Research On The Mechanism Of Exhaust Pollutants Reduction And Energy Saving For Residential Biomass/Coal Combustion

China has been suffered serious air pollution.Residential combustion of solid fuel is widely regarded as one of the major sources of atmospheric pollutant emissions,as well as a great potential for emission reduction.Clean fuel and improvement of combustion technology have been identified as two important emission reduction pathways.This study investigates these two methods.Firstly,aiming to improve the burnout ratio of anthracite coal in residential stoves,this study investigated the effect of addition biomass in anthracite briquettes on combustion efficiency and pollutant emissions.Furthermore,effect of the processing condition for manufacturing bio-coal briquettes on thermal efficiency and pollutant emission was investigated,while the quantitative relationships between biomass content/molding pressure/clay content and PM2.5 emission factor / thermal efficiency were also analyzed.Given that volatile matter is the precursor of PM,which has a direct effect on the PM2.5 emission,this study quantitatively analyzed the relationships between volatile content and PM2.5 emission factor.The experiment results showed that the thermal efficiency and the energy-based PM2.5 EFs were improved 88%-179% and reduced 98.8%-99.5% than those of biomass briquettes,respectively,while these values were 81%-127%,and 61%-67% compared with coal briquettes,respectively.The possible influence of popularization of bio-coal briquette was analyzed quantitatively based on the experimental data.The major synergistic reaction mechanisms on energy saving and pollutant emission reduction between biomass and coal for bio-coal briquettes include:(1)porous structure of bio-coal briquette can provide sufficient oxygen via pore penetration,leading to complete combustion of the anthracite component;(2)Flammable gases produced by the pyrolysis of biomass under low temperature and hypoxic condition can support coal combustion;(3)rich alkali(soil)metals and high H/C of biomass benefit the generation of flammable gases from coal;(4)The burnout and ignition temperature of biomass is lower than coal.And the biomass volume shrinks after burnout,which will further enrich the gas passage to provide coal combustion with more sufficient oxygen;(5)It is pointed out that the volatile matter generated in low temperature pyrolysis of biomass at 200-350 oC is attributed to the effect of volatile matter on PM2.5 emission factor.Secondly,semi-coke and biochar briquettes,made from raw coal and biomass after carbonization treatment to distill out volatile matter,are clean residential fuels.Experimental result in residential cooking stoves showed that the mean PM2.5 EFs of the carbonized briquettes were reduced by 89% and 80% in comparison to raw coal and biomass briquettes,respectively.In addition,the average thermal efficiency were improved from 9.5±0.6% for raw coal and 3.6±0.7% for biomass briquettes to 10.3±0.4% and 4.6±0.5% for carbonized coal and biomass briquettes,respectively.Therefore,residential carbonized solid fuel benefits the reduction of pollutant emissions.Semi-coke,the residue after the recovery of tar and gas in the process of low temperature coal/biomass pyrolysis,can achieve pollutants emission reduction and fully utilization when it is used as the household solid fuel.Thirdly,the under-fire heating technology with changing combustion sequence and gasified combustion cooking technologies with adding secondary air can both significant reduce primary pollutants emissions.Compared with the over-fire heating mode,the under-fire heating technology performed mean reduction efficiencies were ~50% for CO2,79% for PM2.5,97% for EC,85% for benzo[a]pyrene equivalent carcinogenic potency,and 66% for 8 selected toxic elements extracted from PM2.5 samples for unit energy delivered to radiator;The gasified combustion cooking mode presented higher hth and comprehensively lower pollutant emissions than those of the over-fire mode.The hth for crop straw and biomass briquette and pellet were developed from 1.97%-2.29% and 3.83%-4.60% to 4.55%-6.53% and 10.14%-13.66%,respectively.~90% of PM2.5 EFs were eliminated.In summary,energy saving and pollutant emission reduction for residential solid fuel combustion could be achieved by the following two pathways:(1)control fuel quality,i.e.,the bio-coal and carbonized briquette,and(2)advanced combustion technology,i.e.,the development of combustion mode by controlling the burning sequence of the combustible components of solid fuel.