Study On Co-pelleting Mechanism And Control Strategy Of Biomass Pellet Fuel

With environmental problems becoming increasingly serious,renewable energy sources such as biomass energy are receiving widespread attention.Biomass pellet fuel can turn agricultural and forestry waste into treasure,and is one of the effective ways to use biomass energy.The impact of pelleting process on energy consumption and fuel quality restricts its large-scale utilization.Composite biomass can be used to improve fuel quality by taking advantage of different feedstock characteristics.As an important raw material for the preparation of biomass fuels,microalgae not only consume CO₂,but also provide energy from their rich lipids and glycerol,making them an ideal renewable energy source.Biomass energy utilization technology,represented by energy conversion of energy from microalgae and agricultural and forestry waste,has received widespread attention.Currently,technology and cost are the key issues that restrict the large-scale utilization of biomass forming fuels and energy microalgae.In this study,we investigated the interaction mechanism between different biomass substrates,process parameters,fuel quality and energy consumption of forming,and revealed the synergistic forming mechanism between agroforestry waste and energy microalgae.A new way to realize the synergistic utilization of carbon-fixing oil-rich microalgae and agricultural and forestry waste resources is proposed,aiming to help provide biomass clean energy on a large scale and efficiently while solving environmental problems.Specific research includes,1)Four biomass raw materials with different component characteristics were screened as substrate materials for preparing biomass solid forming fuels,namely apple tree pruning branches,corn straw,sunflower seed shells and energy microalgae.Among them,the microalgae raw materials were prepared using a self-built oil-rich microalgae culture experimental system and a new tubular photobioreactor,and the indicators of microalgae biomass yield and CO₂fixation rate of different photobioreactors were compared and analyzed during the culture process.2)A multi-parameter regulated single-pellet forming experimental system was built to control different parameters of pelleting temperature（20℃-160℃）,pelleting pressure（60MPa-200MPa）,and moisture content of substrate material（4%-18%）for four raw materials,and the single biomass pellet fuel was pressed and formed in the experimental system.The interaction mechanisms between different biomass substrates,process parameters and fuel quality and pelleting energy consumption were investigated using fuel density,durability and pelleting energy consumption as evaluation indexes.Based on Design Expert 12 software,a mathematical correlation equation for predicting and describing the relationship between pelleting parameters and physical quality of pellets and pelleting energy consumption was established using response surface multi-objective optimization method（RSM）.The results showed that the pelleting temperature and pressure were positively correlated with the fuel density and durability,and there was an optimal value（about 10-12%）for the effect of water content on physical quality;the temperature and water content variations were basically negatively correlated with energy consumption,and the pressure variations were positively correlated with energy consumption.Comparing with the experimental results,the RSM optimization results of density,durability and pelleting energy consumption of the four fuel samples were consistent with the experimental analysis,and the differences between the predicted and experimental values were less than 5%.Comparing the four fuel samples,apple tree pruning branch pelleting fuel had the highest pelleting energy consumption（43.5 kJ/kg）,corn straw pelleting fuel had the second highest（34.25 kJ/kg）,and microalgae pelleting fuel had the lowest pelleting energy consumption（9.2 kJ/kg）.3)The physical quality,pelleting energy consumption and pelleting mechanism of composite biomass pelleting fuels were explored.The effects of blending different proportions of microalgae on the physical quality and pelleting energy consumption of the samples were investigated at different pelleting temperatures（80-160℃）,pelleting pressures（120-200 MPa）and moisture contents（6%-14%）of the raw materials.It was found that increasing the pelleting temperature,pelleting pressure and blending ratio of microalgae could effectively increase the density and durability of the fuel samples,and the moisture content of the raw material had the best effect on the physical quality of the samples（10%-14%）.In addition,based on the experimental results and microstructural characterization,it was revealed that microalgae act as a"lubricant"and"binder"in the co-pelletization process with agroforestry waste:in the preparation process,carbohydrate components such as starch and protein in microalgae strengthen the bonding between biomass particles.In the process,the carbohydrate components such as starch and protein in microalgae strengthen the bonding between biomass particles and significantly enhance the density and durability of the fuel samples,while the oil and grease content can reduce the energy consumption of the samples during the pelleting process.Therefore,the blending of biomass materials containing oil,starch and protein can effectively improve the physical quality and energy consumption of wood and herbaceous pellet fuels.4)The combustion thermal characteristics and flue gas emission characteristics of the pellet fuels were investigated.Firstly,the produced samples were subjected to thermogravimetric experiments and characterized and analyzed by combined TGA-FTIR.The thermal characteristics and combustion kinetics of the samples were discussed,and it was found that blending microalgae could make the samples achieve lower activation energy levels and higher combined combustion index（CCI）values,and the oil and lipid substances contained in the microalgae intensified the combustion of biomass and played a role in the combustion process of the composite samples,which not only facilitated the ignition but also the more general combustion process.In addition,the flue gas emission characteristics of the samples after combustion were studied using a self-built biomass combustion and post-treatment analysis bench.With the increase of the blended microalgae content,the peak time of SO₂ production and NO peak time of the fuel samples were shifted forward and backward,the SO₂ production and conversion rate were increasing,and the N conversion was stabilized after the first increase,and the SO₂ emission of the composite pelleting fuel was slightly higher,while the NO emission met the standard.5)A biomass pellet fuel production line in Hancheng City,Shaanxi Province,was used as a research object,and the economics of the large-scale utilization of biomass-forming fuel was studied based on actual research by analyzing the various costs of the production process and the cost required per unit of calorific value and other indicators.The results show that although adding microalgae increases the cost of fuel samples,adding microalgae can significantly reduce the cost per unit calorific value of the fuel.Compared with apple tree pruning branch forming fuel and corn straw forming fuel,the addition of 20%microalgae resulted in a significant reduction in the cost required per unit calorific value,with a reduction of about 24.5%and 27%in the cost per unit calorific value,respectively.Therefore,this study is beneficial to the resourceful and synergistic utilization of agricultural and forestry waste and energy microalgae,which can significantly reduce the technical complexity of microalgae utilization on a large scale and provide a reference for the preparation of efficient and clean biomass energy sources.