LABORATORY RESEARCH ON CO-FIRING WOODEN BIOMASS WITH BOSNIAN BROWN COAL AND LIGNITE FOR LARGE UTILITY

摘要

Paper presents an experimental research into ash-related problems and emissions during co-firingwooden biomass with Bosnian brown coal and lignite. The research is aimed to optimize percentage of the switchingcoal by wooden biomass in two large Bosnian coal-based power stations; the Kakanj power station (2×110 MWe,1×230 MWe) supplied by brown coal and the Tuzla power station (1×100 MWe, 2×200 MWe, 1×215MWe) suppliedby lignite. For purpose of the research, an electrically heated entrained pulverized-fuel flow reactor was used. Co-firing trials were running under different conditions in the reactor; varying the process temperature, excess air ratio,and air distribution. The co-firing tests used spruce sawdust in combination with Kakanj brown coal and a ligniteblend consisted of Dubrave and Sikulje lignite. Coal/biomass mixtures at 93:7%wt and 80:20%wt were tested.During the tests, the temperature in the experimental facility varied between 880 ℃ and 1550 ℃, while the excess airratio varied between 0.95 and 1.4. Both for lignite-wooden biomass and brown coal-wooden biomass co-firing, therewere no significant ash-related problems if the blend with 7%wt of wooden biomass was used and if the processtemperature did not exceed 1250 ℃. From the other side, adding biomass to the brown coal for the co-firing attemperature 1550 ℃, which is actual temperature in the boilers with slag tap furnaces in the Kakanj power station,slag flow over ceramic surface was improved. Much lower NO_x emissions were measured at the lower processtemperatures and the lower excess air ratio used in all co-firing regimes. It was identified that not only nitrogencontent in the co-firing blend but also volatile content and particle size distribution of the coal-wooden biomassmixture influenced on NO_x emissions. Reduction of SO_2 emission was detected for all co-firing regimes. In additionto the reduction of SO_2 due to the lower sulfur content in the coal/biomass blends tested, the brown coal-sawdust co-firing generated a further reduction of SO_2 due to the higher sulfur capture rate than for coal alone. Finally,significant reductions of CO_2 found by calculation for the blends tested, due to the low ranking of Bosnian coals. Itwas found that 7%wt of wooden biomass can be used in combination with coal in both power stations without risk tothe combustion process and with benefits through emissions reductions. For the first time in Western Balkan a trialrun of co-firing on a large-scale utility is going to be realized (at the Kakanj unit 5 – 110 MWe) – the preparation for this pilot project is ongoing at the moment and co-firing trial run is scheduled for the summer time of 2010.