A Novel Directional Gasification Of Wet Biomass Process For Hydrogen Production

Both the growing awareness of the decreasing availability of fossil fuels and the increasing pressure on the environment from production and combustion of fossil have nowadays led to a deeper interest in sustainable energy generation using biomass. In the past two decades, hydrogen production from steam gasification of biomass has become the subject of extensive research in the field of biomass utilization. However, there are still many obstacles required to resolve until the commercial breakthrough could be obtained, such as low hydrogen content in gas product, high levels of energy consumption. In this thesis, a novel process of directional gasification of wet biomass for hydrogen production was proposed. In the process, the drying of wet biomass, pyrolysis, gasification, steam production and in-situ CO2 capture are occurred in the same reactor. The removing of CO2 by in-situ capture will change the original equilibriums of gas phase and push reactions, such as, steam reforming of carbonaceous fuel, water gas shift reaction, to take place on the direction of hydrogen production to produce additional H2. Furthermore, the steam auto-generated from the drying of wet biomass is utilized as reactant to react with the intermediate product of pyrolysis to produce more hydrogen. As a result, the pre-drying process and the specific steam generation, which involves lots of energy consumption are removed consequently in this process. For developing the novel process, investigations were conducted in this thesis as following:(1) Mechanism study of biomass pyrolysis in an auto-generated steam atmosphere.Hydrogen-rich gas production from pyrolysis of biomass in an auto-generated steam atmosphere was proposed. The scheme aims to utilize steam auto-generated from biomass moisture as a reactant to react with the intermediate products of pyrolysis to produce additional hydrogen. The effects of moisture content, temperature, heating rate, and residence time on product distributions, gas composition, carbon conversion, and other parameters were investigated experimentally. The results show that heating rate is a key role in the process. Under fast-heating conditions, drying and pyrolysis occurred in a relatively shorter time, which enhances the interactions between the auto-generated steam and the intermediate products of pyrolysis and hence produces more hydrogen. The use of sweeping gas is unfavorable to hydrogen production due to the reduced residence time of both the auto-generated steam and the volatile. Moisture content has a great effect on hydrogen production. The H2 yield and content increases with the moisture content. Under the conditions of fast-heating rate and without the use of sweeping gas, the pyrolysis of wet biomass with a moisture content of 47.4% exhibits higher H2 yield of 495 mL/g, H2 content of 38.1 vol%, and carbon conversion efficiency of 87.3% than those (267 mL/g, 26.9 vol%, and 68.2%) from the pyrolysis of the pre-dried biomass with a moisture content of 7.9%, which represents the conventional biomass pyrolysis.Measurement of the atomic structures of chars produced from pyrolysis of wet biomass at different temperature was carried out with XRD. The presented results indicate that most aliphatic chains (γband) are decomposed before 673K during pyrolysis, while only parts of the aromatic system ((002) band) are decomposed, which is the main structure of char. The increase of X-ray intensity ofγband below 573K and the decrease of aromaticity in the same temperature range confirmed the existence of activated intermediate product during pyrolysis in terms of atomic structures.(2) Directional gasification of wet biomass for hydrogen production with in-situ CO2 capture.The directional gasification of wet biomass for hydrogen production with in-situ CO2 capture proposed in the present thesis was investigated with a thermodynamic analysis and experimental studies, respectively. The thermodynamic analysis was done using Aspen Plus software (version 11.1) and the Gibbs energy minimization approach was followed. The analysis data indicate that the directional gasification of wet biomass for hydrogen production proposed in this theis is feasible. The CO2 absorption by CaO plays an important role in the process, which directionally pushes the reactions to take place towards the direction of hydrogen production. High temperature is unfavorable to the CO2 absorption by CaO. It should be noticed that the decrease of CO2 partial pressure resulted from the occurrence of CO2 absorption and/or the dilutedness by excessive steam significantly restrict the CO2 absorption by CaO.The experimental results show that the effects of moisture content, temperature, and [Ca]/[C] on the process is similar with the results of the analysis. The presence of the sorbent greatly promotes both hydrogen production from biomass gasification and CO2 capture. In this process, CaO plays the dual role of catalyst and sorbent. Furthermore, it is noteworthy that the sorbent reveals a stronger effect on the water gas shift reaction than on the steam reforming of methane. The reactor temperature reveals different effects on hydrogen production and CO2 absorption. High temperature favors enhancing the H2 yield while goes against CO2 capture. The negative effect of high temperature on CO2 capture is also proved by XRD spectrum and SEM image. For the novel method, the optimal operating temperature is in the 923–973K range.(3) Study on the improvement of Ca-based sobent in hydrogen production from directional gasification of wet biomass.Aimed to the incomplete conversion and absorption capacity decay of CaO, a series of study were performed. The results show that h-CaO generated from the dehydration of Ca(OH)2 has larger ABET and Vpore, than its former Ca(OH)2. Although the burning of SR sample can realize CaO regeneration, the regenerated CaO with a totally deteriorated pore networks suggests poor reactivity. The pore less than 25 nm in CaO powder are easier be blocked by CaCO3, which will cause incomplete utilization. The effect of steam reactivation gradually decreased as steam reactivation temperature increases. The optimal steam reactivation temperature is 598K. For water reactivation, the increase of water amount will result in a better reactivation effect. A fast decay of absorption capacity during Ca(OH)2 calcination was observed first time. Results show that the CO2 capture capacity decreases significantly as calcination time increases. At 923K, the decay leads to a 27.3% of the CO2 capture capacity loss, while calcination time increases from 2.5 minutes to 5 minutes. High calcination temperature is helpful to weaken the extent of the decay, whereas the presence of steam during calcination makes the decay deteriorated. During the decomposition of Ca(OH)2, an intermediate product was generated, which is of high reactivity but very unstable.(4) Theoretical study on the directional gasification of wet biomass in a moving bed.A mathematical model on the directional gasification of wet biomass in a moving bed was proposed. The drying of wet biomass, the decomposition of biomass and Ca(OH)2, and the CO2 capture by CaO were considered in the model. The model can predict the time-product (first steam, volatile matter, and second steam) released and time-temperature history of feedstock. The result had a good agreement with the experimental results, as could be beneficial for the optimization of reaction parameter and reactor design.