Release Of Sulfur In Yanzhou Coal And The Effect On Formation Of NH₃ And HCN During Pyrolysis And Gasification

The NOX and SOX are the main gas pollutants during coal combustion. In order to reduce the emission of NOX and SOX, it is necessary to understand the formation of NOX, SOX and their precursors during coal pyrolysis and gasification. The formation and destruction of NOX, SOX precursors during coal pyrolysis and gasification are affected by many factors. Based on the previous work on the formation of NOX precursors during coal pyrolysis and gasification, the main purpose of this study was to experimentally investigate the release rules and mechanism of NH3. HCN and H2S during pyrolysis and gasification of raw coal and desulfurized coal.In this paper, Yanzhou raw coal, which contains high sulfur, was chosen as experimental sample (noted as Y-YZ). After Y-YZ coal was washed with nitric acid, pyrite was removed, and this desulfurised coal sample was noted as S-YZ. Treating S-YZ with NaOH and microwave, a part of organic sulfur was also removed, this coal was noted as S+W-YZ. The formation of NH3, HCN and H2S during fast heating and programmed-heating pyrolysis/gasification by using a quartz fixed-bed reactor coupled with IC analyzer. The release of H2S during coal pyrolysis/gasification was researched. The effect of several factors, such as temperature, feeding method, atmosphere and sulfur in coal, on the formation of HCN and NH3 were also studied systematically.Main results are as follows:(1) Formation of H2S during coal pyrolysis/gasificationThe yields of H2S from three coals pyrolysis with temperature have the same change trend. It increases in the lower temperature range and reaches a maximum value, which is 973K for fast heating process and 873K for programmed-heating process. The fromation of H2S has relationship with sulfur form in coal. Pyrite and unstable organic sulfur compound can decompose into H2S when temperature is lower than 973K. For stable organic sulfur compound in coal, it is hard to form H2S even at higher temperature. The yields of H2S from desulfrised coal pyrolysis were lower than that from raw coal. This shows that inorganic sulfur and unstable organic sulfur were removed after coal was treated with acid and microwave.The yield of H2S from coal pyrolysis in Ar is higher than that from coal gasification in oxidative atmosphere. The higher oxygen content in gas is, the lower yields of H2S is.H2S mainly come from the pyrolysis of coal during temperature heated for programmed-heating process and feeding period for fast heating process. The sulfur in coal have obvious effect on the ratio of H2S formed in the periods of constant temperature and not feeding. This shows that H2S is mainly released from volatiles and the H2S from fresh char is related with the forms of sulfur in coal.(2) Formation of NH3 and HCN during coal pyrolysis/gasificationThe yields of HCN increases with the increase of pyrolysis temperature in fast heating process. The yields of NH3 also increases with the increase of temperature to the maxmum value at 1073K and then deceases. For programmed heating process, The yields of NH3 and HCN increase with temperature increasing and reache the maxmum value at 973K. The feeding ways influence the release of volatile and the interaction of volatile and char. This can affect the availability of H radical, which is necessary for HCN and NH3 formation. As a result, the yields of HCN and NH3 changed.Effect of atmosphere on the release of NH3 and HCN was investigated in CO2,5%O2/CO2 and 10%O2/CO2. The exist of CO2 has two effects on NH3 formation. One is inhibiting NH3 formation. This is mainly due to depletion H radical by CO2 adsorption on coal surface. The other one is favoring NH3 formation. The reason for this is that gasification of coal in CO2 can generate more radicals. These two effects compete when reaction proceeds, and will leads to different results. In 5%O2/CO2 and 10%O2/CO2, NOx will be formed from oxidative reaction of HCN and NH3 with O2.The yield of NH3 from desulfurised coal pyrolysis and gasification was higher than that from raw coal. The result from the model compounds pyrolysis also shows that formation of HCN and NH3 decreases when thiophene exists in reaction system. This indicated that the existence of sulfur has influence on the formation of NH3 and HCN. In other word, formation of H2S competes with formation of HCN and NH3.The yields of HCN and NH3 from the pyrolysis of coal in the period of temperature heated for programmed-heating process and feeding period for fast heating process are dominant. The sulfur in coal have obvious effect on the ratio of HCN and NH3 formed in the periods of constant temperature and not feeding.