| Pulp and paper industry is one of the major industries being blamed for contributing serious pollution to the water environment. The black liquor is regarded as the most harmful contaminant, which has the property of high consistency, deep color, containing a great amount of fiber float, polluting vast water. Neutral Sulfite Semi-Chemical (NSSC) Pulp is the paramount craft of making chipboard paper, the black liquor from which has the characteristic of low organic matter and heat-value. Consequently, the heavy oil and other fuel have to be used as auxiliary fuel in a conventional alkali recovery boiler to sustain stable combustion. Based on the advantage of fluidized bed combustion (FBC) technology in low heat-value waste treatment, FBC technology for black liquor is presented in this paper. This method can not only reduce the pollution effectively, but also decrease the investment and operating costs, which makes the alkali recovery technology more economic efficient in solving the pollution problem. The investment of a FBC alkali recovery system is less than one third of the cost of conventional alkali recovery system as well.In this paper , the investigation is focused on the experiments in a thermogravimetric apparatus and mechanism for the pyrolysis and combustion characteristic and kinetic analysis of the NSSC black liquor, in order to obtain pyrolysis and combustion discipline and kinetic parameters at different heating rates. In order to sutdy on agglomeration mechanism for combustion of the NSSC black liquor in fluidized bed, the optimum combustion conditions for treating the NSSC black liquor are acquired by optimum analysis. Based on above mentioned experiments, circulating fluidized bed alkali recovery combustor has been established. The researches for combustion and alkali recovery of the NSSC black liquor are conducted, in order to obtain the necessary design and operation parameters, provide the foundational knowledge theory and key technology for practical engineering application.Based on the thermogravimetric analysis of dry basis black liquor, the pyrolysis and combustion characteristic has been studied under nitrogen and air atmosphere condition. The result reveals that pyrolysis and combustion behaviors are complicated. The results show that there are three temperature ranges during the pyrolysis process, namely 170~370℃,370~570℃and 570~800℃, in which the black liquor loses weight quickly. The organic volatiles release mostly in the previous two stages, while the alkali metal salts decompose, volatilize and react in the third range and there are also three temperature ranges during the combustion process, namely volatile matter separating process (170~370℃), volatile matter burning process (370~550℃) and fixed carbon burning and alkali metal salts reacting process (550~800℃), in which the black liquor loses weight quickly. Thermal hysteresis is visible with the increasing of the heating rate; the temperature of initiation, termination and maximum reaction rate incrases obviously. The weight loss rate of the samples is respectively 65.47% and 54.73% under nitrogen atmosphere, and 71.90% and 58.39% under air atmosphere with the heating rate of 10℃/min and 50℃/min.By means of the Coats-Redfern, Flynn-Wall-Ozawa and Kissinger methods, the kinetics parameters, the activation energy and the pre-exponential factor are calculated on pyrolysis and combustion processes. The results are discussed and compared with a view of pyrolysis and combustion mechanism. The mechanism functions of the NSSC black liquor pyrolysis and combustion are determined as by means of thermal analytic dynamics method and the most probable mechanism functions in the three temperature ranges. The comparison of the results calculated by the three methods show that the Coats-Redfern and the Flynn-Wall-Ozawa methods give better simulation results than the Kissinger method.Experiments are carried out to investigate the agglomeration mechanism under different temperatures, bed materials, additives and additive dosages during the NSSC black liquor combusting in a bench-scale fluidized bed. Experimental results show that as the temperature increases, the degree of the agglomeration increases at temperatures ranging from 600 to 800℃. Bed materials have a lighter influence on combustion of the NSSC black liquor than the temperature. When additives, such as kaolin, calcium carbonate and alumina were added into the black liquor, it is suggested that these added materials decreased the degree of agglomeration and the effect is the best of with kaolin as the additive. It is suggested that more additive dosage could control the bed agglomeration better. Adding 5% of kaolin added can avoid bed agglomeration and make the NSSC black liquor combust completely at 700~750℃.On the basis of the single factor experiments,the effects of temperature, bed material, additive and additive dosage on combusing the NSSC black liquor are investigated. An orthogonal experiment has been designed. The order of the impact degree is followed as: temperature, additive dosage, additive and bed material. The optimal operating conditions are acquired after the data are analyzed by STATISTICA, The results supplied to fundamental data for industrial amplifieation and in practical application.A 50kg/h CFB alkali recovery combustor has been designed. A large amount of cold and hot experiments are carried out. The effects of the flux of the NSSC black liquor and the flux of the diesel oil on the temperature field, loss on ignition (LOI) and the emission of CO are investigated. The experimental results show that as the flux of the NSSC black liquor increases, the temperature in the dense bed increases and the temperature in the dilute bed decreases, LOI and the concentration of CO increases. As the flux of diesel oil increases, the temperature along the bed height increases, LOI and the concentration of CO decreases. The results show that 153.49g NaOH could be recovered from per 1000g NSSC black liquor on dry basis, the alkali recovery rate is 85.27%. It is an important foundation for the theory and practice for industrial application. |
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