Mechanism Of Low-NO_x Emission In Circulating Fluidized-bed Decoupling Combustion

The technical feasibility of low-NOx circulating fluidized-bed decoupling combustion(CFBDC)has been well proved in an industrial demonstration plant treating distilled spirit lees(DSL).The lowered NOx emission was believed to result from the combined contributions of DSL pyrolysis products including char,tar,and pyrolysis gas(py-gas)to the reduction of NOx via their reburning in the riser combustor of the CFBDC system.In order to further understand the mechanism of NOx reduction in CFBDC,this study is devoted to investigating the capabilities of biomass char,tar and py-gas for NO reduction through experiments in a lab-scale drop-tube reactor(DTR)that simulates the reburning conditions occurring in CFBDC.The work performed pyrolysis of DSL at 500 °C to produce the tested char and tar reactants while py-gas was prepared by mixing pure gases from cylinders according to the analyzed py-gas composition.In order to ensure the sufficient reduction of NO,a total mass feeding rate of reagents,which is 0.15 g/min,was adopted for most experiments.We first investigated in Chapter 4 the variations of acquired NO reduction efficiency(?e)with major parameters including reagent feeding rate,reburning stoichiometric ratio(SR),reaction temperature,residence time,and initial flue gas composition.It was found that tar enabled the best NO reduction in comparison to char and py-gas did at the same mass feeding rate of reagent(0.15 g/min).The presence of CO in py-gas inhibited the homogeneous NO reduction reactions to cause lower ?e.For DSL-derived char and tar,their realized ?e were facilitated through by higher temperatures and higher initial NO and CO concentrations.The main conclusion of this chapter is that the suitable SR values for obtaining the highest ?e by reburning of pyrolysis products were found to be 0.6-0.8.Based on preceding results,the synergetic effects among char,tar and py-gas reagents on NO reduction were evaluated and discussed in Chapter 5.The comparison at given total mass flow rate of NO-reduction reagent indicated that the char/py-gas(binary reagent)enabled the best synergetic NO reduction than the others did.Its realized efficiency elevated with increasing of the py-gas proportion.The tar/py-gas or tar/char mixture caused a positive effect only when the tar proportion was necessarily lowered to about 26%.In addition,there existed obvious interactions between char and some species in py-gas(i.e.,H2,CxHy)for NO reduction by pyrolysis products.The synergetic effects were closely related to the molar ratio of C and H elements in reagents over the fed NO(CH/NO ratio).The NO reduction capabilities of char and tar reagents derived from other fuels such as sawdust(SD)and Xianfeng(XF)lignite were further investigated in Chapter 6.At the specified mass flow rate of reductant,say,0.15 g/min,the SD char or XF lignite char were less efficient than the DSL char did for reducing NO because of the lower contents of ash(containing catalytic matters)in the SD and XF lignite chars.However,the SD tar enabled the highest ?e,among the three tested tars.Above all,tar as an attractive reagent always exhibits the better NO reduction than char does.Testing model tar compounds including phenol,benzene,acetic acid,methyl acetate and heptane for NO removal revealed that phenol plays an important role in enabling the good NO reduction by the SD tar.Our major understanding from testing the NO reduction by tar is that the compounds containing at least one aromatic ring(e.g.phenol,benzene)are the major contributor for reducing NO in either biomass tar or coal tar.In conclusion,the results of this study would be significant in the operation of CFBDC technology treating N-rich fuel.By considering the pyrolysis-generated tar as a dominant factor in lowering NOx emission in a CFBDC system,further studies are suggested to focus on kinetic analysis of the NO reduction by tars and also on the combined action of tar with other reagents.Additionally,the effect of pyrolysis temperature on NO reduction activity by various reagents should be investigated.