Pyrolysis And Gasification Mechanism Of Bagasse Black Liquor

The bagasse black liquor has the characteristics of high silicon content, and high viscosity. This will bring great difficulties to the traditional alkali recovery, and lead to high cost and serious pollution. Development of clean and efficient black liquor gasification technology provides a new way for the treatment of bagasse black liquor. In order to provide theoretical support for the development of bagasse black liquor gasification process, the pyrolysis and gasification mechanism of bagasse black liquor were studied. The main research contents and results are as follows:Through the study of the slow pyrolysis of bagasse black liquor we found that, the black liquor pyrolysis can be divided into three stages:the first stage is below 550 ℃, and the maximum conversion rate could reach to 37.22%. The second stage is 550～750 ℃, and the maximum conversion rate could reach to 61.50%. The third stage is above 750 ℃, and the maximum conversion rate could reach to 69.91%. From the release of the elements, the vast majority of H, 60% of C, and 40% of O were released below 600 ℃. The Na was released above 750 ℃. From the released product we can see that, there is mainly CH4、 H2、CO2 and CO below 600 ℃, and only CO existed when the pyrolysis above 750 ℃. The black liquor pyrolysis was significantly inhibited when CO was added in N2 atmosphere. The degree of inhibition is related to [CO] and pyrolysis temperature. The greater of the black liquor pyrolysis inhibition, while the higher of [CO] and the lower of the pyrolysis temperature.Through the study of sulfide conversion during the process of bagasse black liquor pyrolysis we found that,88.2% of the sulfur enters into gas phase. 12.8% of the sulfur exists in the residue. The conversion rate of organic sulfide is 95.4%, and 92.7% of which entered into the gas phase in the form of CH3SH, CH3SCH3, H2S and CH3S2CH3.2.7% of the organic sulfide is converted to Na2S. The conversion rate of inorganic sulfide is 75.3%, and the conversion rate of Na2S2O3, Na2SO4 and Na2S is 92.7%,53.6%, and 95.2%, respectively.Through the study of kinetics of black liquor char CO2 gasification using the Langmuir-Hugheswood adsorption kinetic model we found that, there is a good linear relationship between the reciprocal of apparent gasification rate and [CO], as well as [CO2]-1,but there is a bad linear relationship with [CO]/[CO2]. Therefore, black liquor char CO2 gasification kinetics should be described by Langmuir-Hugheswood non-catalytic kinetic equation rather than catalytic kinetic equation. Through the Langmuir-Hugheswood adsorption kinetic equation we can found that, CO adsorption constants is -1.0487 m3·mol-1, and CO2 adsorption constants is -0.3238 m3·mol-1. It consistent with the constant hypothesis of the kinetics model. But the rate constant is continuously increasing, which is contrary to the constant hypothesis of the kinetics model. The reason is that the number of active sites in the black liquor char is a function of the M/C molar ratio, and it is not as a constant. When the total carbon black liquor in each of 1% and loss of M of black liquor char surface is 1.044%, the gasificat-ion reaction rate constant is 0.1589 m3·mol·min-1. It consistent with the consta-nt hypothesis of the kinetics model. The average activation energy of the black liquor char CO2 gasification is 153.5 kJ · mol-1, by using the logarithmic of apparent gasification rate linear fitting with the reciprocal of temperature at same atmosphere(95%[CO2]+5%[CO]) and different gasification temperat-ures (750 ℃,800 ℃ and 850 ℃). It is considered to be the activation energy of CO2 gasification reaction with "active molecules" (carbon based alkali metal compounds) after being catalyzed. Bagasse black liquor char CO2 gasification Langmuir-Hugheswood adsorption kinetics equation is expressed as: . There is a good linear relationship between the equation prediction and the experimental value.Through study of the kinetics of black liquor char CO2 gasification using the Arrhenius empirical equation we found that, the apparent gasification rate of black liquor char CO2 gasification is a function of the molar ratio of M/C, the index of [CO] and [CO2]-1. The gasification reaction series of [CO2] is 0.3287, and the gasification reaction series of [CO] is -0.5634. Through the empirical equation fitting with the experimental data, It consistent with the constant hypothesis of the Arrhenius’s empirical equation. The apparent activation energy of bagasse black liquor char CO2 gasification is 186.42 kJ · mol-1, by using the logarithmic of reaction rate constantlinear fitting with the reciprocal of temperature at same atmosphere (95%[CO2]+5%[CO]) and different gasification temperatures (750 ℃,800 ℃ and 850 ℃). It is considered to be the activation energy of CO2 gasification reaction with fixed carbon. Bagasse black liquor char CO2 gasification Arrhenius’s empirical equation is expressed as: . There is a good linear relationship between the equation prediction and the experimental value.Through the study of bagasse black liquor plasma gasification we found that, black liquor pyrolysis in plasma gasifier could produce CO and H2, and the volume content can reach up to 98%. The parameters optimization of coordination between the dosage of working gas, the Plasma power, and the dosage of the black liquor into the gasifier are the key to obtain high quality synthetic gas.Through the study of equilibrium model of black liquor plasma CO2 gasifi-cation we found that, black liquor gasification completely boundary point has the largest cold gas efficiency and chemical exergy efficiency. Point of the black liquor gasification is related to the final temperature and the type of the gasifi-cation agent. Black liquor gasification has the largest cold gas efficiency and chemical exergy efficiency, and the syngas quality is close to the maximum at 1173 K. Increasing temperature is good for improving syngas quality, but the exergy efficiency and cold gas efficiency have different degrees of decline. The chemical exergy efficiency and the low heat value of synthetic gas are increasi-ng, when it from the boundary point of steam gasification to the boundary point of CO2 gasification in the 1173 K black liquor gasification completely the boun-dary, Therefore, the boundary point of bagasse black liquor CO2 gasification completely is the best control point at 1173 K.