A Study Of Charcteristics Of Organic Matters Pyrolysis In Supercritical Water

The biomass energy is abundant in China, and it will play an important role in modifying the energy construction which is mainly composed of fossil fuels. But the utilization ratio of biomass energy is very low in China. The environmental problem caused by the accumulation of waste tyres is becoming more and more serious. The recovering processing of waste tyres will be an unavoidable subject matter, and it is urgent to looking for reusing methods.Pyrolysis in supercritical water is a new technology which can deal with organic matters efficiently with no pollution to the environment. The elementary studies are carried out theoretically and experimentally in this paper.Mathematic models which adapt to the reaction in supercritical water are generalized, including the continuity equation, momentum equation and energy equation. The empirical formulas in supercritical water of several typical organic matters, the describing equation of water flowing through the porous wall, the reaction kinetics and pyrolysis rate of biomass in supercritical water are summarized. The density, coefficient of thermal conductivity, dynamic viscosity and specific heat at constant pressure of supercritical water are fitted to quadratic equations of temperature according to the IAPWS formulation97.Experiments of pyrolysis in supercritical water of wood dust and waste tyre are carried out in discontinuous experimental bench and effects of temperature and pressure on reaction ratio, carbon content in the residue and gaseous products are studied.The wood dust is almost pyrolyzed completely in subcritical condition. Along with the raise of temperature and the pressure, the pyrolysis reaction is becoming more and more complete and carbon content in the residue is becoming higher. But the rate is slowing up gradually.The reaction ratio of waste tyre is only 0.4 at subcritical condition, and it is becoming higher with the raise of temperature and pressure.The pyrolysis is near to completeness at 500℃ and 22.2MPa. The carbon content in the residue heightens at first and reduces afterward. And the average carbon content is higher than 80%. The pressure gives an important impact to the pyrolysis reaction around the critical point.According to the analysis of the gaseous products, wood dust and waste tyre have no SO₂ and NOx production in the pyrolysis in supercritical water. The main gaseous products of wood dust are CO₂, H₂, CO, CH₄ and other hydrocarbon gases. Along with the raise of the temperature, the content of CO₂ increases at first and then reduces, and the content of CH4 increases all along. The content of CO is low and has small variances. With raise of pressure, the content of CO and CH4 reduce and CO₂ and C₂-C₆ hydrocarbon gases increase.CH₄ is the main gaseous product of waste tyre and the content of CH₄ has a rapid rise when temperature raises. Most of CH₄ is produced between 400-500°C .The content of CO₂ is getting higher along with the raise of temperature, but the rate of rise is lower than that of CH₄. The content of CO is also very low,and it has a small increase between 450°C-500°C.The technology of pyrolysis in supercritical water brings no secondary pollution. The solid residue can be modified to substitute for some kinds of activated carbon due to its high carbon content. The liquid and gaseous products can be used as fuel. The technology can recover energy and deal with wastes efficiently with no pollution, and it will have promising prospect.