Experimental And Numerical Investigations On Co-firing Coal Slime With Anthracite In A Down-fired Utility Boiler

With the rapid development of the coal processing industry,the quantity of coal slime has increased significantly.Co-combustion of coal slime in the coal-fired utility boilers is an environmentally friendly approach to coal slime disposal.In this paper,the combustion characteristics,interactions,ash fusion characteristic,NO_x emissions and economic impact of co-firing anthracite with coal slime in a 600 MW down-fired utility boiler were studied by laboratory-scale experiments,numerical simulation and full-scale industrial tests.The combustion characteristic and interaction of anthracite blended with coal slime were analyzed by thermogravimetry.Co-firing coal slime improves the ignition performance but makes the comprehensive combustion worse.There exists a combustion interaction:the more-volatile coal slime combustion firstly can release heat,which promotes the ignition of anthracite,but consume some oxygen,which inhibits the burnout of anthracite.The promotion effect is better than inhibition effect when the blending ratio is below 5%.The ash fusion properties of anthracite blended with coal slime were studied by X-ray diffraction,phase diagram and ash melting point analysis.Blending coal slime has little influence on the slagging potential of blended coal but improves the ash fusion temperature slightly.The combustion characteristics and NO_x emissions of anthracite blended with coal slime in a down-fired boiler at the load of 370MW and 600MW were investigated respectively by numerical simulation.Whether at low load or full load,with the increase of blending ratio,the temperature lowers,the burnout rate reduces,and the carbon content of fly ash increases,but the NO_x emissions decreases.The impact of blending coal slime on combustion characteristics at full load is less than that of the low load.When the blending ratio is more than 10%,the carbon content of fly ash can be decreased by increasing the ratio of air to pulverized coal particle and reducing the particle size,but the NO_x emissions increase.The full-scale industrial tests of co-firing coal slime in a down-fired boiler of the power plant under the load of 370MW was carried out.With the increase of blending ratio of coal slime,the carbon content of fly ash increases from 6.43%to 7.22%and the NO_x emissions at the entrance of SCR device decrease from 893.92 mg/Nm~3 to 809.00 mg/Nm~3,but the boiler efficiency of the power plant decreases from 90.88%to 89.77%.When the price difference between coal slime and anthracite is more than the 16%of the anthracite price,co-firing coal slime is profitable for power plants.