The stockpiling of coal is carried out at a number of points along the transport chain to the end user: at coal mines, coal preparation plants, transport terminals; and the user site, including power plants, integrated iron and steel works, coke works and cement plants. Countries with no domestic coal resources, but which consume large amounts of coal, have to store appreciable quantities for relatively long periods of time in large stockpiles in the open air at harbours or in storage facilities. Assessing gas and dust emissions while storing coal has practical importance in risk assessment of the storage area and prevention of greenhouse gas (GHG) emissions. Although not significant compared to stack emissions, some gases emitted from coal stockpiles are toxic and explosive.
Gases emitted from coal stockpiles are identified as carbon oxides, hydrocarbons, sulfuric gases and hydrogen. Possible sources of gases emitted from coal stockpiles are degassing, low temperature oxidation and, in extreme cases, spontaneous combustion.
Coal beds contain reservoirs of gases, mainly CO2 and CH4. These gases are stored on the internal surface of organic matter (adsorption mechanism) or within the molecular structure of the coal (absorption mechanism).
Gas desorption depends on temperature and pressure. For coal stockpiles, temperatures can be higher (due to oxidation) and atmospheric pressures lower than those occurring in coal beds. These conditions are ideal for degassing. Apart from CO2 and CH4, dimethylsulfide (DMS) is produced from lignite by a desorption process. From the moment that coal is exposed to air, it is subject to low temperature oxidation (weathering) by atmospheric oxygen. This process can be sustained if the
heat produced by the exothermic oxidation cannot be sufficiently dissipated by heat transfer within the stockpile.
Although the exact process of coal oxidation is complex and the mechanisms are not fully understood, some general features of the process are known; that is, the direct burn-off and sorption sequences. The coal oxidation reactions are affected by several factors. Apart from mass transport considerations, these factors can be classified as internal and external variables. The internal variables include composition and physical properties of coal, history of coal weathering/oxidation, as well as particle size. The external variables involve temperature, partial pressure of O2, and moisture content in the gas medium. In terms of coal oxidation in coal stockpiles, the design of the stockpile and configuration of the stockpile also play an important part.
It is difficult to test the gas emissions from a large coal stockpile. The main barrier is sample collection. A lightweight (37 kg) portable sampling unit was developed in the early 1990s to sample the gases found within a coal stockpile and to monitor stockpile temperatures. In this method, a penetrator tube was inserted into a coal stockpile and a vacuum pump was used to get the gas into a glass container. A decade later, a direct sampling method to collect gas from the stockpile surface was established. This method used airtight tents on the stockpile and after a certain time, the emitted gases were collected by air pump at a low flow rate in the tent. The gas samples were taken from the tents in gas sampling bags to the laboratory, where tests of the samples were performed by gas chromatography within 24 hours. Methods of testing fluxes of GHGs from spoil stockpiles of opencast coal mines can also be applied to coal stockpiles.
The sources of carbon oxides emission from coal stockpiles are desorption and oxidation processes.
Temperature and O2 concentration affect the production of CO and CO2. Possible processes for the different hydrocarbon products are desorption of gases that were trapped in the pores of the coal during the latter stage of the coalification process and oxycoal surface reactions.
The evolution of these hydrocarbons is most probably linked to the formation of coal surface oxides or the production of CO2. DMS, carbonyl sulfide (COS) and carbon disulfide (CS2) were detected from coal stockpiles. The concentrations of COS and CS2 are influenced by temperature, and oxygen is required for their formation. The presence of oxygen does
not have an influence on the concentration of DMS in the same way as temperature does. This means that COS and CS2 are emitted via an oxidation process and DMS by desorption. H2 emission is an oxidation-related mechanism and dependent on the amount of O2 consumed by coal. The amount of H2 produced from coal oxidation is quite small. Further, hydrogen is a low molecular weight gas, which is lighter than air, so it is assumed that the hydrogen produced in the stockpile is dissipated. Despite this, hydrogen should still be taken into account when planning the transportation and storage of coal as it may accumulate in certain areas, especially in areas where stockpiles are covered or in confined spaces.
Gas emissions from coal stockpiles have not yet been regulated. However, the Intergovernmental Panel on Climate Change (IPCC) has recognised them. Due to the lack of work, the IPCC cannot nominate a measuring method. In fact, there appears to be very little information available on gaseous emissions from coal stockpiles. More data are needed to assess the emissions. Until more
information is available, it would be difficult to establish a regulation or legislation on gas emissions.
Although gas emissions from coal stockpiles are negligible compared to coal-fired power plant emissions, caution should still be taken since some of the gases are toxic and explosive. This is especially important where the stockpiles are in enclosed areas. Some methods used to prevent coal self-heating and spontaneous combustion can be applied to reduce gaseous emissions from coal stockpiles.
Xing Zhang is a technical author at the IEA Clean Coal Centre. Her full report is available at the IEA Clean Coal Centre Bookshop.
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