EFFECT OF EXCESS AIR RATIO ON FLUE GAS EMISSIONS FOR CO-COMBUSTION OF A LIGNITE COAL AND WOODCHIPS IN A CIRCULATING FLUIDIZED BED COMBUSTOR

摘要

In this study, combustion of coal and biomass was carried out in a circulating fluidized bed combustor. A Bursa-Orhaneli lignite wasused as coal and woodchips was used as biomass in the tests. Woodchips refers to the Rhododendron genus of woody plants inEricaceae family which is mainly cultivated in the Black Sea region of Turkey. In this study, co-combustion of lignite and woodchipsmixtures containing 10%, 30% and 50% woodchips by weight was studied. The experimental setup consists of a circulating fluidizedbed combustor column, a fuel feeding system, electrical heaters, two cyclones and a bag filter. The combustor column has an insidediameter of 108 mm and a height of 6 m. The temperatures along the column are observed with thermocouples located at specificheights. The temperature of the column is kept at 850 °C 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. Aseries of co-combustion tests was performed in order to investigate the effect of excess air ratio on the flue gas emissions. During thecombustion tests, CO_2, CO, O_2, NO, and SO_2, emissions in the flue gas was continuously measured and recorded by ABB-AO 2000flue gas analyzer.The results of the tests showed that as the woodchips ratio in the fuel mixture increases for co-combustion, the combustion takes placemore in the freeboard of the main column. Therefore, the maximum temperatures are seen in the freeboard rather than in the bed. It ispossible to combust Orhaneli lignite and woodchips mixtures in a circulating fluidized bed combustor without any operationalproblems for a long time. In order to get minimum flue gas emissions, the best excess air ratios for the co-combustion tests of fuelmixture including 10%, 30% and 50% woodchips by weight were determined to be 1.37, 1.54 and 1.59, respectively. As thepercentage of woodchips in the fuel mixture was increased, it was required to feed more air into the combustor to get minimumemissions, 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. SO2emissions were above the emission limits when the percentage of woodchips in the Orhaneli+woodchips fuel mixture was 10% byweight. For the other mixtures, SO2 emissions were measured under the limit. Woodchips addition decreased the SO2 emissions as thepercentage of biomass increased in the fuel mixture since the biomass contained almost no sulfur. When the system was operatedunder the condition where NO emission was minimum, it was observed that more CO emission was produced. Therefore, it isnecessary to make an optimization on excess air ratio according to CO and NO emissions. In this set of experiments, CO emission hasthe priority in order to determine the best excess air ratio because it is possible to reduce NO emission by giving secondary air to thecombustor.