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Deliverables
This is a very advanced gasification gas cleaning process. It has been studied, developed and/or applied to biomass gasification in fluidized bed. The core or basis of this application is the use of steam-reforming monoliths downstream from a fluidized bed biomass gasifier. These monoliths can be obtained nowadays from some EU manufacturers such as BASF. The monoliths (catalysts) convert and/or transform the unwanted tar, as well as most of the CH4, the small hydrocarbons (C2 and C3), and the NH3 present in the gasification gas into H2, CO and N2. The gasification gas is then cleaned of tar and NH3.The chemical energy contained in the tar is also transferred to the gasification gas. Tar content in the cleaned gasification gas of 150mg/Nm3 has been achieved by using these monoliths. The great advantage of using these honeycomb structures for gasification gas cleaning is that they accept a gasification gas with particulates in it, avoiding then the use of high temperature filters which could become plugged/ clogged by coke formed from tar cracking in their pores.
The key aspect to have a feasible overall gasification process is the life of the monolith. A high life, no deactivation, of the monolith can be obtained if the upstream gasifier has a good design and operation. An optimised partitioning or distribution of the total air flow used in the whole gasification plant is also important to have/ obtain a good performance of the gas cleaning monolithic reactor.
After five years of testing a one-layer based monolithic reactor, a new, more advanced, 2nd generation, two-layers based, monolithic reactor was tested at small pilot plant scale. It provided excellent results.
This gas cleaning technology can be also applied to fixed bed gasifiers and to other types of feedstocks such as sewage sludge, coal, RDF and plastics from MSW.
Optimal operation conditions for different catalyst materials have been investigated. Effects of various process conditions, like temperature, gas space velocity and gas composition have been found out. Promising new catalysts for ammonia SCO have been identified.
End users of the results are other partners of the consortium and companies and research groups developing catalytic gas cleaning for gasification processes.
The information created is basic knowledge in nature. It will be published in international journals, conferences when appropriate. It can be used as a basis for design and scale-up of catalytic gas cleaning processes.
Different gas phase kinetic models have been evaluated.
A simple four reaction heterogeneous model has been added to the gas phase mechanism and this model replicates very well the behaviour seen over the new catalyst design.
The model is validated over various experimental conditions of temperature, and oxygen concentration, and can predict the conversion of ammonia and hydrogen under these conditions.
The types of species required in an appropriate mechanism have been identified. A thorough literature survey of mechanistic and kinetic data available for the high temperature SCO reaction has been undertaken. Intermediates in the SCO reaction have been identified for the heterogeneous reaction mechanism.
End users of the results are other partners of the consortium and companies and research groups developing catalytic gas cleaning for gasification processes. The result is also applicable to modelling related problems such as hot gas cleaning of coal gasifier gas.
The information created is fundamental knowledge. It will be published in international journals, and conferences when appropriate. It has generated know-how for the partner. It can be used as a basis for design and optimisation of catalysts gas cleaning processes.
The modelling work was performed by University of Leeds (heterogeneous mechanism) and Abo Akademi (homogenous mechanism) and the related experimental work by VTT Processes.
The types of properties required of a catalyst for the oxidation of ammonia have been evaluated and determined. The effects of several properties like surface acidity and metal type and concentration have been studied. Promising SCO catalysts have been developed and prepared on a bulk scale.
End users of the results are other partners of the consortium and companies and research groups developing catalytic gas cleaning for gasification processes.
The information created is fundamental knowledge. It will be published in international journals, and conferences when appropriate. It can be used as a basis for design and optimisation of catalysts gas cleaning processes.
Comparison of the SCO and nickel-monolith systems to filtering only and wet cleaning methods were made for a case where an atmospheric-pressure circulating fluidised-bed (CFB) gasifier (90MWth) is connected to an existing coal/peat-fired boiler. The evaluation indicated that from economical point of view all the studied catalytic gas cleaning concepts were very close to each other. Thus, the selection can be fully based on technical feasibility and on the required level of gas cleaning. All catalytic gas-cleaning concepts are naturally more expensive than the reference case based on filtration only.
The additional cost of catalytic gas cleaning was small compared to basic case where gas is filtered. In some of the cases, the additional investment costs of catalytic gas cleaning devices were partly compensated by lower costs of gas cooler and filter unit.
The best potential for catalytic gas cleaning is in new advanced biomass-to-power/chemicals systems as well as in converting waste fuels into clean gas. Examples of the first category are the turbo-charged gas engines, biomass integrated gasification combined cycle, use of gas in fuel cells and synthesis gas production for liquid biofuels. In the latter case, the driving force is convert contaminated wastes into a clean gas, which can be used as any other fuels in boilers, engines or turbines.
Three-dimensional flow simulation was applied to analyse the combustion processes in a 350MW power plant co-firing pulverised coal and a biomass/waste derived gasification gas. The boiler is located in the city of Lahti, in eastern Finland. It is of a Benson once-through type, and has the dimensions of 50m in height and a base of 10mx7m. Gas combustion takes place in two burners (50MW) in the lower part of the boiler. For coal combustion (300MW), eight low-NOx burners and four conventional burners are used, followed by over-fire-air addition in the middle part of the boiler.
The commercial CFD-code Fluent was used for the simulations. The flow field in the furnace, the main combustion chemistry, heat transfer, and the concentration of nitric oxide (NO) were modelled. The commercial CFD-program Fluent (versions 5 and 6.0) was used for the simulations with the state-of-the-art sub-models for turbulence, radiation, and turbulence-chemistry interaction. Concerning sub-models on chemistry own improvements were introduced.
The aims in this work were:
- To study the combustion process under various operational conditions in order to gain a better understanding of how, where, and why NO is formed in the boiler.
- To come up with recommendations on how to further reduce the NOx emissions from the boiler.
- To illustrate the factors that affect the oxidation of fuel-N (NH3) in the gasification gas to NOx.
End users of the results are other partners of the consortium and companies and research groups developing low emission energy technology based on combustion and gasification.
The information created is basic knowledge in nature. It has been published in international journals and presented at scientific conferences.
Nickel catalysts were studied in granular and monolith forms. These catalysts were tested in product gas lines of small-scale gasifiers. Reactor model for nickel monolith was developed so that the effects of various operation conditions and the behaviour of the monolith could be studied. The most important operational parameters that affected to achievable ammonia and tar conversions were studied. These include air partitioning within gasifier/catalytic reformer system, H2O/C ratio and catalyst inlet temperature. The monolith catalyst was more challenging to operate than was foreseen. However, narrow operating window was successfully found, where over 90% ammonia conversions could be achieved.
A detailed gas-phase reaction mechanism for oxidation of fuel components and ammonia has been developed and validated for both fuel-rich (gasification) and fuel-lean (combustion) conditions.
End users of the results are other partners of the consortium, and companies and research groups developing catalytic/non-catalytic gas cleaning for gasification processes. Further, the results may be applied for development of appropriate low-NOx burners for gasification gas burning.
The information created is fundamental knowledge in nature. It has been published, e.g., in international journals and at scientific conferences.
The procedure for bulk preparation of catalysts has been optimised in terms of impregnation methods, drying conditions, calcinations conditions (heating rate, dwell temperature and dwell time).
End users of the results are other partners of the consortium and companies and research groups developing catalytic gas cleaning for gasification processes. The result has also generated know-how for the partner.
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