Coal slurry

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Coal slurry is a mixture of solids (mined coal or coal waste) and liquids (water or organic) [1] produced by a coal preparation plant.

Contents

Preparation

To transform the coal ash into a slurry, coal is separated from non-combustible components and can be fractionated by particle size as well. Coal slurry can be transferred by pipeline or with specialized pumps such as a progressive cavity pump to pump the highly abrasive, corrosive and viscous coal slurry. [2] More than 7 billion tons of coal are mined per year (2010), using approximately 200 litres of water per ton. [3] However, the amount of water required hinges on the surface characteristics of the coal being used. Most coal slurries require the addition of a surfactant to reduce the viscosity, ergo reduce the stress on pipelines and pumps. [4]

Recent studies have employed new methods of slurry preparation, like using ultrasonic irradiation and a mixture of natural and synthetic surfactants to improve the stability and rheological properties of coal slurry. [5] [6]

Environmental concerns

A 2014 coal slurry spill into a river at Patriot Coal, West Virginia. A straw impoundment has been created across the river to try to hold the spillage. Coal slurry spill at Patriot Coal, WV.jpg
A 2014 coal slurry spill into a river at Patriot Coal, West Virginia. A straw impoundment has been created across the river to try to hold the spillage.

Ideally, coal slurry consists only of crushed coal and water, which can be efficiently separated. In practice, the separation is significantly costly due to the large amounts of water needed and wastewater generated by the process. [8] Furthermore, the slurry consists also of very fine coal dust that results in a waste called blackwater. As blackwater cannot be purified by a water treatment plant, [9] it is stored in large impoundment ponds. Such ponds are susceptible to disastrous releases, such as the Buffalo Creek flood of 1972 or the Martin County coal slurry spill of 2000, which released over 250 million gallons of coal slurry. [10] Coal slurry can contain hazardous chemicals such as arsenic and mercury and can kill aquatic wildlife, as was the case in the Martin County spill. [11] This impounded liquid waste can sometimes total billions of gallons [12] in a single facility.

The scope of coal slurry applications

To date, coal slurry fuels are recognised to have low energy density and therefore can only be successfully combusted in high compression engines such as diesel or gas turbine power plants (large engines with low energy density requirements). Other engine systems include slow speed diesel engines and turbines used as power plants for shipping and stationary electricity production. [13] However, in the combustion market, for small and medium power plants ranging from 20 kW (27 hp) to 5 MW (6,700 hp), the utilisation of CSs will require boiler’s retrofitting. [14]

Other applications found for these slurries are in systems such as boilers, gasifiers and stationary engines with specific requirements divided into two main areas: chemical and physical as shown in table below.

AspectParameterApplication
Fuel chemistry [15] Low N < 0.6 wt% (db.)

Low S < 0.1 wt% (db.) (limit for corrosion)

Boiler
Ash < 0.01% wt% (db.)Diesel engine (No. 2 diesel fuel)
Bulk density 0.6-0.9 ton/m3 or higherBoiler and gasifier
Physical and handling property [15] Viscosity ≤1000 mPa at shear rate of 100s−1 at 25 °C (77 °F) (desirable for fuel handling).Boiler

See also

Related Research Articles

<span class="mw-page-title-main">Surfactant</span> Substance that lowers the surface tension between a liquid and another material

Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word "surfactant" is a blend of surface-active agent, coined c. 1950. As they consist of a water-repellent and a water-attracting part, they enable water and oil to mix; they can form foam and facilitate the detachment of dirt.

Thermal depolymerization (TDP) is the process of converting a polymer into a monomer or a mixture of monomers, by predominantly thermal means. It may be catalysed or un-catalysed and is distinct from other forms of depolymerisation which may rely on the use of chemicals or biological action. This process is associated with an increase in entropy.

<span class="mw-page-title-main">Pyrolysis</span> Thermal decomposition of materials

The pyrolysis process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere.

<span class="mw-page-title-main">Gasification</span> Form of energy conversion

Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2). This is achieved by reacting the feedstock material at high temperatures (typically >700 °C), without combustion, via controlling the amount of oxygen and/or steam present in the reaction. The resulting gas mixture is called syngas (from synthesis gas) or producer gas and is itself a fuel due to the flammability of the H2 and CO of which the gas is largely composed. Power can be derived from the subsequent combustion of the resultant gas, and is considered to be a source of renewable energy if the gasified compounds were obtained from biomass feedstock.

<span class="mw-page-title-main">Tailings</span> Materials left over from the separation of valuable minerals from ore

In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.

