| As a kind of renewable energy,biomass combustion is considered to have zero carbon emissions and it is highly valued by society.Large-scale use of coal will inevitably cause a large amount of nitrogen oxides,sulfur oxides and carbon dioxide emissions.The development of clean energy such as biomass energy has become the only way for our country to save energy and reduce carbon.With the proposal of carbon peaking and carbon neutrality,our country’s current coal-fired way is actively looking for an effective reduction path with carbon emission.Coal-biomass mixed briquette technology is an important clean coal technology that can bring huge environmental and economic benefits.The development of new solid fuels is the main way to save energy and reduce emissions.The combustion characteristics and pollutant release of solid fuels are analyzed by the combination of experimental research and numerical simulation.In this paper,based on low-rank pulverized coal and waste biomass as solid fuel,the co-combustion process of low-rank coal and biomass was studied in detail from multiple angles.Firstly,the combustion characteristics and combustion mechanism of coal and biomass were studied by various methods such as TG-FTIR-GC-MS,which provided the basis for further exploration of the raw material ratio of briquette fuel.Secondly,the combustion pollutant emission test and analysis of biomass briquette fuel was carried out in a civil furnace,and the carbonization fuel briquetting method was used to achieve the purpose of emission reduction.Finally,using the research method combining experimental research and numerical simulation,the combustion performance and pollutant release of biomass briquette were analyzed from the perspective of heat and mass transfer.The effect of furnace structure on combustion emission reduction was also studied.The research contents were as follows:1、The combustion performance and kinetic analysis during the co-pyrolysis,co-combustion for raw materials and co-combustion of carbonized materials were studied.In the process of co-combustion of low-rank coal and biomass,the pyrolysis temperature of biomass was relatively low and more small-molecule low-flame-point gases are generated in the low temperature section,such as CH4,CO and C2H4,which promoted the combustion of low-rank coal.It was conducive to the co-combustion of the mixture.The more biomass added,the larger the comprehensive combustion characteristic index Sn and the better the combustion performance.In the co-combustion process of low-rank coal chemical material and straw carbonized material,the addition of straw carbonized material could significantly improve the ignitability and combustibility of low-rank coal chemical material,and then provide a certain ratio for the best choice of carbonized material in civil furnaces.With the increasing rice straw carbonized material,the synergistic effect was first produced,and then the antagonistic effect was enhanced,so the mixing ratio should not be too much.For the combustion of mixed carbonized materials,when the addition ratio of straw carbonized materials was 20%,the total activation energy of combustion is low and the synergistic effect was strong.The most effective model for the combustion of straw carbonized material was the first-order chemical reaction model O1,and the most effective model for the combustion of carbonized coal was the three-dimensional diffusion D4.The most efficient mechanisms in the combustion of mixtures were diffusion-controlled reactions（D3and D4）.2、Using low-rank raw coal and straw as raw materials,clean biomass briquette was prepared.The emission factor,morphology and composition of the collected particulate matter PM2.5were studied in detail.The effects of different furnace types（normal firing furnace and semi-gasifier）,different ignition methods（upper ignition and lower ignition）and biomass blending on the PM2.5emission pollutants from biomass briquette combustion were analyzed quantitatively.A quadratic function relationship was established for the correlation of volatile matter,ash content and particulate matter PM2.5emission factors.It was also found that ash in the samples promotes the formation of polycyclic aromatic hydrocarbons in PM2.5particles during combustion.The best conditions for the combustion of biomass briquette to emit the least particulate matter were semi-gasifier,upper ignition mode combustion,and 20%of straw.Under these conditions,the emission factors of 16 PAHs,Ba P and water-soluble anions and cations were the lowest and the effect of emission reduction was the best.It was found that the collected PM2.5particles were diverse in morphology and structure by scanning electron microscopy.The energy spectrum analysis showed that the main elements contained in PM2.5particles were C,O,Na,Zn,K,Al,Ti,Ca,Cl,Te,Hg.3、Methods of