Combustion Characteristics And Numerical Simulation Of Biomass Briquettes

With the continuous demand and growth of fossil fuels,excessive emissions of greenhouse gases have seriously affected climate change and public health,while also increasing the national economy.China is currently in the stage of energy transformation to get rid of its dependence on fossil fuels.Compared with other renewable energy sources,biomass energy can obtain abundant supply from various sources and has the key advantage of being close to carbon neutral.Using biomass formed fuel in rural areas where the transition to electricity or natural gas is not yet popular can alleviate resource pressure and promote carbon neutrality.However,there are design and structural defects in the current pellet stoves on the market,which do not adequately consider the air volume and power ratio,resulting in incomplete combustion and reduced thermal efficiency.This leads to environmental pollution and health hazards,hindering the promotion of biomass formed fuel.Therefore,this study examined the combustion characteristics of two types of wood biomass pellets and automatic pellet stoves,and analyzed the pollution emissions under different operating conditions.The study also considered the influence of power and air volume on particle fuel combustion and pollutant emissions.Numerical simulation was used to analyze the internal flow field of the pellet stove,providing a basis for optimizing the design of pellet stoves.The thermogravimetric experiments on the two types of pellet fuel showed that the comprehensive combustion characteristic index of the Pinus sylvestris pellet was higher than that of the redwood pellet.The combustion of Pinus sylvestris fuel in the temperature range of 238-370℃followed a three-dimensional diffusion mechanism with D6,and was dominated by first-order chemical reaction kinetics in the temperature range of 409-450℃.The combustion of redwood pellets followed a three-dimensional diffusion mechanism with D3 in the temperature range of 240-343℃,and was dominated by first-order chemical reaction kinetics in the temperature range of 343-430℃During the startup phase of a particle furnace under different operating conditions,it was found that the temperature rise rate was faster and more stable under high power compared to medium power.The combustion temperature and emissions are the main factors affecting combustion performance.The maximum CO₂value depends on the intensity of combustion.Under high power,the CO emission factors of camphorwood and redwood particles were the smallest,at9.92±0.65 and 9.69±0.3 mg/kg,respectively.NOx was lowest under medium power,at 3.71±0.06 mg/kg and 3.41±0.08 mg/kg,respectively.The results of the variance analysis showed that power had a significant impact on emission factors,and only under high power conditions did wind speed have a significant impact on emission factors.Interpolation fitting was performed on pollutant concentrations,induced air flow rates,and excess air coefficients during the stable combustion stage under different operating conditions.The results showed that excess air in the fixed bed was the main control parameter for pollutant formation,especially under low and medium power conditions.Excess air under high power had a greater optimization effect on the oxygen and fuel mixture,while induced air flow rate was used to fine-tune the combustion distribution between the fixed bed and the front combustion zone to optimize and reduce pollutant emissions.Numerical simulation studies have shown that the dense ventilation holes in the upper section of the air distribution plate lead to lower pressure compared to the two ends,causing the flue gas to not mix fully with the air and being trapped in the air distribution channel.In addition,the excessively fast flow rate in the air distribution channel also results in a shorter residence time for the flue gas,leading to incomplete combustion.