Investigation and Control Technology on Excessive Ammonia-Slipping in Coal-Fired Plants

Xuan Yao,Hao Kong,Man Zhang,Junfu Lyu,Hairui Yang

doi:10.3390/en13164249

Abstract

After the implementation of the ultra-low emissions regulation on the coal-fired power plants in China, the problem of the excessive ammonia-slipping from selective catalytic reduction (SCR) seems to be more severe. This paper analyzes the operating statistics of the coal-fired plants including 300 MW/600 MW/1000-MW units. Statistics data show that the phenomenon of the excessive ammonia-slipping is widespread. The average excessive rate is over 110%, while in the small units the value is even higher. A field test data of nine power plants showed that excessive ammonia-slipping at the outlet of SCR decreased following the flue-gas process. After most ammonia reduced by the dust collector and the wet flue-gas desulfurization (FGD), the ammonia emission at the stack was extremely low. At same time, a method based on probability distribution is proposed in this paper to describe the relationship between the NH3/NOX distribution deviation and the De–NOX efficiency/ammonia-slipping. This paper also did some original work to solve the ammonia-slipping problem. A real-time self-feedback ammonia injection technology using neural network algorithm to predict and moderate the ammonia distribution is proposed to decrease the NH3/NOX deviation and excessive ammonia-slipping. The technology is demonstrated in a 600-MW unit and works successfully. The excessive ammonia-slipping problem is well controlled after the implementation of the technology.

Highlights

NOX is an atmospheric trace gas with a short lifetime
The ammonia slip in the flue gas will cause the plugging problem of the air preheater (AH) and electrostatic precipitator (ESP)
In a given selective catalytic reduction (SCR) device neglected catalyst activity condition, if the target De–NOX efficiency is required to reach 85%, the recommended deviation coefficient of NH3 /NOX should be less than 5% to maintain the ammonia-slipping less than 3 ppm

Summary

Introduction

NOX is an atmospheric trace gas with a short lifetime. It can participate the formation of tropospheric ozone and secondary aerosols in the air, which may harm human health and affect the climate [1,2]. Since 2015, the government published a new regulation called “ultra-low emission regulation for coal-fired powerplants”, which requires all coal-fired boiler units to meet the new standards (particulates < 10 mg/NM3 , SO2 < 35 mg/Nm3 , NOX < 50 mg/Nm3 ) by 2020. The ammonia slip in the flue gas will cause the plugging problem of the air preheater (AH) and electrostatic precipitator (ESP). We investigated the operating data of more than 200 domestic coal-fired plants to verify the excessive ammonia in the SCR process. The entire process of ammonia concentration in the flue gas was measured in nine plants All these works can reveal the actual situation of ammonia-slipping problem in the coal-fired plants. The real-time technology used neural network algorithm model to reduce the NH3 /NOX deviation instantly and realized a good demonstration in a 600-MW unit

Statistics Data of Ammonia Slip in Coal-Fired Plants

Field Test of Ammonia Variation in the Flue Gas

Findings

Solutions to Alleviate the Excess Ammonia-Slipping

Conclusions