Fundamental Study On Drying And Torrefaction Pretreatments To Produce High High-quality Biomass Feedstok

The total quantity of biomass resources is huge, but the quality is low. Too much moisture in raw materials always leads to delay pyrolysis reaction, increase the heating cost and destruct the stability of pyrolysis liquid products. In addition, the biomass has many adverse characteristics, such as strong hydrophilicity, high oxygen contents, low energy density, not easy to store and wide dispersion. These drawbacks cause higher costs in the transport and storage, which limits the further development of biomass utilization technology. Based on this background, the research was carried out to obtain high quality biomass pyrolysis feedstock by dry ing and torrefaction methods. In this work, the heat and mass transfer mechanism of biomass drying was first systematically researched, and then two convenient methods were proposed for determination of effective moisture diffusivity, finally torrefaction mechanism of biomass and the effect of drying and torrefaction on biomass pyrolysis were in-depth studied.(1) The heat and mass transfer mechanism of biomass isothermal and non-isothermal dryingThe drying of biomass is a complicated heat and mass transfer process. Under isothermal condition, the whole process can be divided into three stages:rising rate period, first falling rate period and second falling rate period, which is closely related to the bonding force between water and biomass. Midillokucuk model shows the best fit to experimental dying data of rice straw under isothermal condition, while the Page model is the best model for describing the non-isothermal conditions of sawdust, cotton stalk and rice straw. There is an internal relation between heat and mass transfer, and the total heat flow can be decomposed into three components:heat flow from the water evaporation, heat flow from the heat capacity of unevaporated water and heat flow from the heat capacity of dry base biomass. The main part of the heat requirement of drying is corresponds to water evaporation. The simulative results agree well with the experimental ones, showing the reliability of the drying kinetics and the thermal simulation.(2) The measuring method of the effective moisture diffusion coefficient in biomassThe isothermal two-step method and the non-isothermal one-step method were proposed for measuring the moisture diffusion coefficient. In the isothermal two-step method, Deff can be obtained with the Fick diffusion law, and the drying activation energy can be calculated by the relation of the ln(Deff) and1/(T+273.15). This method overcomes the difficulty of the isothermal condition which is difficult to achieve. In order to reduce the influence of the thermal hysteresis phenomenon and get a more accurate calculation result, this paper has made some improvements to the isothermal two-step method. The rising rate period which is controlled by the surface vaporization is excluded and the falling rate period which is dominated by the internal diffusion was used to calculate the Deff.The results show that the improved isothermal two-step method is more reliable. The calculation results of the isothermal two-step method is relatively close between the before improvement and the after improvement. In the non-isothermal one-step method, the phenomenon of the biomass thermal hysteresis is eliminated. When the heating rate is2～8℃/min, the result of the non-isothermal one-step method was included in the isothermal two-step method (30and100℃). The non-isothermal one-step method has some advantages, such as, fewer material requirements, less experiments, reducing the thermal hysteresis, and the calculation result reliable. It is a better measuring method of the biomass drying parameters.(3) Torrefaction research for preparation high-quality biomass feedstockTorrefaction is a slow pyrolysis process that occurs at atmospheric pressure and in the200～300℃temperature range in the absence of oxygen. The experimental method of torrefaction on biomass was improved in this study. Residence time could be determined more easily, so that the biomass material was torrefied under pre-determined temperature. The textural structure of rice husk was destroyed and the bulk was gently shrunk after the torrefaction pretreatment. Besides, the grinding performance was improved and the hydrophobicity was largely enhanced. The solid product with favourable pore structure, namely minimum average pore size and maximum specific surface area, was attained at260℃.The volatiles gradually decreased while the ash and fixed carbon substantially increased with the temperature increase. The sulfur and nitrogen contents remained unchanged, the hydrogen content slightly decreased, the carbon content gently increased, and the oxygen heavily decreased in the process of torrefaction. Both the energy and solid mass yleld decreased with the increase of operating temperature and residence time. The hemicellulose was mostly decomposed, the cellulose remained unchanged, and the lignin increased greatly after torrefaction. The results of TG-FTIR indicated that the change of physicochemical property of rice husk was mainly owed to the decomposition of the hemicellulose. The organic functional groups of solid product were simplified as a result of the breakage and removal of some oxygen-containing functional group. The liquid product contained plentiful water and a little ethanol and the gaseous product was composed of CO2and CO.(4) Effect of drying and torrefaction on biomass feedstock pyrolysisBiomass pyrolysis has three stages:moisture evaporation, main devolatilization, and continuous slight devolatilization. Moisture loss peak, the shoulder side peak formed by the hemicellulose pyrolysis, and the sharp main peak formed by cellulose pyrolysis are the three different peaks in the DTG curve. The drying pretreatment improves the overall rate of straw pyrolysis and volatile matter yield, but also improve the calorific value of the bio-oil, reducing the moisture content in the bio-oil. The Py-GC/MS analysis showed that the moisture content in rice husk had little effect on thermal decomposition products. The relative reduction of pyrolysis products result from the relative reduction of the organic content of dried biomass.The weight loss peak of hemicellulose is disappear when the torrefaction temperature is high (260and290℃). As the torrefaction temperature increases, the moisture in the bio-oil and the acidity are significantly reduced while the calorific value is gradually increased. This helps bio-oil storage and high-value use. The baking process for the generation of non-condensible gas and coke, baking, liquid (bio-oil and baking liquid) is still the main product in the low temperature (less than260℃), while in the high temperature (290℃) baking, non-condensible The gas is the main product. The Py-GC/MS show that baking has no effect on the types of fast pyrolysis products, but improving the content of phenolic compounds in the lignin pyrolysis products.