Study on the effect of limestone on NOx emission in circulating fluidized bed combustion and particle size optimization
SHANG Manxia1, LI Yiran1, JIANG Ling1, LI Dongfang2, HUANG Zhong1, ZHNAG Man1, LÜ Junfu1, KE Xiwei1
1. Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China; 2. School of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
Abstract：[Objective] In the context of increasingly stringent pollutant emission standards, more and more circulating fluidized bed (CFB) boilers are pursuing low-NOx combustion and high-efficiency desulfurization in the furnace. How to decouple the contradiction between the two is one of the research hotspots. Previous studies found that when the separator efficiency is suitable, a reasonable adjustment of the limestone particle size can significantly improve the desulfurization efficiency in the furnace while effectively inhibiting the negative effect of limestone on NOx emission. However, this phenomenon is only at the experimental stage and lacks theoretical analysis.[Methods] In this paper, a commercial 550 MWe ultra-supercritical CFB boiler was calculated with the help of the established comprehensive one-dimensional two-phase hybrid mathematical model for CFB combustion to obtain the rules and the effect of limestone particle size on 烧NOx emission desulfurization in a furnace. The comprehensive mathematical model for CFB combustion consists of three parts, namely, material balance (including ash, limestone, raw coal, and coke), gas balance, and energy balance. Moreover, this model specifically considers the local gas/solid fluidization state and gas/heat transfer conditions in different regions of the CFB combustor. Some two- or three-dimensional problems, such as bubble breakage over dense bed surface, secondary air injection, core annular flow structure, and particle clusters in freeboard, are also considered in the modeling. For nitrogen-containing catalytic reactions on the limestone surface, CaO has significant catalytic activity for many nitrogen-containing reactions, which is one of the important reasons for the increase of NOx emission caused by limestone desulfurization in the CFB furnace. This paper considers four types of nitrogen-containing catalytic reactions on the surface of CaO particles, including oxidation of NH3, hydrolysis of HCN, oxidation of CO, and catalytic reduction of NO by CO. The integral CFB model is validated against the field test data obtained from the commercial CFB boiler. Favorable comparisons were conducted between the calculated and measured results, involving the median diameter of each ash sample and the concentration of the main flue gas components.[Results] The calculation results showed the following:(1) Due to the significant catalytic effect of CaO on each nitrogen-containing reaction, increasing the calcium-sulfur mole ratio could improve the desulfurization efficiency in the furnace but could also lead to an increase in NOx emission. (2) In addition to the negative effects on NOx emission, increasing the calcium-sulfur mole ratio also had different degrees of negative effects on boiler efficiency and flue gas dedusting, among others. (3) With the reduction of limestone particle size, the total CaO effective reaction area in the furnace and near the coal feeder was reduced, which inhibited the overall rate of relevant catalytic reactions and was conducive to alleviating the negative effect of limestone on the reduction of NOx emission.[Conclusions] By calculating and analyzing the relationship between the limestone desulfurization in the furnace and NOx emission, a new idea is provided to decouple the contradiction between desulfurization and low-NOx combustion. That is, under the same calcium-sulfur mole ratio, appropriately reducing the limestone particle size or even using ultrafine limestone particles matched with the high-efficiency separator can effectively improve the in-furnace desulfurization efficiency and alleviate the negative effects of limestone on the reduction of NOx emission.
尚曼霞, 李怡然, 蒋苓, 李东方, 黄中, 张缦, 吕俊复, 柯希玮. 石灰石对循环流化床燃烧NOx排放影响及粒度优化研究[J]. 清华大学学报（自然科学版）, 2023, 63(12): 2033-2041.
SHANG Manxia, LI Yiran, JIANG Ling, LI Dongfang, HUANG Zhong, ZHNAG Man, LÜ Junfu, KE Xiwei. Study on the effect of limestone on NOx emission in circulating fluidized bed combustion and particle size optimization. Journal of Tsinghua University(Science and Technology), 2023, 63(12): 2033-2041.
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2023, Vol. 63 
