| Gasification is one of the most promising conversion routes for biomass utilization, but traditional gasification technology is flawed in some ways, such as high tar content in fuel gas and difficulty in waste water treatment. A pivotal factor that prevents biomass gasification technology from massive commercial application is incomplete purification of fuel gas. To develop stable and efficient low-tar gasification process becomes an acknowledged difficulty. Heat destruction for tar removal is a simple and effective method which is easy to be engineered, but now it still lack perfect research result and industrialized running device, therefore, developing and perfecting effective low-tar gasification technology and apparatus is of significant engineering value for promoting the massive application of biomass energy. Due to the complexity of the tar composition, current numerical computation on biomass gasification process seldom takes the tar composition into consideration, which makes it impossible to analyze the cracking of tar in the gasifier. Hence, building up a mathematical model of biomass gasification process that contains tar as an impact parameter will make theoretical guidance to study on low-tar gasification technology. In this paper, to take downdraft fixed bed for an example, many critical issues in making plant biomass achieve low-tar gasification were investigated by means of experiment and numerical computation, such as low-tar gasification device, mathematical model with tar crack process and so on.Pyrolysis plays a very important role in biomass gasification process, for tar is produced during pyrolysis process and pyrolysis products are reactants of the following oxidiation and reduction processes. In order to organize redox processes well and gain low-tar gasification technology, it is neccesary to matser the rules of tar production and distribution properties of pyrolysis products. A small pyrolysis experiment platform was built up. The pyrolysis property of plant biomass was studied on the platform. The distribution rules of the gaseous, liquid and solid pyrolysis products at different reaction temperature and time were obtained, and their compositions were analyzed. The results showed that the reaction temperature had a strong impact on the products distribution while the reaction time showed very little. It was also found that the yields of liquid products and solid products decreased obviously as the reaction temperature rising, while the yields of gaseous products clearly increased. The mass ratios of solid, liquid and gaseous products took the percentages of 40-45%,45-50%,7-10% respectively at about 400℃, and they went to 20-30%,12-18% and 55-60% respectively as the reaction temperature rose to 800℃. It could be concluded that the uncondensable gaseous products mainly contained H2, CO, CO2, CH4 and light hydrocarbon (CmHn). As the temperature increased from 400℃to 800℃, the volume content of H2 rose obviously, those of CO content and CO2 both decreased distinctly, the CH4 volume content increased slightly, and there was no visible change in CmHn content. The yield of tar reached its peak at the pyrolysis temperature of 500-600℃and decreased as the temperature rose.In order to achieve the low-tar gasification process, a stable high-temperature condition which should separate from pyrolysis and reduction stages is essential. A step-by-step method can carry out the low-tar gasification. In this paper, with the use of downdraft fixed bed reactor for example, a simple and maneuverable low-tar step-by-step gasification technology was proposed, which made pyrolysis and oxidation become two separate processes by physical space. Besides, a corresponding device was built up, and the low-tar gasification technology was validated through extensive experiments. As the results showed, in the step-by-step fixed gasification device, the component, heat value, and tar content of the fuel gas were nearly bound up with the temperature of oxidation zone which can be directly influenced by pyrolysis temperature and equivalence ratio (ER). With other conditions remaining the same, as the pyrolysis temperature rose from 390℃to 550℃, the temperature of oxidation zone increased, the reaction intensified, tar decomposed more completely, the residual ash content decreased and the carbon conversion ratio increased. Under the conditions that air ER was 0.25, pyrolysis temperature was above 450℃and the temperature of oxidation zone was above 950℃, the raw tar content of the fuel gas was less than 20 mg/Nm3, and the low heat value(LHV) was about 5MJ/Nm3, while the gasification efficiency was higher than 72%, and the carbon conversion ratio was over 90%. As ER increased from 0.23 to 0.3 at the pyrolysis temperature 450℃, the temperature of oxidation zone rose, the volume contents of CO and CH4 in the fuel gas decreased while the H2 content increased slightly, both the gas calorific value and the tar content reduced. Under the conditions that ER was from 0.25 to 0.3 and pyrolysis temperature varied from 400℃to 500℃, the gas calorific value