Sub-bituminous coals fired in boiler designed for bituminous coals

In the last two decades there has been little capacity added to coal-based power plants. However, much of the existing plants had to comply with the Clean Air Act amendments. Using sub-bituminous coals has become an important solution for emissions compliance due to their unique constituents and combustion characteristics; these coals are often referred to as enviro coals. The considerable advantages of these coals, like Melawan, Adaro or PRB coals, is their low sulfur compared to typical bituminous coals, which makes its burning more economic as scrubbers or other SO₂ reduction technologies are not required. Low nitrogen and ash content as well as their high volatile matter are other advantages of these coals. Hence, firing sub-bituminous coals alone or as blends with bituminous coals is deemed economically attractive. Power generation plants were originally designed to operate on a particular bituminous coal. In order to fire subbituminous coals or their blends some modifications are required in the firing modes. These modifications may affect boiler reliability and as result to reduction of the power plant availability and hence increasing operation and maintenance cost. In order to prevent such undesirable effects we initiated a study to understand the influence of using sub-bituminous coals on the capacity, limitations of furnace size, heat transfer surfaces, firing systems, pulverizers, fans and airheaters. The present paper discusses issues connected with each of these issues on the combustion system. We also present recommendations for reliable burning of various subbituminous coals and their blends in a 575 MW tangentially-fired boiler. For example, we found that firing Indonesian subbituminous coals (Adaro and Melawan) considerably reduced NO_x (30% reduction) and SO_x (reduced to 200 mg/dNm³@6%O₂) emissions without post combustion measures. We also tested various blends of sub-bituminous coals with bituminous coals and found positive and negative synergism in these blends with regard to NO_x emissions. We used in the present study a series of experiments in a test facility and computational fluid dynamic codes.