| Coal fire,caused by long-term air exposure and subsequent spontaneous combustion of coal and gangue,has emerged in almost all coal producing countries around the world since the Industrial Revolution.In China,it is often recognized as?the coal fire in the northern China‘,due to the geographic distribution of coal deposit.One typical example is Wuda coal fire in the Wuda coalfield at the center of Inner Mongolia Autonomous Region.It is commented by international community that the coal fire in the world may continuously endanger global environment,ecology and human health.However,till now,there still lacks direct scientific evidence and pertinent fundamental studies on environmental chemistry of coal fire.This thesis,based on Wuda coal fire,targets the coal fire smoke,aerosols and coal fire sponge?CFS?,aims to differentiate the basic chemical composition of coal fire pollutants through on-site measurement and lab analysis,and provide scientific evidence for further detailed understanding of typical coal fire pollutants,the genesis,and environmental fate.Three types of coal fire were studied in this thesis:spontaneous combustion of underground coal seam,spontaneous combustion of giant gangue hill stood on the ground surface,and spontaneous combustion of residual coal and landfill gangue in the coal pit reclamation site.They are also the most common coal fire types of Wuda coal fire and of most coal fires in northern China.The emission gas and the derivatives of above coal fire types were studied in this thesis;the exact physical forms include four categories:gaseous species such as Hg and SO2,aerosols such as total suspended particles?TSP?and their inclusions,liquid and water-soluble species such as condensate in the coal fire sponge?CFS?,and solid such as surface soil.Lumex RA-915+mercury analyzer?Russia?was employed after method research on the typical coal fire area of No.8 coal seam of Wuhushan mining area,Wuda coalfield.The average mercury concentration in the emission gas of surface cracks and vents is 464ng/m3?117-1239ng/m3,n=30?.The same measure was taken on a giant gangue hill in Suhaitu mining area,and the average mercury concentration at the vent outlets is 5908ng/m3?1022-31750ng/m3,n=46?.This two concentrations are 221 times and 2813 times of background level of atmospheric mercury?1.4-2.8ng/m3?,respectively,which confirms that both the underground coal fire and gangue hill coal fire release mercury significantly,between which the gangue hill coal fire releases mercury with higher intensity.Furthermore,on-site and lab measurement combined also suggest a certain level of elevation in mercury concentrations in ambient air,surface soil and plantation in coalfield and adjacent downwind Wuda urban area,as a result of diffusion and propagation of coal fire emission gas.This result confirms that coal fire releases mercury,which has posed impact on local environment,other than entering atmospheric circulation.After thorough field survey and investigation,an A field in the above No.8 fire area was selected,and eight types of cracks and vents zones were marked due to different surficial phenomena caused by different level of spontaneous combustion of 9#and 10#coal seams 40m below.They are:no trace zone,wax zone,sulfur zone,sodium sulfate zone,sulfate-disappearing zone,high temperature zone,cooling zone and cracking?of heat-altered rock?zone,referring to the progressing stages that the surface temperature rises from ambient temperature to high temperature?300°C?and then drop to close ambient temperature,in accordance with ignition,burning at full capacity and burnt out of underneath coal seam.Targeting these eight stages,Picarro molecular hydrogen fluoride?HF?analyzer was employed,and the average HF concentration in the coal fire smoke directly released from coal fire surface cracks is 5069ppb?77.25-41042ppb,n=96?.The sensible HF emission started from sulfur zone,rose significant at sodium sulfate zone,reached climax at sulfate-disappearing zone and then dropped quickly.The average HF concentrations at above three zones are 1325ppb?161-6149ppb,n=33?,3956ppb?1915-6602ppb,n=18?and 18375ppb?1950-41042ppb,n=18?respectively,and the corresponding SO2 levels and smoke temperatures are 14ppm/106°C,27ppm/130°C,and 86ppm/231°C,respectively.The average HF concentration in the near-surface ambient air?1.2m?in the whole area?8000m2?is elevated to 228ppb?108-643ppb,n=12,t=144h?,and that in the peripheral area is 53ppb?44-67ppb,n=5,t=60h?.The HF concentration in the ambient air of Wuda urban area 10km downwind of the coalfield is about 2.8ppb.In comparison,the HF concentration in Beijing in the same season?autumn?is about 0.28-1.00ppb.The results suggest that underground coal fire significantly releases molecular HF,and the HF emission from coal fire is synchronized with sodium sulfate release,which is in direct relationship with the relatively mild oxidation and thermal decomposition of pyrites in coal,and irrelevant to the fast decomposition of organic phase of coal under high temperature or open fire combustion.Total suspended particle?TSP?in the ambient air of coal fire area were