Research On Key Technology Of Producing Crop Straw Densification Fuels Via Rotary Extrusion

A proper way to mitigate the energy crisis and air pollution in China is to use the energy in agricultural crop straw,which can also ensure a good environment for the economic and social development of the country.With good industrialization potential,making densified biomass fuels by rotary extrusion technology is a direct and simple way to use the energy in agricultural crop straw.The products can be used for not only electricity generation,heating,and cooking by direct burning,but also producing biochar,biogas,biodiesel,and bioethanol combined with other technologies.However,China have lagged behind the US,Japan and European countries in related technologies.There are several problems existing in crop straw densification process,including high energy consumption,unstable product quality,and serious wear,which have severely restricted the development of related industries.This dissertation mainly focuses on the energy consumption and product quality of crop straw rotary extrusion densification process.Based on theoretical analysis and experimental investigation,several key technologies are studied to obtain the effects of different factors on the densification process.These key technologies mainly include the mechanism of rotary extrusion densification process and the design of test platform,pelletizing properties of crop straws,improvement of modification on the densification process,and interactions between the densified materials and the mechanical surface.The results of this work can provide data and theoretical bases for crop straw densification process and help to facilitate the development of relevant industries.Main research works in this dissertation are as follows:The mechanism of biomass rotary extrusion densification process was analyzed.Motion of the biomass,ring die,and roller was investigated,and a mathmetical model of biomass velocity was established.On the basis of this model,mechanical analysis of the biomass in a single die hole was carried out,and a mathmetical model of compression force was established.By analyzing these two models,factors affecting the densification process were obtained.Based on this,the need of the test platform was analyzed,and a test platform for biomass densification which could provide a hardware for subsequent research was designed and built up.Effects of raw material particle size on densification process was investigated firstly with different sized milled and chopped rice straw,based on which,the rationality of choosing milled materials as the test material was analyzed.A method to get specific sized test materials with similar particle size distribution by sieving was proposed.Using this method,pelletizing properties of a mixture including wheat straw and rice straw were studied based on a response surface design.The experimental data were processed by the methods of regression analysis and variance analysis,and regression models were finally built and verified.On the basis of these models,effects of the temperature,pressure,moisture content,blending proportion,and particle size on the specific energy consumption,pellet density and compressive strength were studied considering the interaction between other factors.The significance of modification for biomass materials was analyzed,and the principle for modification was proposed.The effects of different modification methods were compared with each other,based on which,waste engine oil was chosen as the additive,and the effect of additive content on pyrolysis properties of wheat straw was investigated.With the developed densification platform,the effect of additive content on densification process was studied,and regression models of the specific energy consumption,density,compressive strength,and heating value were established respectively.Combined with theoretical calculation,heating values of the wheat straw and waste engine oil were obtained,which showed a good accordance with general knowledge.Results of this chapter could provide a new thinking for the improvement of crop straw densification process.The significance of studying the interaction between biomass and mechanical surface was analyzed.Elastic stress distribution of the ring die was investigated,and the relationship between the pressure on inner die wall and the strain of outer die wall was obtained.Based on this,a test system for testing mechanical properties during the densification process was developed.Taking wheat straw,rice straw,and wood shave as the object,the effects of material type,particle size,compression velocity,moisture content,and modification on inner wall pressure,pre-stressing pressure,friction force,and friction coefficient were investigated with the system.Results of this chapter could provide theoretical,data,and technological bases for further investigation on the interaction between biomass and mechanical surfaces.