CFB gasification of biomass residues for co-combustion in large utility boilers: Studies on ash control and gas cleaning

Atmospheric-pressure gasification of biomass and waste fuels and co-firing of pro-duced gas in large utility boilers is a promising and cost-effective method for re-ducing the CO2 emissions of coal-based energy production in short and medium time scale. The commercialisation of this co-utilisation technology will also pro-mote the development and commercialisation of advanced biomass-based power production systems, such as IGCC, which in the longer run have a key role in the global renewable intensive energy scenarios. The woody residues of mechanical and chemical wood industry as well as other high-grade woody biofuels can be co-combusted with coal in existing fluidised-bed boilers or even in pulverised combustors (1, 2) without major problems. On the other hand, extensive full-scale test programmes carried out with straw in Denmark (3) and with various agrobiofuels in the United States (4) have clearly shown the limitations of this type of co-combustion methods. Many potential biomass feed-stocks, such as straw, have a problematic ash melting behaviour, which causes sintering and fouling problems in combustors. Straw and many fast-growing energy crops as well as industrial and municipal waste fuels often contain high amounts of chlorine and alkali metals, which have a tendency to cause severe corrosion prob-lems in coal-fired boilers. This problem can be assumed to be more severe in the modern plants where supercritical steam values are utilised. Perhaps the most critical factor controlling possibilities for direct co-firing of these problematic biofuels in large PC boilers is the usability of coal ash for cement in-dustry and construction purposes. Demolition wood waste is another example of a locally important renewable feedstock, which is difficult to be introduced into ordi-nary coal-based combustion plants due to the relatively high content of heavy met-als (Zn, Pb, Cd) and chlorine. In principle, there are three main technical solutions to avoid sintering, corrosion and ash problems in co-utilisation of problematic biofuels in large coal-fired power plants: · Construction of a smaller separate boiler with low steam values for the biofu-els and superheating the steam of the biomass boiler in the coal-fired boiler. · Production of pyrolysis oils from the biomass, and · Gasification of biomass and combustion of cleaned product gas in the boiler. The concept based on gasification has several advantages over the other alterna-tives. First of all the gasification reactor, bubbling or circulating fluidised-bed, is simpler and much cheaper than the complete fluidised-bed boilers with steam cy-cles or complete pyrolysis oil production plants. Secondly, the product gas can be cleaned from trace metals, chlorine and other harmful contaminants prior to leading into the coal-fired boiler. Depending on the feedstock quality and on the require-ments for contaminant control different gas cleaning methods from simple hot cy-clones to effective low-temperature filtration can be used. Thirdly, the separation of gasification and gas combustion makes it possible to maintain stable high-temperature combustion in order to minimise risks for the formation of dioxins and other chlorinated organic compounds. Finally, the biomass-derived gas may even have positive effects also on the NOx emissions if it is introduced to the coal-fired boiler as a reburning feedstock (5). At present, there are a lot of on-going activities in Europe aiming at the demon-stration and commercialisation of this promising co-utilisation concept (6). Al-most all activities are based on Circulating Fluidised-Bed gasifiers. Series of studies on the critical technical questions of this co-utilisation concept are car-ried out by VTT Energy in the PROGAS R&D programme. The results obtained with wheat straw, demolition wood and fresh wood are outlined in this paper.