<span class="mw-page-title-main">Slurry</span> Mixture of solids suspended in liquid

A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal pump. The size of solid particles may vary from 1 micrometre up to hundreds of millimetres. The particles may settle below a certain transport velocity and the mixture can behave like a Newtonian or non-Newtonian fluid. Depending on the mixture, the slurry may be abrasive and/or corrosive.

The Fischer–Tropsch process (FT) is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The Fischer–Tropsch process is an important reaction in both coal liquefaction and gas to liquids technology for producing liquid hydrocarbons.

Coal liquefaction is a process of converting coal into liquid hydrocarbons: liquid fuels and petrochemicals. This process is often known as "Coal to X" or "Carbon to X", where X can be many different hydrocarbon-based products. However, the most common process chain is "Coal to Liquid Fuels" (CTL).

<span class="mw-page-title-main">Synthetic fuel</span> Fuel from carbon monoxide and hydrogen

Synthetic fuel or synfuel is a liquid fuel, or sometimes gaseous fuel, obtained from syngas, a mixture of carbon monoxide and hydrogen, in which the syngas was derived from gasification of solid feedstocks such as coal or biomass or by reforming of natural gas.

<span class="mw-page-title-main">Biomass to liquid</span>

Biomass to liquid is a multi-step process of producing synthetic hydrocarbon fuels made from biomass via a thermochemical route.

Pyrolysis oil, sometimes also known as bio-crude or bio-oil, is a synthetic fuel with limited industrial application and under investigation as substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500 °C (900 °F) with subsequent cooling, separation from the aqueous phase and other processes. Pyrolysis oil is a kind of tar and normally contains levels of oxygen too high to be considered a pure hydrocarbon. This high oxygen content results in non-volatility, corrosiveness, partial miscibility with fossil fuels, thermal instability, and a tendency to polymerize when exposed to air. As such, it is distinctly different from petroleum products. Removing oxygen from bio-oil or nitrogen from algal bio-oil is known as upgrading.

Blackwater is a form of pollution produced in coal preparation. In its purification, coal is crushed in a coal preparation plant and then separated and transported as a coal slurry, From the slurry, incombustible materials are removed and the coal can be sized. After the recovery of the coal particles from this slurry, the remaining water is black, contains very fine particles of coal. This blackwater cannot be processed in a water treatment plant.

<span class="mw-page-title-main">Biochar</span> Lightweight black residue, made of carbon and ashes, after pyrolysis of biomass

Biochar is a carbon-rich residue derived from the pyrolysis of biomass and stands at the intersection of sustainability, agriculture, and environmental stewardship. This versatile material, characterized by its stable carbon composition, emerges as a promising tool in addressing pressing challenges such as soil degradation, carbon sequestration, and agricultural productivity enhancement.

Second-generation biofuels, also known as advanced biofuels, are fuels that can be manufactured from various types of non-food biomass. Biomass in this context means plant materials and animal waste used especially as a source of fuel.

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<span class="mw-page-title-main">Superheated water</span> Pressurized liquid water at temperatures between the boiling and critical points

Superheated water is liquid water under pressure at temperatures between the usual boiling point, 100 °C (212 °F) and the critical temperature, 374 °C (705 °F). It is also known as "subcritical water" or "pressurized hot water". Superheated water is stable because of overpressure that raises the boiling point, or by heating it in a sealed vessel with a headspace, where the liquid water is in equilibrium with vapour at the saturated vapor pressure. This is distinct from the use of the term superheating to refer to water at atmospheric pressure above its normal boiling point, which has not boiled due to a lack of nucleation sites.

Coal-water slurry fuel is a combustible mixture of fine coal particles suspended in water. It can be used to power boilers, gas turbines, diesel engines and heating and power stations.

<span class="mw-page-title-main">Hydrothermal carbonization</span>

Hydrothermal carbonization (HTC) is a chemical process for the conversion of organic compounds to structured carbons. It can be used to make a wide variety of nanostructured carbons, simple production of brown coal substitute, synthesis gas, liquid petroleum precursors and humus from biomass with release of energy. Technically the process imitates, within a few hours, the brown coal formation process which takes place in nature over enormously longer geological time periods of 50,000 to 50 million years. It was investigated by Friedrich Bergius and first described in 1913.

Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet biomass, and other macromolecules, into crude-like oil under moderate temperature and high pressure. The crude-like oil has high energy density with a lower heating value of 33.8-36.9 MJ/kg and 5-20 wt% oxygen and renewable chemicals. The process has also been called hydrous pyrolysis.

