Coal is used primarily as a fuel. While coal has been known and used for thousands of years, its usage was limited until the Industrial Revolution. With the invention of the steam engine, coal consumption increased. In 2020, coal supplied about a quarter of the world's primary energy and over a third of its electricity.[5] Some iron and steel-making and other industrial processes burn coal.
There are several international standards for coal.[46] The classification of coal is generally based on the content of volatiles. However the most important distinction is between thermal coal (also known as steam coal), which is burnt to generate electricity via steam; and metallurgical coal (also known as coking coal), which is burnt at high temperature to make steel.
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The development of the Industrial Revolution led to the large-scale use of coal, as the steam engine took over from the water wheel. In 1700, five-sixths of the world's coal was mined in Britain. Britain would have run out of suitable sites for watermills by the 1830s if coal had not been available as a source of energy.[65] In 1947 there were some 750,000 miners in Britain[66] but the last deep coal mine in the UK closed in 2015.[67]
A grade between bituminous coal and anthracite was once known as "steam coal" as it was widely used as a fuel for steam locomotives. In this specialized use, it is sometimes known as "sea coal" in the United States.[68] Small "steam coal", also called dry small steam nuts (DSSN), was used as a fuel for domestic water heating.
During gasification, the coal is mixed with oxygen and steam while also being heated and pressurized. During the reaction, oxygen and water molecules oxidize the coal into carbon monoxide (CO), while also releasing hydrogen gas (H2). This used to be done in underground coal mines, and also to make town gas, which was piped to customers to burn for illumination, heating, and cooking.
When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler (see also Pulverized coal-fired boiler).[97] The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity.[98] The thermodynamic efficiency of this process varies between about 25% and 50% depending on the pre-combustion treatment, turbine technology (e.g. supercritical steam generator) and the age of the plant.[99][100]
A few integrated gasification combined cycle (IGCC) power plants have been built, which burn coal more efficiently. Instead of pulverizing the coal and burning it directly as fuel in the steam-generating boiler, the coal is gasified to create syngas, which is burned in a gas turbine to produce electricity (just like natural gas is burned in a turbine). Hot exhaust gases from the turbine are used to raise steam in a heat recovery steam generator which powers a supplemental steam turbine. The overall plant efficiency when used to provide combined heat and power can reach as much as 94%.[101] IGCC power plants emit less local pollution than conventional pulverized coal-fueled plants; however the technology for carbon capture and storage (CCS) after gasification and before burning has so far proved to be too expensive to use with coal.[102][103] Other ways to use coal are as coal-water slurry fuel (CWS), which was developed in the Soviet Union, or in an MHD topping cycle. However these are not widely used due to lack of profit.
About 8000 Mt of coal are produced annually, about 90% of which is hard coal and 10% lignite. As of 2018[update] just over half is from underground mines.[110] More accidents occur during underground mining than surface mining. Not all countries publish mining accident statistics so worldwide figures are uncertain, but it is thought that most deaths occur in coal mining accidents in China: in 2017 there were 375 coal mining related deaths in China.[111] Most coal mined is thermal coal (also called steam coal as it is used to make steam to generate electricity) but metallurgical coal (also called "metcoal" or "coking coal" as it is used to make coke to make iron) accounts for 10% to 15% of global coal use.[112]
Consideringthe environmental concerns and process efficiency,electrolysis is found to be more promising compared to steam reformingof fossil-based resources. Currently, the most commonly used electrolysistechnology is alkaline water electrolysis.78 In contrast to low efficiency, biomass gasification is consideredto be a promising method to obtain hydrogen-rich syngas.79
Chen et al. have evaluated the effect of experimental conditionson the production of optimal H2 and other gases such asCO, CO2, and CH4 through the gasification ofmunicipal solid waste (MSW).107 In thestudy, temperature, equivalence ratio (ER), and residence time werechosen as the independent variables in the central composite designto investigate the yield of gases, char, and tar. The optimized H2 yield of 41.36 mol % efficiency occurred when experimentalconditions were held at 757.65 C with an ER of 0.241 for 22.36min. Based on statistical analysis and experimental results, usingair as a gasifying agent effectively resulted in both qualitativeand quantitative products. For instance, for a steam-to-biomass ratioof 1, mole fractions of CO and H2 are 0.52 and 0.15 at650 C, while 0.27 and 0.58 mole fraction of CO and H2 is reached at 900 C, respectively.108 Singh and Yadav studied steam gasification of mixed food waste at700 C.109 They employed torrefactionas a pretreatment method to improve the physicochemical propertiesof the mixed food waste. In the study, the steam-to-biomass ratiowas chosen as 1.25, and the steam ratio was held constant at 0.625.Their results showed that syngas production increased with the increasingtemperature of torrefaction. Torrefied food waste at 290 C gavethe highest hydrogen yield with 2.15 m3/kg.
