This will contribute towards improving technology and reducing costs. The Networks have played a major role in collating and disseminating information throughout Europe and to other parts of the world. They have led to many new collaborative ventures at research and development stages and made the entire industrial and political community aware of the what bio-energy has to offer. The evidence for this, lies in the large number of papers offered and the high attendance at the meeting, as indicated in the following contents list. The inclusion of contributions from outside the EU together with the results from these networks makes this report a comprehensive record of the state of the art, essential for those interested in biomass thermal processing.

List of papers presented
The proceedings contain over 50 presentation divided over the following topics

1. OVERVIEW (6 papers)
2. GASIFICATION
   1. Gasification technology (12 papers)
   2. Gas cleaning (9 papers)
   3. Applications (3 papers)
3. PYROLYSIS
   1. Fast pyrolysis (10 papers)
   2. Upgrading (5 papers)
   3. Characterisation and analysis (4 papers)
   4. Chemicals (3 papers)
   5. Applications (2 papers)
   6. Systems (1 paper)

The last section deals with the conclusions from the meeting and makes some recommendations concerning further research and development needs. These are abstracted below.

Conclusions and Recommendations

Gasification
Considerable progress has been made in very specific areas of biomass gasification in recent years. But up to now, a technical and economical optimum has not been found between simple reactor design, effective gas cleaning and efficient utilisation of the gas. Also the optimisation of plant capacity considering technical and economic constraints and the availability of the biomass for each location is still at a very early stage. In particular the following points therefore require more attention:

• Realisation of additional demonstration projects in different power ranges to optimise the overall process and to gather more experience with the technology of biomass gasification. This is especially true for fixed bed gasifiers. But each new demonstration project should be carefully screened in the light of the existing experience obtained with the demonstration projects of the past. Only if a new project brings really new and / or improved technology should it be approved.
• Continuing evaluation of both successful and less successful projects concerning resolved and unresolved problems to ensure better transfer of existing knowledge and experience.

The overall conclusion was that there are still many problems concerning aspects of the technology and system. In each part of the gasification process there are unresolved problems and in particular the optimum combination of the different system elements is still very difficult. The long history of biomass gasification inside and outside Europe and the various failed projects in the last few decades may suggest that a rapid solution of these R,D&D problems is impossible, particularly considering that there are finite public resources available to support the development of biomass gasification.

To make biomass gasification commercially successful it is necessary to target available funds to the more promising alternatives rather than adopt widespread allocation of funds. Therefore the public funds should be allocated to the most promising gasification concepts with a high short and medium term market potential although there does need to be some allocation to new ideas and innovative and promising ideas to take advantage of innovation potential. One of the advantages of this strategy is that the probability of successful implementation is thereby improved which is necessary for providing the necessary evidence of technical and commercial success to convince decision makers to spend more public money in this area. Only when the technology is developed for markets with a high potential, it is likely that a satisfactory market penetration can be achieved.

Gasification of biomass is a promising option especially for electricity generation. Within the overall electricity market two niches could be identified where this technology fits well. These are:

• plants up to around SMW electrical capacity based on disposal of wood waste and related materials for small and medium sized enterprises in the wood processing sector
• plants up to around 20MW electrical capacity, or as large as possible dependent on biomass availability, to meet the demand for green electricity to be covered by utilities.

The most promising option at the moment for the first niche market appear to be fixed bed gasifiers with an IC engine and for the second niche market atmospheric CFB gasifiers with a gas turbine in a combined cycle system. But these technologies are currently still in the developing stage. However, there are some projects on the way which are based on these technologies. Therefore, the short term needs are to support these projects by accompanying measures and to solve the problems which occur during the construction and operation of these plants as well as the already known problems and obstacles which have to be solved. The results of these projects then have to be evaluated and new projects have to be started with concepts solving the problems still remaining after this phase.

Fixed bed gasifiers:

• The reactor design for fixed bed gasifiers need to be optimised to minimise dust and especially tar in the producer gas; recent results show that there might be a chance to reduce the tar content to such values that the producer gas can be used in engines without an additional tar reduction,
• Gas cleaning of the producer gas needs to be improved to run also in the long term with satisfying results without producing too much waste (i.e. waste water),
• In engines where producer gas is used as fuel, a higher tolerance to contaminants, particularly dust, needs to be developed,
• The interaction of different system elements within the whole system "biomass gasification" (i.e. gas production, gas cleaning, gas utilisation) needs to

CFB gasifiers

• CFB reactor design and operation need to be improved to reduce tar content in the gas; the latest findings show that the promised technical possibilities are in most cases not yet fully realised; and it appears to be possible to reach a tar content in the producer gas which makes a further gas cleaning unnecessary,
• If the tar content could be reduced to sub-critical values, the focus has to be placed on reducing the dust content of the producer gas; here more efforts should be placed on the development and optimisation of filter units for dust removal preferably based on ceramic candles,

Producer gas utilisation in gas turbines causes among others two fundamental problems: firstly the producer gas needs to be compressed to be fed into the burning chamber of the gas turbine and secondly dust in the gas could cause erosion on turbine blades; therefore work should be concentrated on optimisation of the gas compression and adaptation of existing (robust) gas turbines to producer gas (i.e. improvement of tolerance towards dust), The interaction of different system elements within the whole system "biomass gasification" needs to be optimised. The most urgent R,D&D needs on the long term are summarised as follows.

