Research On High Temperature Air Combustion Of Herbaceous Biomass

Because of the low bulk density, caloric value and the unique ash properties, there are problems in combustion chamber firing straw, including the higher investment for transport, more complex structure of unit to prepare stock, lower heat output, lower stream parameters, lower combustion efficiency and more severe problems relevant to ash. Hereby a scheme was provided to apply High Temperature Air Combustion (HiTAC) to straw in view of its high combustion efficiency and low pollution emissions. And then research was implemented under both cold and thermal condition. Furthermore, ignition was investigated to identify the dominant factors in the start of combustion of volatile composition in straw.Firstly, a comparison concerning the combustion properties between straw and coal was made by quality and by quantity respectively. It is resulted that the bulk density of straw is much lower than coal and the higher ratio of volatile to fixed carbon, the lower demand for combustion air and yield of flue gases, the lower yield of NOx. However, the measures of De-NOx in combustor fired coal is unfit for straw because of them different formation path to NOx, which mainly follows NH3path in straw. Furthermore, it is better to reflect ash properties of straw according to the index such as ash component based on heat value, ratio of elements Si to Al and alkali index, etc.Secondly, the thermal events were investigated by means of thermogravimetric (TG) method during pyrolysis of both wheat straw and corn stalk, and its kinetics was investigated at the same time. The TG experiment was carried out in N2environment and4kinds of heating rate. According to the data from TG, the key parameters, including the onset, the final and the peak temperature, all increase along with increase of the heating rate. The kinetic research was conducted by means of combining the model method with the model-free method, and referring to the idea of dual extrapolation method as well. It is concluded that the mechanisms of pyrolysis follow2-order reaction for wheat straw and2.5-order for corn stalk. Moreover, the results from model accord with them from TG in general.Thirdly, a conceptual design of combustor firing pulverized straw based on HiTAC idea was put forward in view of the problems in existing combustion chamber firing straw. The design is based on the fuel/air parallel flow, a significant method to realize HiTAC, and the principle to separate combustion and heat transfer. Firstly, a pilot cone is located in the output to obtain the secondary combustion air with high temperature and low oxygen concentration. Secondly, the measures of cyclone and liquid slagging are taken to purify the exhaust gases and lower char loss. Finally, the measures of the low oxygen concentration and the rebuming are taken to suppress the formation of NOx. The main parameters for the combustor are chosen as follows, thermal output50kW, operation pressure1atm, the combustion intensities135W/(N-m) per volume and100W/N per section area and the global excess air ratio1.3. Fourthly, the cold investigation, including experiment and computer modeling, was carried out to know the flowing field, analyze the influence of the degree of the pilot cone and provide evidence for choosing appropriate turbulence model for thermal modeling. There are9working conditions in experiment and both standard k-ε and RNG k-ε are taken in simulation. The results indicate several conclusions as follows.(1), the Rankine vortex is predicted from the2turbulence models.(2), the velocity and vortex center predicted from the2turbulence models are all in accord with the metrical to a great extent, and the results from RNG k-ε excel standard k-s in precision.(3), the maximum of minus axial velocity increase and its range enlarge along with increase of the degree of the pilot cone in the ring region.(4), the comprehensive dynamics performances are optimized under working conditions corresponding to the thermal out kept in50kW, where the global excess air ratio1.3and degree of the pilot cone10°.Fifthly, the thermal experiment and CFD-based simulation have been carried out to know performances of the combustor and optimize its operation parameters. The scaled combustor has the degree of pilot cone10°, primary excess air ratio0.1and global excess air ratio1.3. The simulation has been performed using FLUENTcode. It is mainly resulted as follows. Firstly, the flame volume increase, its luminescent part reduce, the high temperature region locate in the intersection between the cylinder part and the cone part, along with the input of secondary air approaching to the rating value gradually. Secondly, the predicted temperature accords with the metrical in general, though the former is overestimated to some extent. Thirdly, the tangential velocity demonstrates M shape similar to the cold condition. Fourthly, the axial velocity demonstrates alternate minus value with plus value, which is similar to the cold and strengthened further in thermal condition in the ring region. At last, the maximum temperature increases at first and then reduces along with increase of the thermal output.Finally, the ignition of volatiles from straw has been investigated under condition of lower oxygen concentration. It was carried out through the PSR of CHEMKIN4.0code and the mechanisms GRI3.0, and a sensitivity analysis was made at the same time. The results indicate the the ignition delay increases exponentially with respect to the decrease of concentration of oxygen. Furthermore, the composition, H2, is of importance in determining the start of combustion of the volatiles. Especially, the reaction, H+O2(?) OH+O, play a more dominant role along with the oxygen concentration becoming lower.