Simulation on operation efficiency and pollutant emissions of coal-fired boiler with bio-gas co-firing

Abstract: As a kind of rich renewable energy in China, biomass has become an important research direction for energy conservation and emission reduction and sustainable development. Most of biomass utilization can be used to generate power. In recent years, biomass co-firing with coal combustion technology has been widely developed. Because biomass gasification technology is more mature and has many applications, the biomass gas co-firing technology in a coal-fired boiler becomes the new research direction of biomass which can be used to generate electricity. Additionally, as one of the most abundant biomass energy in China, straw is the main object in this thesis. Meanwhile, the different combustion characteristics of straw gas and coal entail certain changes in combustion process and pollutant emissions. In order to solve the problem of boiler operation caused by biomass fuel performance during the direct co-firing of biomass and coal, the co-firing process of biomass gas and coal is introduced, and based on the design and operation data of a 330MW pulverized coal boiler, the models of straw gasification and straw gas co-firing with coal were established by Aspen Plus, and the models were reasonably verified. Due to that the fuel characteristics of biomass have changed after the biomass gasification, the influence of coal-fired boiler co-firing straw gas on boiler operation performance and the change of pollutants were studied, so as to provide valuable reference for research work of biomass used in boiler co-firing. The whole model consists of the following 3 processes: the gasification process of straw, in which the air is selected as gasification agent and straw gas and straw ash will be obtained from the gasifier and the energy conservation will be realized in this process; the co-firing process of straw gas and coal in coal-fired boiler; the heat exchange process of heat transfer in tail. In this whole process, the heat exchange between the medium in super-heater, re-heater, economizer and air pre-heater and the flue gases and the air leakage in the furnace exit and economizer tail were taken into account. Meanwhile, in order to guarantee the stable operation of the boiler, under the conditions of the constant total calorific value entering the boiler system, the combustion processes cases, such as straw gas (produced by the condition of the best air to biomass ratio which ensures the highest gasification efficiency) co-firing ratio changing from 10% to 30% and straw moisture content changing from 5% to 30%, excess air ratio changing from 1 to 1.25, and the pure coal combustion were simulated, and then the change rules of the performance and pollutant emissions at the air pre-heater outlet were studied. The results show that, compared with pure coal combustion case, when the straw gas co-firing ratio and straw moisture content increase from 10% to 30%, the furnace combustion temperature decreases gradually and drops by 89.3 ℃ to the greatest extent, the flue gases density increases gradually, and the flue gas mass flow rate decreases at 10% straw moisture content cases with the increase of co-firing ratio and the rise of straw moisture content. Meanwhile, the flue gas temperature decreases by 1.87 ℃, but the temperature increases with the rise of co-firing ratio and reaches 11.78 ℃ at the most extent. Meanwhile, the boiler efficiency change range is 92.72%-93.71% and the system efficiency (the whole process efficiency including biomass syngas generated from gasifier and biomass syngas co-firing in the boiler) change range is 88.75%-92.62% when straw gas co-firing ratio is changing from 10% to 30% and straw moisture content is changing from 5% to 30%. Therefore, the influence of the co-firing process on the boiler efficiency is slight when co-firing ratio and moisture content change within the range of 30%, which provides the possibility for the high efficient utilization of straw. Additionally, NO and SO2 emission concentration at outlet of the air pre-heater decreases gradually with the increase of co-firing ratio from 10% to 30%, and when the furnace excess air ratio changes from 1 to 1.25 under the condition of 10% co-firing ratio, NO emission concentration increases firstly and then decreases slightly, while SO2 emission concentration decreases gradually, and the drop reaches a maximum of 1 052.6 and 219.8 mg/Nm3 respectively for NO and SO2 at the 30% straw moisture content and 30% co-firing ratio.