Fixed bed gasifiers

• Design and development of new concepts for fixed bed gasifiers and especially for gas cleaning,
• Investigation of "new" feedstocks like herbaceous biomass,
• Development and further improvement of engines to use producer gas,
• Improvement of the interaction of different system components to ensure a more efficient plant operation.

CFB gasifiers

• Improvements in catalytic hot gas cleaning techniques,
• Investigation of "new" feedstocks (like herbaceous biomass),
• Improvement of the interaction of different system elements for realising more efficient plant operation,
• Adaptation and operational improvement of existing gas cleaning technologies to operation under pressure,
• Adaptation and improvement of existing gas utilisation concepts to operation under pressure.

Pyrolysis
Fast pyrolysis for liquid is less developed than gasification but offers the considerable advantage of a liquid product that can be stored or transported, i.e. de-coupling the conversion technology from the utilisation of the product. As with gasification the system has to be considered in totality and the component needs and system needs are identified and described. The heart of the fast pyrolysis process is the fast pyrolysis reactor. Many types are currently operational and no best method has emerged. Indeed, further innovation should be expected, with new concepts and both radical and minor modifications to reactors to reduce costs, improve yields, improve product quality and improve ease of scale up. The use of catalysts to enhance the pyrolysis process and improve the product quality is likely to increase. A key feature of further development will be increasing attention on the fundamentals of pyrolysis - modelling, design, mechanisms etc. which are essential for further process improvement and optimisation.

Product collection
Efficient collection of the liquid product in terms of yield and product quality is still a major problem. Heat exchange is not sufficient by itself as the product is largely in aerosol form which, like cigarette smoke, is very difficult to capture. Quenching and electrostatic precipitation are currently the preferred options, but there is a need to design more effective and efficient devices. Selective condensation or fractionation can be explored again to improve product quality and possibly enhance yields of chemicals.

In addition to collection problems, there is the problem over char in the product which is too small to be collected in the cyclones and accumulates in the liquid. This can be removed by hot vapour filtration which affects product yield and quality or by liquid filtration or centrifugation which is difficult and gives high losses. The third option is to adapt the application to accept liquid with a significant char content. All these solutions have advantages and disadvantages and require further R&D and subsequent evaluation to identify the preferred route.

Product characterisation and quality
Pyrolysis liquid is a fuel oil but with significantly different characteristics from conventional fuel oils in that it is water-miscible and relatively unstable through viscosity changes and occasionally phase separation and is temperature sensitive. The important characteristics for each potential application need to be defined and steps taken to satisfy requirements such as by addition of modifiers or enhancers. Characterisation of this liquid is difficult and standard fuel tests may not be appropriate. New tests may have to be developed to measure key parameters including stability in particular. Chemical analysis is complex from the wide range of different functional groups and the very large number of individual chemicals. Analysis is important to aid process analysis and hence process optimisation and requires continued support to ensure that ongoing research projects are properly supported.

Product modification:
Upgrading to a product that is compatible with conventional fossil fuels will remain of interest even though the chemistry and economics of the conversion of biomass or bio-oil, with its high oxygen content, to hydrocarbons with no oxygen, is not easy. Upgrading by hydro-processing type methods to hydrocarbons and transport fuels is technically successful but uneconomic due to the hydrogen requirement and high pressure processing. Zeolite cracking is potentially more interesting as low pressure is required without any hydrogen and the process is integrated into the fast pyrolysis process, but yields are lower and a more complex integrated catalytic regenerative process is needed. This makes useful research difficult to perform in small scale laboratory systems. This area of integrated catalytic pyrolysis is deserving of further study for process and product optimisation. The stability problems that have been reported with the liquid are believed to be caused by polymerisation/condensation type reactions between carbonyl groups in aldehydes for example and the phenols in the acid environment. Modification of the more reactive functions in the liquid would lessen or minimise such reactions. This is being studied through modification of the pyrolysis process either within the reactor or close coupled, and also by modification of the liquid after its production. Neither route yet has enough results to offer guidance. The interface between fast pyrolysis and the liquid product application needs more careful assessment for a variety of topical applications so that the bio-oil requires minimum modification.

Applications

Heat:
Combustion of pyrolysis liquid has been successfully demonstrated but problems remain with handling and stability, start-up and shut-down, atomisation, emissions, co-firing with other fuels, although some success has been reported with emulsions, flame temperatures and patterns.

Power:
Both engine and turbine show promising results with some improvements over conventional fuels but require long term operation and optimisation in order to assess corrosion, deposits and offer warranties. The anticipated problems of instability etc. have not materialised.

Chemicals:
Chemicals offer the high value-added opportunities that this industry requires to be viable in the short term. The markets require development however, both for product substitution a well as new products.

System:
It is important to consider the complete system in terms of energy integration and utilisation of the gas and char by-products.

Conclusions on pyrolysis:
Fast pyrolysis offers the unique advantages of a liquid that can be stored and/or transported and which can be used for fuel and/or chemicals production. There are still many challenges to be met including provision of consistent bio-oil in sufficient quantities for thorough testing in various applications, scale-up of fast pyrolysis processes with concomitant cost reductions from process development, and development of norms and standards for the liquid product particularly in terms of quality so that it can be marketed successfully. Table 2 summarises the strategic requirements .

Publication: The 550 page hardcover book of proceedings is available (price 150 ecu) direct from CPL Scientific Ltd (CPL Press) or from normal bookshops or other outlets for EC publications (ISBN 1 872691 71 4, EUR 17788) as Biomass Gasification and Pyrolysis, M Kaltschmitt and A V Bridgewater eds, 1997.