reducing pollutants in the combustion process of civil fuel were discussed.On the basis of the research on biomass briquette,the low-rank coal chemical material and straw carbon material were obtained by carbonization to remove volatile matter.Then a cleaner solid fuel was prepared.At the same time,the carbonized material with low volatile content could be burnt out fully in the stable combustion of the briquette,so that the PM2.5emission value of the generated particulate matter was much lower than that of original biomass briquette.When the blending ratio of straw carbonized material was 20%,the particle reduction effect of block fuel was the best,and the emission factor of PM2.5was 3.7 mg/g for the ignition method under the semi-gasifier.The emission factors of other particulate matter TSP and PM1were also the lowest,and the higher combustion thermal efficiency could be achieved at 9.6%.In addition,the mass-based emission factors of carbonaceous components organic carbon and elemental carbon,16 kinds of polycyclic aromatic hydrocarbons and benzopyrene emitted in the flue gas were also reduced accordingly.The emission factors of heavy metals（Pb,Mn,Cr,Ni,As,Se,Cd,Ba,Co,Ag,Hg）,SO2,and NOXin the energy base of PM2.5particles were also be reduced.From the results of scanning electron microscopy,the number of particles collected by the combustion of carbonized briquette in the unit area of the quartz film was significantly less than that formed by the combustion of primary briquette.Through the tube furnace experiment,it was found that the original material and the original low-rank coal would indeed produce some particulate matter in the flue gas of the carbonization process.The total emission factor of particulate pollutants from straw was larger than that of low-rank coal.Some polycyclic aromatic hydrocarbons were produced,which was harmful to the environment.Therefore,it was necessary to strengthen the treatment of flue gas in the carbonization process.4、The relationship between single-block briquette and multi-block briquette combustion in civil furnaces was explored.A heat and mass transfer model for multi-block briquette combustion was established.Firstly,taking the single block briquette as the research object,using commercial CFD simulation software,the heat transfer phenomenon inside the block was simulated.The temperature distribution in different time periods inside the block was obtained through numerical simulation calculation and a two-dimensional and steady-state heat transfer model was established.Its internal heat transfer characteristics were compared under different conditions.In the heat transfer model of the block,based on the temperature gradient of the particle layer,the interior of the particle was divided into a heating zone and an unheated zone from the outside to the inside.The heating zone was further divided into a combustion zone,a pyrolysis zone,and a non-pyrolysis zone.If the air velocity outside the biomass briquette was too large,the oxygen supply would increase.The part of the fuel would be more fully burned,but the cold air would reduce the temperature of the part of the briquette.The heat transfer effect would be worse.The change of the external temperature of the block had a great influence on its internal heat transfer,which was mainly affected by the external convection and heat radiation of the particles.When the external temperature was high,the internal heat transfer rate of the molding block increased.The internal carbon and volatile matter could be fully burned to achieve the purpose of emission reduction.According to the heat transfer model of three different materials of coal,biomass,coal and biomass,the heat transfer rate inside the biomass block was the fastest,which would easily cause the pyrolysis rate in the pyrolysis zone to be faster.It led to an accelerated rate of volatile matter escape,which may cause some combustible substances to be discharged out of the combustion chamber without burning in time,causing air pollution.Secondly,based on the simulation of a single biomass pellet in a civil furnace,a numerical model of the combustion of multiple biomass pellets was studied in depth.Furthermore,the distribution of the flow field,temperature field and gas concentration field inside the furnace of biomass briquette under different conditions（different ventilation rates,with or without exhaust air and optimized combustion chamber structure）were compared.Studies showed that if CO was used as the pollutant emission index,the emission was less at the the transition flow for Reynolds number of the outside air,the exhaust air for upper part of the furnace and the trumpet-shaped structure of furnace.When the NOXcalculated by the post-processing was the pollutant emission index,the emission was less at the waist drum-shaped structure of furnace. |