K is one of the alkalimetals that is used as a catalyst in gasification.It is considered the most active catalyst among the alkali metals.K accelerates the diffusion of the gasifying agent into carbon andthus leads to the formation of microstructures and thereby resultsin an increased reaction rate.166 Zhanget al. studied sorption-enhanced gasification of tobacco stalks byusing steam as the gasification agent in a fixed-bed reactor.173 The effects of temperature, catalyst type,and catalyst loading were evaluated for hydrogen production. Whenusing the selected catalysts of K2CO3, CH3COOK, and KCl, increasing the temperature from 600 to 700C and the loading of K2CO3 and CH3COOK enhanced the effects of the catalyst on the gasificationof biomass for hydrogen production. On the contrary, increasing theloading of KCl resulted in a decrease in hydrogen yield and carbonconversion because of the inhibition of the gasification process.The maximum carbon conversion efficiency of 88.0% and hydrogen yieldof 73.0% were achieved by applying 20 wt % of K loading in the K2CO3 precursor at 700 C.
In order toimprove syngas quantity and reduce the tar content,alkali and alkaline-earth catalysts are applied to algal biomass (Cladophora glomerata L.) through the steam gasificationprocess.174 NaOH, KHCO3, Na3PO4, and MgO commercial catalysts were used inthe process, of which NaOH was found to be superior for hydrogen productionand also contributed to the conversion of char and tar decomposition.Increasing the temperature from 700 to 900 C resulted in decreasingthe tar content in the produced gas and increasing hydrogen yield.
Thanks to its high mechanical strength, olivine can be used asa primary catalyst to decrease tar content. Olivine ((Mg, Fe)2SiO4) includes natural iron oxides that stronglyinfluence the olivine catalytic activity. Rapagnà et al. studiedcatalytic steam gasification of almond shells in a bubbling fluidized-bedgasifier by using 10 wt % of Fe/olivine catalyst synthesized via theimpregnation method.190 The experimentswere performed at temperatures between 800 and 830 C, and blank(olivine) was used for comparison of the effect of a catalyst on gasand hydrogen yield. According to the results, a 10 wt % Fe/olivinecatalyst exhibited superior stability and increased both gas and hydrogenyield by almost 40% and 88%, respectively. Furthermore, a 16% reductionof CH4 was achieved. Both reforming activity and reductionin tar concentration were realized by the Fe/olivine catalyst. Whenboth economic and environmental causes are considered, the Fe/olivinecatalyst is a good option for eliminating tar formation in the gasificationprocess.
Peng et al. studiedair-steam gasification of the wood residue using a research-scalefluidized bed. Two different types of metal catalysts (Ni/CeO2/Al2O3) at different catalyst loadings(20, 30, and 40%) were examined for catalytic activity. To investigatethe effect of process parameters on the catalytic activity, differentresidence times (20, 40, and 60 min) and gasification temperatures(750, 825, and 900 C) were examined. In parallel, noncatalyticexperiments were also carried out to decide the optimal conditionsthat increase tar cracking and enriched hydrogen/syngas production.According to their results, the high temperature (900 C) andhigh catalyst loading (40%) are suitable for tar cracking and enrichinghydrogen/syngas production.197
Not only is the char surface an essential aspect of tar reformingbut also the porous structure of char is an important factor. Buentello-Montoyaet al. investigated the porous structure of regular char obtainedby pyrolysis and activated char that was activated physically usingCO2 for tar reforming at temperatures between 650 and 850C.216 Their results show that highertar conversion was achieved using activated char at 650 and 750 C,while it presented more deactivation than the regular biochar. Ata higher temperature (850 C), two biochar catalysts exhibitedthe same performance, and tar (mixture of benzene, toluene, and naphthalene)removal efficiency reached 90% within the 3 h experiment duration.In contrast, the mesoporous and microporous chars exhibited higherinitial tar conversion, but coking occurred due to rapid deactivation.The study proved that meso- and macroporous biochars are applicablealternatives for tar steam reforming. One of the advantages of charis its good catalytic activity due to its active sites, carbon structure,and alkali and alkaline-earth metal content. Furthermore, when manyactive metal oxides were loaded on char, it creates char-supportedcatalysts, and the catalytic activity of char is enhanced.156 2ff7e9595c
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