The Safety And Economic Study Of 600MW Lean Coal Supercritical Bolier On Co-firing Bituminous Coal

The designed coal of Wangqu power plant is lean coal.But subject to the coal market and to improve the economic performance,the power station fires mixed coal blended of lean coal and bituminous coal.The previous combustion mode was grinding different kind of coal in different pulverizer and co-firing them in the furnace in the way that different coal firing in different burners(named in-furnace co-firing).But in this way,some problems like coking in the furnace and blocking of the slag discharge opening arised.The analysis of the fired coal showed that the ash fusion point of the bituminous coal is very low,which may cause the coking around the burners which burned the bituminous coal only.And to mitigate the coking of the furnace,a new combustion mode,which mixed the lean coal and bituminous coal.in the pulverizers and fired them in the same burners(named out-furnace co-firing),was tested.And first the theory of co-firing was studied.The coal quality parameters and coking degree of the coal mixed by Lu-an lean coal and Hong-yan bituminous coal with different mix ratio were calcluted,and the results showed the the mixed ash softening temperature of the mixed coal is higher than that of the bituminous coal,which may mitigate the coking of the furnace.The boiler thermodynamic calculation showed that when fired mixed coal with a high bituminous coal ratio,the heat absorption characteristics of the heating surface were similar to the designed situation,and the exhaust gas temperature increased a little with the increase of bituminous coal ratio in the mixed coal.The metal temperature calculation of the outer tube of the platen superheater showed that the metal temperature in this point will not exceed the allowable value when firing mixed coal with a high bituminous coal ratio.The acid dew point calculation of flue gas showed that when the bituminous coal ratio in the mixed coal increases,the acid dew point increased too,and the temperature difference between the acid dew point and the lowest air preheater metal temperature decreases,which means a higher possibility of low-temperature corrosion in this position.And then the out-furnace co-firing test were performed at the power station,and the varying characteristics of the furnace coking degree,the NOx concentration of the boiler and the carbon in fly ash were measured.The test showed that the out-furnace co-firing mode can mitigate the coking of the furnace and the blocking of the blocking of the slag discharge opening.The coking around the burners will be more serious with a higher bituminous coal ratio in the mixed coal,and the bituminous coal ratio should be limited to 60%.The NOx concentration of the boiler will be lower with the increase of the bituminous coal ratio in the mixed coal,and at high boiler load,the drop of NOx of the boiler will be more.The economic statistics of this test include three parts,the cost of removing coke in the furnace,the saving NH3 for denitration,and the money saved by firing more low-price bituminous coal.The calculation showed that if the combustion mode was changed to outfurnace co-firing,it will be saved more than 20 million yuan a year for the unit.