Co-firing high ratio of woody biomass with coal in a 150-MW class pulverized coal boiler: Properties of the initial deposits and their effect on tube corrosion

Abstract

A demonstration test on the co-firing of a high-ratio of woody biomass (25% energy basis) with coal in a 150-MW class pulverized-coal boiler was conducted in Japan for the first time. To investigate the effect of the co-firing on the corrosion of the boiler heat-transfer tubes inside furnace wall and superheater, we collected the initial layer of the ash deposits (initial deposits) by inserting water-air-cooled probe simulating heat-transfer tubes during the test. Based on the results of the analysis of the collected initial deposits, we then prepared synthetic deposits and used them to conduct high-temperature laboratory-scale corrosion test for 500h. Carbon steel (STB410) and low-alloy steel (STBA22) were used to represent the furnace wall tubes, while two austenitic stainless steels (Ka-SUS304J1HTB and Ka-SUS310J1TB) were used to represent the superheater tubes. The co-firing was found to produce deposits in the furnace wall containing twice as much sulfates (mainly Na2SO4) compared to coal firing. A relatively high ratio of unburned carbon was also observed in the initial deposits of co-firing, indicating a strongly reducing atmosphere in the furnace wall. We found that the co-firing produced a higher corrosion rate in the furnace wall tubes at 500°C compared to coal firing. The corrosion risk, however, can be mitigated by decreasing the reduction capacity of the atmosphere. Further, the co-firing was found to produce two to three times as much alkali sulfate, but nearly the same low level of unburned carbon in the initial deposits obtained in superheater compared to coal firing. However, the increased sulfates concentration in the deposits had negligible effect on the corrosion of austenitic stainless steel tubes in the oxidizing atmosphere of the superheater at 650°C.