A solid bowl centrifuge is a type of centrifuge that uses the principle of sedimentation. A centrifuge is used to separate a mixture that consists of two substances with different densities by using the centrifugal force resulting from continuous rotation. It is normally used to separate solid-liquid, liquid-liquid, and solid-solid mixtures. Solid bowl centrifuges are widely used in various industrial applications, such as wastewater treatment, coal manufacturing, and polymer manufacturing. One advantage of solid bowl centrifuges for industrial uses is the simplicity of installation compared to other types of centrifuge. There are three design types of solid bowl centrifuge, which are conical, cylindrical, and conical-cylindrical.

References

  1. ↑ Shin, Yu-Jen; Shen, Yun-Hwei (1 June 2007). "Preparation of coal slurry with organic solvents". Chemosphere. 68 (2): 389–393. Bibcode:2007Chmsp..68..389S. doi:10.1016/j.chemosphere.2006.12.049. ISSN   0045-6535. PMID   17276487.
  2. ↑ Admin. "Transferring coal slurry made easy". www.globalpumps.com.au. Retrieved 11 April 2019.
  3. ↑ The Coal Handbook: Towards Cleaner Production. Volume 1; Coal Production. Osborne, D. G. Cambridge: Woodhead Publishing Ltd. 2013. ISBN   9780857097309. OCLC   875224821.{{cite book}}: CS1 maint: others (link)
  4. ↑ Guo, D (1 February 1998). "Rheological behaviour of oil-based heavy oil, coal and water multiphase slurries". Fuel. 77 (3): 209–210. doi:10.1016/s0016-2361(97)00172-5. ISSN   0016-2361.
  5. ↑ Das, Debadutta; Dash, Uma; Meher, Jibardhan; Misra, Pramila K. (1 September 2013). "Improving stability of concentrated coal–water slurry using mixture of a natural and synthetic surfactants". Fuel Processing Technology. 113: 41–51. doi:10.1016/j.fuproc.2013.02.021.
  6. ↑ Guo, Zhaobing; Feng, Ruo; Zheng, Youfei; Fu, Xiaoru (1 July 2007). "Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation". Ultrasonics Sonochemistry. 14 (5): 583–588. doi:10.1016/j.ultsonch.2006.12.001. ISSN   1350-4177. PMID   17236802.
  7. ↑ Conlon, Kevin (12 February 2014). "Officials: Coal slurry spill blackens 6 miles of West Virginia creek". CNN. Retrieved 2 May 2017.
  8. ↑ Andrews, Graham F., and Karl S. Noah. "The Slurry-column Coal Beneficiation Process." Fuel Processing Technology 52.1-3 (1997): 247-66. Print.
  9. ↑ Shiao-Hung Chiang and James T. Cobb "Coal Conversion Processes, Cleaning and Desulfurization" in Kirk-othmer Encyclopedia of Chemical Technology Wiley-VCH, 2000. doi:10.1002/0471238961.0312050103080901.a01
  10. ↑ Kilborn, Peter T. "A Torrent of Sludge Muddies a Town's Future." The New York Times. The New York Times, 25 Dec. 2000. Web. 25 Apr. 2019.
  11. ↑ Leung, Rebecca. "A Toxic Cover-Up?" CBS News. CBS Interactive, 01 Apr. 2004. Web. 25 Apr. 2019.
  12. ↑ "Coal Sludge Impoundments, West Virginia." NASA. NASA, 18 Mar. 2006. Retrieved 10 Apr. 2019.
  13. ↑ Gary K Ellem (12 December 2023). "A NOVEL 2ND GENERATION LIQUID BIOFUEL PRODUCTION CONCEPT USING BIOMASS CHAR". Chemeca 2010: Engineering at the Edge; 26–29 September 2010, Hilton Adelaide, South Australia.
  14. ↑ Bridgwater, A. V.; Grassi, G. (6 December 2012). Biomass Pyrolysis Liquids Upgrading and Utilization. Springer Science & Business Media. ISBN   978-94-011-3844-4.
  15. 1 2 Abdullah, Hanisom; Mourant, Daniel; Li, Chun-Zhu; Wu, Hongwei (21 October 2010). "Bioslurry as a Fuel. 3. Fuel and Rheological Properties of Bioslurry Prepared from the Bio-oil and Biochar of Mallee Biomass Fast Pyrolysis". Energy & Fuels. 24 (10): 5669–5676. doi:10.1021/ef1008117. ISSN   0887-0624.
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