Research Information System
29th Annual International Pittsburgh Coal Conference, Pittsburgh, United States Of America, 15 - 18 October 2012, pp.1213-1228, (Full Text)
In this study, combustion of coal and biomass was carried out in a circulating fluidized bed combustor. A Bursa/Orhaneli lignite was used as coal and woodchips was used as biomass in the tests. Woodchips refers to the Rhododendron genus of woody plants in Ericaceae family which is mainly cultivated in the Black Sea region of Turkey. In this study, co-combustion of lignite and woodchips mixtures containing 10%, 30% and 50% woodchips by weight was studied. The experimental setup consists of a circulating fluidized bed combustor column, a fuel feeding system, electrical heaters, two cyclones and a bag filter. The combustor column has an inside diameter of 108 mm and a height of 6 m. The temperatures along the column are observed with thermocouples located at specific heights. The temperature of the column is kept at 850 oC during the combustion tests. The pressure drops along the combustor column, cyclone, and downcomer are continuously measured and observed in order to determine the solid mass flux within the combustor. A series of co-combustion tests was performed in order to investigate the effect of excess air ratio on the flue gas emissions. During the combustion tests, CO2, CO, O2, NO, and SO2, emissions in the flue gas was continuously measured and recorded by ABB-AO 2000 flue gas analyzer.
The results of the tests showed that as the woodchips ratio in the fuel mixture increases for co-combustion, the combustion takes place more in the freeboard of the main column. Therefore, the maximum temperatures are seen in the freeboard rather than in the bed. It is possible to combust Orhaneli lignite and woodchips mixtures in a circulating fluidized bed combustor without any operational problems for a long time. In order to get minimum flue gas emissions, the best excess air ratios for the co-combustion tests of fuel mixture including 10%, 30% and 50% woodchips by weight were determined to be 1.37, 1.54 and 1.59, respectively. As the percentage of woodchips in the fuel mixture was increased, it was required to feed more air into the combustor to get minimum emissions, especially CO emission.
Woodchips addition to Orhaneli lignite made the CO emission worse and caused CO emission to exceed the Turkish emission limit. NO emissions were under the limits for all cases except for the case of co-combustion of Orhaneli lignite with 50% woodchips. SO2 emissions were above the emission limits when the percentage of woodchips in the Orhaneli+woodchips fuel mixture was 10% by weight. For the other mixtures, SO2 emissions were measured under the limit. Woodchips addition decreased the SO2 emissions as the percentage of biomass increased in the fuel mixture since the biomass contained almost no sulfur. When the system was operated under the condition where NO emission was minimum, it was observed that more CO emission was produced. Therefore, it is necessary to make an optimization on excess air ratio according to CO and NO emissions. In this set of experiments, CO emission has the priority in order to determine the best excess air ratio because it is possible to reduce NO emission by giving secondary air to the combustor.