Study On Catalytic Gasification And Hydrogen Production Of Alkali Lignin In Supercritical Water

It is renewable and in accordance with the view of sustainable development to get energy from biomass. Therefore, with the energy and environmental problems become increasingly serious, more and more people pay attention to gain energy from biomass. Biomass gasification in supercritical water is a newly developed energy conversion and utilization technology, and it has attracted much attention because of its advantages of fast response, high gasification rate, cleaning process.The lignin content in nature is only inferior to cellulose. Utilization of cellulose is good, however, lignin has not yet been large, widely use due to the lack of effective utilization method. The lignin by-product produced from paper industry was up to600billion tons in our country every year, which contains a lot of alkali lignin, Causing biological resource waste and environmental pollution problems. Therefore, making full use of lignin and transforming it into fuel gas can avoid wasting. Lignin has stable chemical properties because its molecular structure containing aromatic polymers. It is a very worthwhile energy conversion technology to explore and promote to gasify lignin in supercritical water because supercritical water has strong solubility on organic compounds.The article studied alkali lignin gasification in supercritical water in316-stainless steel mini-batch reactors with alkali lignin as raw material and producing hydrogen as the purpose. The research results and conclusions include the following:(1) The experimental results showed that, the temperature and raw material concentration are the main influence factors. With Ni/Al2O3-SiO2, higher temperature, longer reaction time, higher water density, lower raw material concentration and a certain catalyst amount was in conducive to gasification of alkali lignin. In a certain temperature range, with the increase of temperature, the lignin gasification rate and hydrogen yield increased significantly. The increase in raw concentration did not favor the lignin gasification, and hydrogen production capacity significantly decreased. Regardless of high or low temperatures, the gas product yields were basically stable to a certain reaction time. It shows that the reaction between the gases reached equilibrium state for a certain amount of reaction time. Gaseous and carbon efficiency of alkali lignin is relatively low in a general state of water, and it increased significantly with the increase of water density and reached to sub-supercritical water. But in supercritical water conditions, the gasification rate increase is not obvious with the increase of water density. Accordingly, hydrogen yield of alkali lignin was less than3mmol/g in the general state of water, and it reached4.03mmol/g in the sub-supercritical water, and increased obviously with the increase of water density. The lignin gasification process was basically not affected when addition amount of Ni/Al2O3-SiO2is small, it haven’t shown its catalytic activity. Its catalytic activity began to show out with adding0.5(g/g alkali lignin), and the gasification rate and carbon gasification rate of lignin significantly improved.(2) Gasification yield (GY) and carbon yield (CY) of lignin were dramatically improved by four metal catalysts. The order of gasification efficiency from high to low is as follows:Ru/C> Pt/C> Pd/C> Ni/Al2O3-SiO2. The reason might be that catalysts can reduce lignin and help opening-ring, which lead to efficient gasification of lignin in SCW, but further research is necessary. Four catalysts have some different selectivity to different gases, for H2, the selectivity is as follows:Ru/C> Pd/C> Pt/C> Ni/Al2O3-SiO2. Also, the reactions finally reach the equilibrium and gaseous compositions are still similar as catalysis can’t change the reactions equilibrium. The catalytic effect of Ni/Al2O3-SiO2is slightly lower than the precious metal catalyst, and its cost is low compared with, so it is good catalyst and can be applied in supercritical water system.(3) The reaction order of alkali lignin gasification reaction in supercritical water was1. Both of reaction rate constant of gas generation and coke formation increased with the increase of temperature. Activation energy of gas generating reaction and coke formation reaction were36.68±0.22(kJ/mol) and9±2.4(kJ/mol), respectively. The pre-exponential factors were29.84±1and1±0.2. And the kinetic model can describe and predict the gasification process very well.(4) Phenol and benzene hydrocarbon compounds were the most contents of the liquid products of alkali lignin gasification in SCW. And alkali lignin can be quickly degraded into small molecular structure of compounds at500℃. Phenol is an important intermediate of lignin in gasification process.(5) Gaseous and carbon efficiency of alkali lignin gasification showed a downward trend with repeated use of Ni/Al2O3-SiO2. But at the same time, the yield and the molar ratio of H2was increased. Reduce of gasification rate was because the reused catalyst had higher crystallinityand there were large granular appearance. And the catalyst poisoning by sulfur or its active phase structure changed. The H2yield increases may be due to the increase in the proportion of Ni elements and NiO generation and promotes the water gas shift reaction.