Experiment And Mechanism Research On Microwave Pyrolysis Of Agricultural Straw

Biomass has always been an important energy source and plays an irreplaceable role in energy system, which is highly likely to be one of the important components of the future sustainable energy system. At present, biomass could be used in a clean and efficient way through various conversion technologies. Pyrolysis is one kind of thermochemical conversion methods, through which biomass can be converted into energy products of gas, liquid, solid. At the background of fossil fuels reducing increasingly, conversion of biomass into alternative fuels and chemicals by pyrolysis, becomes a hot research focus at home and abroad. However, as an important part of biomass, some negative factors exist in the utilization of agricultural straws, such as: lower bulk and energy density, some difficulties during collection, transportation and storage, as well as the higher requirements of pretreatment, leading to high energy consumption and complex processing. The factors above restrict the large-scale utilization of agricultural straws.Microwave heating has the characteristics of immediacy, integrity, selectivity and efficiency. If applied to biomass pyrolysis, the pretreatment requirements of agricultural straw could be greatly decreased, then costs of pretreatment could be reduced significantly, while the comprehensive performances and economic value of pyrolytic products would not be dropped, or even greatly improved.With the development of microwave technology, producing alternative energy sources through microwave pyrolysis of biomass has become an important way of biomass utilization, which attracts extensive attentions and many researches have been carried out.Currently, studies of microwave pyrolysis of biomass are mainly focusing on the influences of such factors as material type and particle size, microwave power, catalyst type & ratio on the pyrolysis process. While the volume/size of sample used is usually small, then the advantages of microwave pyrolysis cannot be fully displayed. In addition, researches on the temperature rising and weight loss characteristics of pyrolysis are still little, but these parameters are very important for in-depth study of microwave pyrolysis.Also, microwave pyrolysis carried out at the fixed temperature is almost not concerned. However, it has been proved that temperature is the key factor for conventional pyrolysis.Besides, for practical application, it is nevertheless desirable to further improve the process performance by better design of microwave pyrolysis system. In particular, reduction of specific power consumption should be of significance.Based on the research status of microwave technology used in the energy conversion field, combined with the National Natural Science Foundation project"Basic research on the directional transformation process of biomass into synthesis gas by microwave pyrolysis",one apparatus using microwave as heat source was developed for the directional transformation of biomass into various target products by microwave pyrolysis, which had the function of thermogravimetric and combined the technologies of pyrolysis, catalysis and restructuring of catalyst togather. Not only the special heating mechanism and characteristics of microwave, but also the special roles of microwave on chemical reactions, were introduced. In view of studying the impacts of temperature on microwave pyrolysis, the temperature control function was added, which was proved to meet the experimental requirements properly.Systematic researches on the temperature rising and weight loss characteristics of wheat straw during microwave pyrolysis were carried out on the apparatus above, and the following conclusions were obtained. Firstly, if microwave power was small, drying and some dehydrated pyrolysis of sample could only be achieved. The total weight loss was not over 40wt.% and the final temperature did not exceed 400℃. The internal sample pyrolyzed first. When the transformation of physico-chemical characteristics of internal sample reached a critical point, its ability to absorb microwave had a sudden increase. As a result, the sample's temperature rose fastly and the external sample pyrolyzed caused by the heat transfer. The temperature rising of internal sample and transformation process of physico-chemical characteristics of material, as well as the heat transfer between the internal and external sample reduced with the increase of microwave power. However, when microwave power was over a threshold, the pyrolysis process was controlled by the external heat transfer conditions. The heat and mass transfer had a significant impact on the process of microwave heating and weight loss. When microwave power was small,pyrolysis of sample inside and outside was carried out separately, which was controlled by internal heat transfer conditions. Pyrolysis reactions were promoted by adding microwave absorbents and catalysts appropriately, particularly for the pyrolytic char. Heating curves of sample under different microwave power were in very good regularity:with the increase of microwave power, the time of sample entering every stages of pyrolysis brought forward gradually, but the final steady temperature distribution had the same tendency with microwave power, that was, the final temperature rose with the increase of microwave power.Pyrolysis product distribution of wheat straw under different conditions was obtained through effective separation and analysis. Compared with the conventional electric heating method, it was found that a great difference in the formation mechanisms of products existed in pyrolysis using microwave heating method.As capacity of absorbing microwave of pure straw was poor, the sample could only be dried and had a lower conversion ratio during the whole process. The total gas products was few and concentration of CO2 in gas products was close to 50vol.