By Ashwani Kumar
| October 22nd 2009 11:18 PM | Print
Biomass is one of the main available source of renewable energy. Its thermochemical upgrading can be performed in gasification (in the presence of air, O2, steam) or in pyrolysis (inert atmosphere) processes. The aim of gasification is to produce gases (CO, H2) . Biomass fast pyrolysis, characterized by high heating rates has been invented in the 70th with the objective to produce maximum fractions of gases. In last decade fast pyrolysis has been mainly carried out for the conversion of biomass into condensed vapours commonly named bio-oils . However, only few papers mention the possibility to produce gases, by fast pyrolysis [3, 4]. Most of the usual pyrolysis processes are primarily designed according to a given selectivity. It should be theoretically possible to control the gas phase reactions so as to favour either condensed vapours or gases production in a single reactor. This is the case with the multifunctional cyclone reactor [5, 6]. Indeed, several steps of a process can be simultaneously performed inside a single vessel : efficient heating and fast reaction of the solid biomass particles in the vicinity of the heated walls, separation of the solids from the gaseous products and possible quenching or thermal cracking of the vapours in the gas phase depending on operating conditions.
Fluidised-bed gasification of biomass residues and clean waste-derived fuels has been successfully demonstrated for a wide variety of feedstocks. A power plant concept consisting of a gasifier connected to a large coal-fired boiler with a high efficiency steam cycle offers an attractive and efficient way to utilise local biomass and waste sources and to lower the CO2 emissions of power production at relatively low cost. The 60 MW circulating fluidised-bed gasifier has been in reliable operation in Lahti, Finland, since early 1998. Another example of successful large-scale gasification has been realised in Varkaus, Finland, where 40 MW of aluminium-containing plastic wastes are gasified in a specially designed fluidised-bed gasifier. These projects are examples of economically sound demonstration projects, where the technical risks have been in good balance with the know-how of technology supplier and it's R&D partners. These commercially operating gasifiers also offer excellent sites for bringing the gas cleaning R&D from laboratories to actual field testing without creating unacceptably high technical or economical risks. Low-pressure fluidised-bed gasification process has also been developed to high-alkali biofuels (e.g. straw) and contaminated wastes, such as demolition wood, MSW, sewage sludge and autoshredder residues. In these applications, the gas cooler design and gas filtration play the key role. The process has been tested at MW-scale pilot plants with a wide range of fuels.
The next phase of gas cleaning R&D to be industrially applied is the catalytic removal of tars and ammonia. Technologies based on nickel and zirconium catalysts have already been field-tested in a slip-stream facility at the Lahti gasifier. After successful commercialisation in small-scale fixed-bed gasifier engine plants, this technology will also offer new markets for large-scale fluidised-bed gasifiers. Complete tar destruction makes it possible to utilise conventional gas coolers and cleanup technologies. The new gas cleaning technologies, firstly demonstrated in low-pressure fuel gas applications, can also be applied to significantly improve the performance of the simplified integrated gasification combined cycle process developed and demonstrated in Finland and Sweden in 1990's. Catalytic gas treatment is also one of the key technologies in developing second generation synthesis gas plants for producing methanol, FT-liquids, synthetic natural gas, hydrogen or chemicals from biomass and waste fuels. Keywords: gasification, demonstration, gas cleaning
Gasification technologies play an important role in increasing the share of biomass utilisation for energy production. Most of the high-efficiency power cycles as well as processes for the production of liquid biofuels or hydrogen are based on gasification. The main drivers for the development and commercialisation of biomass and waste gasification processes can be listed as follows:
•Requirement for sustainable waste utilisation methods with increased material recycling and improved energy production efficiency. •Need for small-scale power production from local biomass sources.
•Targets for increasing the share of renewable power, where biomass integrated gasification combinedcycle systems and other high-efficiency gasification power plants have a large potential.
•EU and US targets for extensive production of liquid biofuels for transport sector, which at present is the topic of intensive studies and R&D projects. •The development of fuels cells and long-term visions to hydrogen economy, which on the longer run offer interesting markets for biomass gasification technologies.