Study On Pyrolysis Gasification Of Reed Pulp Black Liquor In Fluidized Bed

Black liquor is a kind of serious industrial pollution sources. It has thecharacteristics of high chroma, high alkalinity and high organic content, accounting forabout90%of the total papermaking wastewater COD emissions, which is harmful to theenvironment. Currently, the main treatment for pulp black liquor is traditional Tomlinsonrecovery boiler, but it isn’t suitable for straw pulp black liquor due to its high viscosityand high silicon content. A new idea of using black liquor gasification (BLG) technologyto deal with straw pulp black liquor is proposed in this paper. A series of experimentswere carried out, including pyrolysis kinetics, pyrolysis characteristics testand pyrolysis&gasification experiment in a fluidized bed rig, with the representative reed pulp blackliquor as object of study. The results can provide theoretical basis for thedevelopment and application of black liquor gasification (BLG) technology to deal withstraw pulp black liquor.Firstly, The pyrolysis characteristics of reed pulp black liquor was studied by usingTG/DTG techniques to obtain its pyrolysis characteristic parameters. And pyrolysiskinetics model was established to determine its kinetic parameters. The results showedthat the process of reed pulp black liquor pyrolysis can be divided into three stages. Inthe first and the second stages, which are183~350℃and350~560℃, the most of thevolatile organic compounds volatilized out, while the decomposition, evaporation andreaction of alkali metal salt occurs in the third stage, which is between560and800℃.The maximum weight loss rate displayed in the first stages. Significant thermalhysteresis appears in pyrolysis process with the heating rate increasing and the pyrolysisrate and devolatilization rate increase. the solid conversion rate is elevated when heatingrate is lower than10℃/min and then decreased.Then, a lab-scale fluidized bed equipment was adopted to perform pyrolysisexperiment at N2atmosphere and combustion experiment at air atmosphere for reed pulpblack liquor particles and droplets at different temperatures, namely530℃,580℃,630℃,680℃,730℃and780℃. The gaseous products were real-time onlineanalyzed and the Na, K, Cl ion concentrations in gas were measured. Results indicatesthat both the total volume of gas products and the combustible gases in it are increaseswith operating temperature increasing, wherein the gaseous products yield of combustionis more than that of pyrolysis at the same operating temperature, but the combustible gasquotient in the total gaseous products of pyrolysis reaches2.6times than that ofcombustion, approximately. In combustion experiment, the proportion of each gascomponent in descending order is: CO2> CO> CH4> H2> CnHm, on which the effect of temperature is not obvious. In pyrolysis experiment, the contents of CO2, CH4and CnHmdecrease while the contents of H2and CO increase as the operating temperature rises.The operating temperature has little effect on the migration of Na, K element in thecombustion experiment and the pyrolysis experiment of the reed pulp black liquordroplet but greatly affects the Na, K elements content in pyrolysis experiment of the reedpulp black liquor particles with temperature changes, especially when the temperature isabove680℃, the Na, K elements content much higher than the other three treatmentways. Cl release rate of the particles pyrolysis increases significantly as the temperatureincreased, while that of the other three treatment ways are not affected by the operatingtemperature.Finally, gasification experiment with continuous reed pulp black liquor feeding wascarried out in lab-scale fluidized bed under different bed temperature. the main gasproducts releasing characteristics were real-time on-line analyzed and Na, K, Clmigration were measured. Studies show that, according to the laws of the gaseousproducts release, reed pulp black liquor gasification process can be divided into threestages: rapid increase stage, slowly increase stage and balance stage. As bed temperaturerise, the first and the second stages became shorter and the H2yield is higher in the thirdstage. Bed temperature has little effect on the equilibrium ratio of each component, whichfrom high to low is:53%H2,22%CO2,12%CO,11%CH4,2%CnHm. The precipitationamount of K is almost no change as the temperature rise, while the precipitation amountof Cl increased slightly; the precipitation amount of Na slightly decreased from530℃to580℃but increased rapidly between580℃and680℃, which is almost twice as muchas that of530℃. Therefore, it is necessary to choose a appropriate bed temperature toincrease the rate of producing gas and reduce the release of Na element at the same time.