Abstract
For coal semi-char as a by-produced of low-temperature pyrolysis, combustion for power generation is one of the effective utilization methods to realize its clean and efficient utilization. However, the coal semi-char combustion process has a difficult ignition, unstable combustion and low burnout rate. The co-firing of the semi-char with biomass under oxy-fuel conditions can improve the combustion behavior and reduce fossil CO2 emissions. In this paper, the combustion behavior of Shenhua coal semi-char (SHC) co-firing with straw (ST) in O2/CO2 mixture is investigated using thermogravimetric analysis. The results show that each curve lays between those of the individual fuels and presents three peaks (i.e., three stages). The thermogravimetric curves of SHC co-firing with ST can be divided into three stages: the volatile combustion of ST, the co-combustion of SHC and ST fixed-carbons and the SHC fixed-carbon combustion and the decomposition of the difficult pyrolytic material of ST. Blending ST into the SHC can significantly decrease the ignition temperature and improve the comprehensive combustion behavior of blended samples. In increasing the proportion of ST from 25 to 100%, the change of the blended ignition temperature is slight, but the burnout temperature decreases greatly. Kinetic parameters of combustion are calculated by using the Coats–Redfern integral method. Compared to that of stage I and stage III, the activation energy of stage II is significantly lower. As increasing blending ratio from 25% to 100%, the activation energy increases at stage I and decreases at stage III. Furthermore, the O2 concentration obviously affects stage III of 50% SHC + 50% ST, and the thermogravimetric curves at this stage are obviously shifted to the lower temperature zone as the O2 concentration increases. The activation energy of 50% SHC + 50% ST increases as the oxygen concentration increases. Besides, the activation energy shows that the combustion characteristics cannot be determined only by the activation energy obtained by the Coats–Redfern method. These findings can provide useful information for semi-char co-firing with biomass.
Highlights
In China, the low-rank coal accounts for above 50% of the total (141,595 million tons at the end of 2019 from BP Statistical Review of World Energy 2020) [1,2]
These findings can provide useful information for semi-char co-firing with biomass
Zheng et al [11] performed an experimental investigation on ignition and burnout characteristics of coal semi-char and bituminous coal blends, and the results show that the ignitability index and the comprehensive combustion characteristic index of the blends decrease as increasing the blending proportion of coal semi-char
Summary
In China, the low-rank coal (such as lignite and subbituminous coal) accounts for above 50% of the total (141,595 million tons at the end of 2019 from BP Statistical Review of World Energy 2020) [1,2]. Pyrolysis is one of the most effective methods to realize the high and clean, efficient utilization of low-rank coal [3,4,5]. Co-combustion technology of coal semi-char with high-volatile fuels is one of the effective methods. Co-firing with coal semi-char under oxy-fuel conditions may be one of the effective ways to improve the combustion characteristics and reduce fossil CO2 emissions. There are few studies on the coal semi-char co-firing with biomass under oxy-fuel conditions. It is necessary to investigate the co-firing behavior of coal semi-char and biomass in the oxy-fuel atmosphere. The combustion behavior of coal semi-char co-firing with straw in O2 /CO2 mixture is investigated using thermogravimetric analysis, which can provide useful information for coal semi-char co-firing with biomass in oxy-fuel conditions
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