ranged from 4.2 to 5.3 MJ/Nm3 while using air as the gasification agent, and the value changed from 7 to 9.5 MJ/Nm3 while using oxygen-enriched gas in which the volume concentration of oxygen was 90% as the gasification agent. When air was used as the gasification agent, as adding the steam, the temperature of oxidation zone reduced, the H2 content increased and the CO content decreased, while the CH4 content and the gas calorific value added up slightly.Oxidation zone is the key to tar crack. In order to know the influence of reaction conditions on the tar crack process, a dynamic mathematical model of reaction process in oxidation zone was established, which made the reaction processes visualize, and the changing rules of various reactants in oxidation zone could be analyzed. It was concluded from the calculation results that ER had a great effect on both the temperature value and the substance concentration in oxidation zone. As ER growing from 0.17 to 0.32, the average temperature of the oxidation zone increased, while the mole ratios of C(S), H2O, C2H4 and tar all decreased at the outlet of oxidation zone, in contrast to the increase of the ratios of N2 and CO2. The content of CO and H2 declined after their initial growth. The growth of ER showed a little effect on the content of CH4. The results also indicated that the higher the pyrolysis temperature was, the higher the average temperature of the oxidation zone was and the faster the tar cracked. With the increase of the inlet velocity of gasification agent, the reactions in the field worked more perfect and tar crack became more complete for intensification of gasification agent disturbance. While the pyrolysis temperature and ER remained unchanged, the two gasification methods with separately air and oxygen-enriched gas as the gasifying agent, in which the volume concentration of oxygen was 90%, showed slight influence on tar crack but great effect on concentration field. Seen from the axial cross-section of the oxidation zone, it showed that all the components changed along the axial direction. Combustion reactions and tar crack reactions mainly took place at the gasifying agent inlet surface down to about 200mm and it was some reforming reactions in the horn part and the lower area of the oxidation zone. The cracking speed of tar was mainly influenced by temperature.In order to systematically investigate the relationship between reactants and products of gasifier in theory, this paper established a thermodynamic mathematical model of biomass gasification processs with tar on the bases of mass balance, energy balance and chemical reaction balance. The results were in good accord with those the predecessors got, but had some difference in the content of CO and CO2 with the experiment results of step by step fixed gasification bed. The difference was mainly resulted from that some reactions couldn’t achieve the balance in experiments while in numerical simulations they were supposed to be balanced. The simulation results actually indicated the ideal trends. Therefore they could be used to make theoretical guidance to the practice on the view of macroscopic. The results showed that under the conditions that air ER was 0.25 and reaction temperature was 1000℃, after the reactions achieved their balance, the volume of CO and H2 in the gas added up to more to 40% while CH4 took about 0.8% to 2.5%. The tar content was really a little and the LHV of fuel gas ranged from 6 to 7 MJ/Nm3. As the air preheating temperature went up, the volume ratios of CO and H2 increased, while the volume contents of CO2, H2O, CH4, N2 and tar all reduced, as a result, LHV of the fuel gas improved. Increasing moisture content in raw material could bring down the volume ratios of CO, H2 and N2, and raise the ratios of CO2, H2O, CH4, while making little effect on the content of tar and the gas calorific value. The entry of water vapor could increase the volume contents of H2, CO2, H2O and CH4 while decrease the contents of CO and tar, as a result, which could make LHV of the fuel gas rise. As ER increased from 0.2 to 0.3, the contents of CO, H2O, CH4 and tar decreased clearly, and the contents of CO2 and N2, increased obviously, while the H2 content varied inconspicuously and the gas calorific value decreased.In summary, this subject comprehensively and systematically studied the key issues in plant biomass low-tar gasification technology with the use of theoretical analysis, experiment research and numerical calculation. In this paper, the low-tar gasification processing parameters were found through a step-by-step gasification method. Besides, a dynamic mathematical model of oxidation zone in downdraft fixed bed and a thermodynamics mathematical model for the whole gasification process were established, in which the tar was both considered as effect parameters, and the changing rules of various reactants in the furnace were obtained. The work done by this research can offer theoretical support and practice guidance to the plant biomass gasification industry. |
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