Experimental Study And Numerical Simulation Of Zero Emission Of Desulfurization Wastewater Coupled With Mercury Removal From Coal-fired Flue Gas

In the process of coal combustion,chlorine can promote the oxidation of elemental mercury.There are a lot of chloride ions in desulfurization wastewater of coal-fired power plants.It is of great significance to combine zero emission of desulfurization wastewater with reduction of mercury emission in flue gas to reduce mercury emission in coal-fired power plants.In this paper,the mercury emission characteristics of raw coal and raw coal after chlorination combustion were studied by tube furnace combustion experiment and sedimentation furnace combustion experiment;Aiming at a 630 MW supercritical coal-fired unit in a power plant,a zero emission desulfurization wastewater coupled with flue gas mercury removal test device was built to increase the concentration of chlorine in coal to reduce mercury emission;At the same time,numerical simulation software is used to simulate and analyze the combustion and mercury speciation distribution in the boiler.The experimental results of tubular furnace show that with the increase of combustion temperature,the mercury precipitation rates of three kinds of coal also increase.When the temperature reaches 1100℃,the mercury precipitation rates of three kinds of coal are 89.8%,82.1%and 91.0%respectively.The proportion of zero valent mercury Hg⁰in total gaseous mercury in coal-fired flue gas is relatively high.After adding different proportion of chlorine solution,the mercury precipitation rate of coal is further improved.After chlorination,the proportion of Hg⁰in coal-fired flue gas decreased and the proportion of Hg²⁺increased to a certain extent.The proportion of Hg⁰in the flue gas of three kinds of coal in the settling furnace is less than that in the tubular furnace.The proportion of Hg⁰in the flue gas of coal A is 59.3%in the settling furnace experiment and 72.1%in the tubular furnace experiment;The proportion of Hg⁰in the flue gas of coal B is 65.5%and 76.5%respectively;The proportion of Hg⁰in the flue gas of coal C is 54.9%and 69.3%respectively.After adding chlorine to three kinds of pulverized coal,the proportion of Hg⁰in flue gas decreased.The composition of CO₂,NO_xand SO₂in coal-fired flue gas has a certain influence on the speciation distribution of mercury in flue gas.A demonstration test of zero emission of desulfurization wastewater coupled with mercury removal from flue gas was carried out for a 630MW coal-fired power plant.The results show that the total gaseous mercury concentration is not affected by the spraying amount of desulfurization wastewater;With the increase of wastewater spraying amount,the oxidation efficiency of SCR system to Hg⁰in flue gas is also increasing;The removal rate of particulate mercury and Hg^pin the flue is basically unchanged;The removal efficiency of Hg²⁺in flue gas by WFGD system was improved to a certain extent after spraying wastewater;Different spraying amount of desulfurization wastewater has little effect on mercury removal efficiency of wet ESP system;When the spraying amount of desulfurization wastewater is 0t/h,3t/h,6t/h,9t/h and 12t/h,the total mercury removal efficiency of the pollutant removal system is 77.5%,79.1%,80.5%,83.6%and85.0%respectively.The combustion numerical simulation of a 630MW tangentially fired boiler unit in a power plant was carried out.The gas flow spirals up in the furnace and forms a swirl low speed zone in the center of the furnace and a tangential circle with good shape;The main combustion zone has high velocity and strong airflow disturbance,which is more conducive to the combustion of pulverized coal;The overall temperature distribution of the boiler is uniform,the temperature in the central area of the furnace is low,and the temperature near the tangential circle is high;In the region of burnout wind,the temperature distribution presents a“Hump”distribution with high temperature in the middle and low temperature around.In the main combustion zone,the concentration of O₂is very low and the concentration of CO is high;In the region of burnout wind,CO concentration decreases continuously;The distribution of CO₂is opposite to that of CO along the height of furnace;NO_xis mainly generated in the main combustion zone.Based on the numerical simulation of combustion,the distribution of mercury and its chloride in the furnace was simulated.Temperature is an important factor affecting the distribution of mercury species in flue gas.In high temperature environment,mercury mainly exists in the form of Hg⁰;Hg⁰in the flue gas is mainly distributed in the high temperature areas such as the area near the burner and the center of the furnace.With the decrease of flue gas temperature,Hg⁰gradually decreases.The most Hg⁰is distributed in the area around the burner,and its mole fraction is about1.2×10^-6,there is also a certain content of Hg⁰in the lower part of the main combustion area and the cold ash hopper area,which may be because mercury is a heavy metal and is caused by gravity;The mole fraction of Hg Cl₂is lower near the burner and in the center of the furnace,which is opposite to that of Hg⁰,the mole fraction increased gradually;The oxidation atmosphere is conducive to the conversion of Hg⁰to Hg²⁺in flue gas;H₂O in flue gas can promote the oxidation of Hg⁰and the removal of mercury;For the flue gas containing certain concentration of chlorine components,the presence of NO and SO₂will have a certain impact on the proportion of mercury species in the flue gas.