| China is a large agricultural country,we can obtain fuel with high energy density,high calorific value,easy transportation,convenient application and 'zero emission' of CO2 during combustion process by the densification procedure.However,the fuel quality cannot be improved by merely increasing the energy density during densification.The large content of alkaline substances and low ash melting point limit the application of biomass resources,especially for straw biomass.Research at home and abroad is mainly directed to single biomass or single additive,but a variety of biomass will be used in actual biomass industrial boilers,and study in this area is not yet in-depth.In response to these problems,this paper uses a variety of biomass materials to develop composite biomass briquettes,upgrading fuel quality and combustion performance from the molding process.Moreover,briquettes are blended with composite additives to control pollutant emission and modify ash fusion synergistically.The following work was carried out:(1)The wheat straw and tree bark briquettes were formed by laboratory molding device to investigate its physical properties and combustion characteristics.SEM found that the micro-morphology of briquette was shored up by cylindrical cellulose structure,and the gap was covered with small particles.The morphology of two materials are obviously distinct due to the different lignin content.Wheat strow skeleton has smooth surface and ordered strip-like layers which shows that the bonding method is mainly by the bridging effect of solid particles.The surface of tree bark skeleton is rough and in a'melted' state,and the broken small particles are disorderly and irregularly connected by the bonding of lignin,indicating that the bonding method is mainly by the cohesive force of softening lignin.The curve of thermogravimetric analysis shows that biomass briquette combustion is divided into drying stage,volatile combustion stage,transition stage,char combustion stage,and burnout stage.The ignition temperature of these fuels is about 280 ?,while the burnout temperature of tree bark is significantly higher than others.Two briquettes collected from the heating boiler have similar parameters,indicating that the raw materials and blending ratios are not much different.A consistent conclusion can be drawn from the average weight loss rate,ignition index,and comprehensive combustion characteristic index.Fuel performance shows that wheat straw is the best,and tree bark is the worst.The activation energy E and the pre-exponential factor A were obtained by calculation of kinetic parameters.These parameters illustrate that the effective collision of wheat straw is much higher than other molding fuels,resulting in the highest reaction rate of wheat straw,therefore the fuel performance of wheat straw is better than others.(2)Sulfur and nitrogen emissions were studied on a fixed bed system to reduce pollutant emissions of briquettes.The prepared five kinds of prototypes have high sulfur fixation rate at 800 ?,and the desulfurization characteristic does not show a linear relationship with the composite ratio,indicating that the composite molding between tree bark and wheat straw is not a simple physical mixing.A discipline can be acquired after comprehensive analysis of sulfur and nitrogen emission:the SO2 release rate of five kinds of samples at 800 ? is less than 5%,and the NO release rate is less than 0.9%.Wheat straw is low in sulfur precipitation and has good sulfur self-retention characteristics,but the release rate of nitrogen is high;the sulfur precipitation of tree bark is high,while the release rate of nitrogen is low.When the mass ratio of wheat straw/tree bark is 25/75,the release rates of sulfur and nitrogen are at a low level,which represent the best blending ratio.The compound additive and the single additive have the same regularity:when the phosphorus-based additive content is constant,SO2 emission decreases with the increase of calcium-based additive dosage,and CaO desulfurization effect is better than that of CaCO3.The addition of phosphorus-based additive slightly increases the sulfur and nitrogen precipitation.The addition of calcium-based additive has little effect on the amount of nitrogen deposition.When the combustion temperature reaches 1000 ?,the amount of sulfur released from wheat straw and tree bark is obviously increased.The curve changes from a single peak to multiple peaks,and the discharge time is prolonged.The sulfur precipitation of the two raw materials is reduced after mixing at 75:25 with ratio of 9.01%,which indicates that the biomass has good sulfur self-retention characteristics at this blending ratio.The sulfur precipitation is further reduced to 7.21%after the addition of compound additive.The nitrogen release rates at different temperatures are always less than 0.9%,and the increasing temperature has tendency to decrease the precipitation of nitrogen.(3)In order to solve the problem of ash corrosion caused by excessive alkali metal content of biomass,the ash fusion performance of biomass was studied.Wheat straw is worse than bark.The characteristic temperature and curve trend of the composite fuels of three ratios are similar,and they are not much different from each other.Furthermore,they are all between the bark and the wheat straw,indicating that the composite ratio has a certain linear relationship with ash melting point.The ash melting point is increased after the addition of the additive,and the effect is more obvious as the amount of the additive raise.What's more,the composite additive is also better optimized for the ash melting point than the single additive,thereby improving the ash fusion property.The macro-morphology of ash sample at different temperatures shows that the forming fuel shrinks internally while maintaining the cylindrical shape during combustion,and is agglomerated into a rigid honeycomb at 1000 ?.After adding composite additive,the ash of forming briquettes at different temperatures is softer than the original.The SEM micro-morphology further indicates that the addition of composite additive can significantly reduce the particle size of the agglomerates by inhibiting the agglomeration of ash particles,and explain the causes of the macro-morphology of the original rigid honeycomb.XRD crystal phase analysis can find out whether blending additives have a great influence on the composition of ash components.The original ash is mainly silicate.Potassium is basically precipitated into the gas phase under high temperature.After adding compound additive,the ash is more complicated.The components are calcium silicate,calcium phosphate,magnesium phosphate and K-Ca-P compound,which have stabilized form at high temperature.Potassium element can be fixed in ash after the addition of additives,thereby improving the ash fusion property.The results of this paper provide a theroretical basis for efficient and clean combustion of biomass,and have important value and practical significance for improving the quality of biomass briquette and large-scale industrial applications. |
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