collected and studied.The average TSP concentration of three location closest to coal fire vents was 14367?g/m3?12600-17300?g/m3,n=3?.Among them,the average concentration of sulfur alone was 2717?g/m3?2340-3300?g/m3,n=3?,about 19%of TSP mass,and that of total carbon?including organic carbon and elemental carbon,TC=OC+EC?was36?g/m3?14-79?g/m3,n=3?,only about 0.25%of TSP mass.The average mass concentration of TSP of all other location in the vents concentrated area was 4847?g/m3?2090-7620?g/m3,n=6?,while that of sulfur was 792?g/m3?208-1680?g/m3,n=6?,about 16%of TSP,and that of TC reached 101?g/m3?6-140?g/m3,n=3?,about 2%of TSP.In the peripheral area,the average concentration of TSP was 419?g/m3?393-466?g/m3,n=3?,and that of sulfur dropped to 37?g/m3?19.5-48.4?g/m3,n=3?,about 9%of TSP,and that of TC was 31?g/m3?14-63?g/m3,n=3?,about 7%of TSP.The above results suggest that coal fire releases sulfur-rich aerosols.Further elemental analysis and source analysis?PCA?suggest that the TSP samples collected can trace back to three sources:the oxidation decomposition of pyrite in coal,the decomposition of organic phase of coal,and environmental dust.In addition,five source sampling methods of coal fire aerosols were attempted in the field and discussed in this thesis.Currently there still lacks commercial instrument that can detect sulfuric acid?fog??H2SO4?at molecular level.Therefore,this thesis targets a natural collector of coal fire pollutants—coal fire sponge?CFS?,which is a type of fluffy and porous,mushroom-liked bulge of surface soil at the coal fire vents openings.33 pieces of CFS samples were collected on a reclamation area filled with residue coal of 9#/10#coal seam and gangue as reclamation material.Chemical volumetric analysis suggests that thenatural remainder liquidcontains[H+]=5.60mol/L?p H=-0.75?and[SO42-]=3.17mol/L?304mg/g?,with a stoichiometric condensed structural formula of N0.23H1.77SO4,where N0.23 represents normalized monovalent metal cation that substitutes the hydrogen ion in the reaction that acid in smoke meets clay.The result suggests that underground coal fire releases sulfuric acid?hydrate?in macro-level.The analysis on CFS also suggests enrichment in insoluble fluoride?1449?g/g?and water-soluble fluoride?774?g/g?,volatile HF?260 ppb?,mercury?16830ng/g?,arsenic?20.9?g/g?and polycyclic aromatic hydrocarbons?PAHs?(?16PAHs=21933 ng/g).A comparison between CFS and surrounding surficial soil samples suggests that the long-term existence and weathering and diffusion of CFS have led to a common pollution of acid,Hg and PAHs in local soil body.Based on above findings,this thesis proposes an“active HF intermediate”theory.The main contents include:the relatively mild oxidation-thermal decomposition of reduced sulfur such as pyrite in coal and gangue in coal fire processes may not only release SO2,sulfur and sodium sulfate,but more importantly sulfuric acid.This sulfuric acid through diffusion may activate the fluorine in the insoluble and refractory fluoride with high melting and boiling point such as calcium fluoride and generate active intermediate—HF with a b.p.of merely 19.8°C.This intermediate,then,migrates and propagates through air or water passageways until meets with other substance?e.g.CaCO3?and forms back into insoluble fluoride?e.g.CaF2?,which results in fluoride enrichment in the new environment.All above steps are spontaneous,while above mentioned molecular HF and unusual enrichment of sulfuric acid and fluoride in the CFS provide three key scientific evidences for this theory.Based on a coexistence system of sulfur and fluoride,plus the spontaneous processes being the initial drive,an extended application of this theory may cover terrestrial volcanic system,supergene geochemical processes in the key stratum,mild and slow-speed anthropogenic coal-burning system,such as fireplace coal-burning?about 350°C?in traditional endemic fluorosis area in southwest of China and domestic stove coal-burning?<350°C in sealing period or at unignited area?.The results of this thesis suggest that the CFS may change the environmental fate of at least a portion of coal fire pollutants from entering the atmosphere to stay in-situ contaminating local soil and water body,which calls for more awareness and action from local community and government with more strict legislation and policy-making.On the other hand,the CFS may also serve involuntarily as a pollutant collector as in coal-fired power plants,which may provide new thoughts for coal fire pollutants control and treatment.At last,this thesis presents the concept of“common state underground coal fire”,the range of which is preliminary defined as starting from mild oxidation-thermal decomposition of coal seam and ending at the coking of coal seam.This concept distinguishes coal fire from industrial coal burning.It is suggested that the understanding and treatment of the environmental impact of coal fire should not be limited by current knowledge of industrial coal burning;the subject of liability of coal fire should be clarified,and treatment of common state underground coal fire be normalized. |
PDF Full Text Request