%.Liquid products was the main products when using CuO and Fe3O4 as microwave absorbent, and under certain operating conditions, the amount of liquid products exceeded 50wt.% or more.A higher content of H2 and CO in gas products were obtained using CuO and Fe3O4 as microwave absorbent, respectively. The condensation reactions of larger molecular organics were promoted under high temperature by alkali metal salts. As a result, production of bio-oil decreased, while more char and gas products were obtained when adding alkali metal salts. Mainly due to the gasification reaction of pyrolytic char with CO2 at high temperature, lower volume content of CO2 and higher concentration of CO were obtained when adding residual char. As pyrolytic char was obtained and reused easily, taking pyrolytic char as microwave microwave absorbent is the best choice for producing more gas products. The volume content of H2 in gas product was generally more than 40vol.% when adding alkali carbonate, while the volume of CO2 and CO concentration was relatively low. The volume content of syngas (CO+H2) in gas product was more than 70vol.% when using residual char and alkali carbonate, so microwave pyrolysis has an advantage in the syngas production. Temperature was the most important factor affecting the pyrolysis process, and high temperature promoted the production of gas, as well as the production of combustible gas, especially for H2.In addition, the energy balance analysis for this system was calculated and analyzed. It was found that due to the small scale, energy loss of heat and transformation of this system was large, but it could be reduced by expanding the scale and energy efficiency would be improved.Combined the pyrolytic characteristics with product analysis, the microwave pyrolysis and product formation process could be speculated as follows:microwave penetrated the sample and decayed, which was transformed into heat. Due to the heat loss on the surfaces of sample, the internal temperature of sample was higher and heat was transferred to the external continuously. Pyrolysis were carried out layer by layer from inside to outside, and parts of biomass particles heated rapidly were decomposed into char and volatiles, while the volatiles released would pass through the low temperature zone, then the occurrence chance of secondary cracking reactions was reduced greatly. In consequence, the yield of liquid products in microwave pyrolysis was relatively higher. However, when microwave absorbent was added, due to the effects of "hot spot", the reactions of high-temperature char and CO2, water vapor would be promoted significantly, as well as the thermal cracking of volatiles, which promoted to obtain more combustible gases, especially for H2.In order to investigate the microwave pyrolytic characteristics of large-sized materials, a microwave pyrolysis device for pyrolysis of straw bale was designed and built. The temperature rising and weight loss characteristics of straw bale, as well as the influences of straw type, microwave power on product distribution and components were investigated. Also, the power consumption and energy balance of microwave pyrolysis process were studied. The results showed that:uniformity of temperature field using microwave heating was better, but due to the impacts of heat transfer and product diffusion, some differences existed in the internal three-dimensional temperature distribution. More uniform internal temperature and shorter lasting time of moisture evaporation and devolatilization plateau was obtained with higher microwave power. Under the same microwave power, more uniform temperature field was obtained with the more uniform arrangement of microwave irradiation. Gas product was mainly composed of CO, CO2, H2, CH4, C2H6, and a small amount of unsaturated hydrocarbons. The maximum content of H2 was higher than 35vol.% and the maximum content of syngas (H2+CO) was more than 50vol.%.Finally, based on the parameters and experimental data obtained, the numerical simulation of temperature distribution during microwave pyrolysis process of straw bale has been performed by joint consideration of microwave heating, pyrolysis reactions and heat transfer process during microwave pyrolysis together, and experimental verification were carried out. It was found that the materials close to microwave incident plane absorbed microwave most, so its temperature increased significantly. The surface's temperature exceeded 700℃, and part of the position was even higher than 1000℃.Microwave decayed continuously during transmission to the inner layers leading to the temperature of inner material slowing down gradually. The temperature distribution inside the material was asymmetric,which might be caused by non-uniform grid, but it cannot be ignored that microwave heating had the features of heating unevenly and "hot spot".The temperature of "hot spots" rose fastly, and thermal runaway occurred at the end of heating process, whose temperature quickly rose above 1100℃.The temperature of"cold spots"rose slowly throughout the heating process, just rising to 300℃at the end. Due to the impacts of shape or sizes of sample, microwave power and not perfect of mathematical model itself or not precise enough of measuring data, some differences existed between numerical simulation and experimental results. But the simulation results can reveal basic laws of heat transfer during microwave heating process.However, mathematical model itself needs to be further improved to describe the microwave heating process of large material better.