Formation And Emission Characteristics Of Volatile Organic Compounds During The Pyrolysis And Combustion Of Chinese Coals

Volatile organic compound(VOC)emissions are increasing continuously.These emissions will produce foul odors,and participate in photochemical reactions in the atmosphere,leading to the formation of secondary organic aerosol(SOA)and ground-level ozone.Due to their impact on public health and air quality,there is an urgent need for comprehensive and strict control of VOCs.Coal combustion is one of the most important anthropogenic souces of VOCs,with the initial stages of coal pyrolysis and coal combustion playing a key role in the formation of VOCs.The high volatility,reactivity,decomposability and temperature sensitivity of VOCs make it quite challenging to conduct direct experimental measurements.Therefore,research on the formation mechanism and emission characteristics of VOCs during coal combustion is very scarce.In this dissertation,the emission of VOCs in the pyrolysis and combustion of Chinese coals was measured.The effects of pyrolysis and combustion conditions on the formation and evolution of VOCs were studied.The VOCs emissions from coal-fired boilers in a real power plant were also tested to investigate the effects of conventional pollutant control equipments on VOC reduction.Firstly,The VOC emssions during the pyrolysis of bituminous(SH coal)and anthracite(XT coal)coals using both online(SPI-TOFMS)and off-line(GC/FID/MS)techniques were measured.It is found that high-volatile low-rank bituminous coal releases more short-chain aliphatic and oxygen-containing compounds,while low-volatile high-rank anthracite coal releases more aromatic and sulfur-containing compounds under the same temperature.Total concentrations of VOC emitted from SH coal are also higher than that from XT coal.As temperature increases,the signal intensities of most products increase accordingly.The higher the temperature,the earlier and faster the release of VOCs regardless of coal type.For bituminous coal,most products have a single-step relase behavior and started to release at approximately the same time,while aliphatics and aromatics with low molecular weight exhibite an interesting two-step relase behavior.Secondly,experiments were carried out on the Hencken flat flame burner,using sorbent tubes to collect gas samples and GC/EI-MS as the offline analysis method to study the formation characteristics of organic products during the initial stage of coal combustion.Comprehensive investigattions were performed on the effects of coal type and combustion conditions such as combustion temperature,oxygen content and residence time on VOC emissions.The results show that the organic matter produced in the initial stage of coal combustion are mainly VOCs and IVOCs.Similar to the experimental results of coal pyrolysis,as coal rank increases,the proportion of aliphatic species decreases,while the proportion of aromatic species.The influence of combustion conditions on different kinds of volatile organic products was given and explained in detail by various mechanisms such as pyrolysis,pyrosynthesis and oxidation.Based on the experimental results,the interpolation algorithm is further used to improve the CPD model,the improved model was proved to be able to accurately predict the release process and final yield of important volatile organic products in coal pyrolysis in a wide range of coal types and pyrolysis temperatures.Finally,systematic on-site measurements of VOCs in different streams of flue gas were carried out on a real coal-fired power plant using gas bags and canisters to collect gas samples,filters to collect particle samles,and GC/FID/MS,GC/MS as the offline analysis method.It is found that gas phase VOCs are mainly aliphatics,followed by aromatics and halogen-containing species,and a small amount of oxygen-containing and nitrogen-containing species.The total mass concentration of the tested 102 species is13456±47?g/Nm~3.The change in VOC concentrations along the way indicates that conventional pollutant control equipments have a limited effect on VOC reduction.