Rice husk, the by-product of rice processing factory, is an important agriculture and forestry biomass resource. Therefore, it is necessary to develop rice hull biomass utilization technology in our country, which has important significance to effectively guarantee the safety of energy supply, improve environmental pollution and reduce carbon emissions. Because of its advantages in the process and economy, biomass pyrolysis polygeneration technology is in line with China’s biomass energy development needs at this stage. However, due to the low quality of pyrolysis products (non-condensable gases, bio-oil and char), direct use of these products is difficult which can limit the further development of biomass pyrolysis polygeneration technology. Based on this, this thesis carried out the basic and applied research on rice husk pyrolysis polygeneration and its pyrolysis product modification for utilization.
(1) The quality of biomass raw materials was regulated by combined acid washing and torrefaction pretreatment, and the mechanism of pretreatment process on the physical and chemical properties of rice husk samples was revealed. The fuel characteristics, surface functional groups, crystal structure, microstructure, grindability and hydrophobicity of rice husk samples before and after pretreatment were analyzed. The results indicated that combined pretreatment not only removed a large amount of inorganic mineral elements from rice husk samples, especially alkali and alkaline earth metals (AAEMs), but also improve the fuel characteristics of rice husk samples. The grindability and hydrophobicity of rice husk samples were also improved by combined acid washing and torrefaction pretreatment. By comparing the yields of solid, liquid and gas products and the quality of torrefaction liquid and gas products during the torrefaction process of rice husk samples, it was found that the acid washing pretreatment process had a significant effect on the subsequent torrefaction pretreatment process. The solid yield of torrefaction process after acid washing was improved, meanwhile, the relative content of acetic acid in torrefaction liquid was reduced and the relative content of sugars was increased.
(2) The influence mechanism of combined acid washing and torrefaction pretreatment on pyrolysis kinetics and gas, liquid and solid three-phase pyrolysis products was revealed by thermogravimetric analysis and fixed-bed pyrolysis reactor. The pyrolysis kinetics analysis showed that activation energy E of all rice husk samples increased with the increase of conversion rate α. The torrefaction process slightly increased the value of average activation energy. With the increasing of torrefaction temperature, the value of average activation energy was gradually increased. Model-fitting method and isoconversional method were used to obtain the reaction model function. It was found that the main pyrolysis process could be described by reaction-order model function. The effect of combined pretreatment on three-phase pyrolysis products of rice husk samples was analyzed using a fixed-bed pyrolysis reactor. The results showed that acid washing process increased the yield of bio-oil, but reduced the yields of char and non-condensable gases products. However, the subsequent torrefaction process reduced the yield of bio-oil and increased the yield of char. Combined pretreatment resulted in a significant increase of pH and calorific values along with a reduction of the water content of bio-oil. It also has an important effect on the chemical composition of bio-oil, which resulted in a significant increase in the relative content of phenols and sugars in bio-oil along with a reduction of acids, ketones, aldehydes and furans. After combined pretreatment, the volume fraction of CO, CH4 and H2 in non-condensable gases was increased, resulting in the high calorific values of gas product. The specific surface areas of rice husk char from all the rice husk samples were in the range of 215.8~300.9 m2/g, and the relative content of SiO2 in the ash of rice husk char was increased to 97.76%~ 97.98% after pretreatment.
(3) The influence mechanism of combined acid washing and torrefaction pretreatment and on-line catalytic cracking of pyroysis volatiles on the upgrading of bio-oil were studied using two-stage catalytic pyrolysis system. It was found that the yield of bio-oil decreased gradually with the increase of Fe loading, but the content of aromatic hydrocarbons in bio-oil gradually increased. With the increase of Fe loading, the selectivity of benzene, naphthalene and naphthalene derivatives was enhanced, but the selectivity of xylene, ethylbenzene and trimethylbenzene was reduced. Comparing the results of different Fe loading in ZSM-5 zeolite catalyst, it was found that the high loading reduced the bio-oil yield. Therefore, 4Fe/ZSM-5 zeolite catalyst was considered as the best catalyst. Coupling pyrolysis volatiles on-line catalytic cracking with combined acid washing and torrefaction pretreatment improved the relative content of aromatic hydrocarbons in bio-oil and further enhanced the selectivity of benzene, toluene and xylene. Considering the factors such as the yield and selectivity of aromatic hydrocarbons, it is considered that the torrefaction condition, i.e., the torrefaction temperature of 240 ℃, was more suitable.
(4) Pyrolysis rice husk char was used as raw material, high performance porous carbon materials were prepared by NaOH-KOH mixed alkali activation modification. The physical and chemical properties and capacitance characteristics of obtained porous carbon materials indicated that NaOH-KOH mixed alkali activation combined with the characteristics of NaOH and KOH activation. The obtained porous carbon material not only exhibited a specific surface area as high as 3046.06 m2/g with total pore volume of 1.69807 cm3/g, but also had the microporous/mesoporous developed pore structure with the mesopore percentage of 46.74%, meanwhile the pore size of this porous carbon material was 2.22986 nm. Electrochemical test results showed that the porous carbon material prepared by NaOH-KOH mixed alkali activation has an excellent rate capability, charge and discharge reversibility and cycle stability. At the current density of 0.2 A/g, the specific capacitance of obtained porous carbon material reached 312 F/g, and the electric double layer supercapacitor fabricated with this porous carbon material delivered a high energy density of 10.8 Wh/kg.
(5) A process route for simultaneous preparation of silica and porous carbon material by two-stage desiliconization and activation of rice husk char was proposed. Furthermore, the effect of SiO2 in rice husk char on the formation of pore structure during the process of mixed alkali activation was studied. By optimizing the preparation conditions of silica, the features of the silica prepared were superior to that of precipitated hydrated silica according to national standard. Compared with direct activation of rice husk char, the porous carbon material prepared by activation of rice husk char after desiliconization has a larger surface area (3255.83 m2/g) and total pore volume (1.75426 cm3/g). Furthermore, it has a higher a high degree of graphitization. Electrochemical test results showed that the porous carbon material obtained by activation of rice husk char after desiliconization exhibited excellent capacitance performance. The specific capacitance of this porous carbon material reached 345.6 F/g at the current density of 0.2 A/g, and the energy density of corresponding electric double layer supercapacitor was 12.0 Wh/kg. In general, it was found that simultaneous preparation of silica and porous carbron material by two-stage desiliconization and activation of rice husk char was an ideal upgrading method of rice husk char.