Editing Coke (fuel) (section)

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[[Bituminous coal]] must meet a set of criteria for use as [[Metallurgical coal|coking coal]], determined by particular [[coal assay]] techniques. These include moisture content, ash content, [[sulphur]] content, volatile content, [[tar]], and [[plasticity (physics)|plasticity]]. The goal is to achieve a blend of coal that when processed will produce a coke of appropriate strength (generally measured by [[coke strength after reaction]]), while losing an appropriate amount of mass. Other blending considerations include ensuring the coke will not swell too much during production and destroy the coke oven through excessive wall pressures.<ref>{{Cite web |last=Brandcodax |date=2023-07-19 |title=Understanding Volatile Matter in Coal |url=https://www.nationalcoal.co.za/post/volatile-matter-in-coal |access-date=2024-12-12 |website=National Coal |language=en}}</ref>

The greater the volatile matter in coal, the more by-product can be produced. It is generally considered that levels of 26–29% of volatile matter in the coal blend are good for coking purposes. Thus, different types of coal are proportionally blended to reach acceptable levels of volatility before the coking process begins. If the range of coal types is too great, the resulting coke is of widely varying strength and ash content, and is usually unsaleable, although in some cases it may be sold as an ordinary heating fuel. As coke has already lost its volatile matter, it cannot be coked again.<ref>{{Cite web |last=Brandcodax |date=2023-07-19 |title=Understanding Volatile Matter in Coal |url=https://www.nationalcoal.co.za/post/volatile-matter-in-coal |access-date=2024-12-12 |website=National Coal |language=en}}</ref>

Coking coal is different from thermal coal, but arises from the same basic coal-forming process. Coking coal has different [[macerals]] from thermal coal, i.e. different forms of the compressed and fossilized vegetative matter that compose the coal. The different macerals arise from different mixtures of the plant species, and variations of the conditions under which the coal has formed. Coking coal is graded according to its ash percentage-by-weight after burning:

* Steel Grade I (Ash content not exceeding 15%)
* Steel Grade II (Exceeding 15% but not exceeding 18%)
* Washery Grade I (Exceeding 18% but not exceeding 21%)
* Washery Grade II (Exceeding 21% but not exceeding 24%)
* Washery Grade III (Exceeding 24% but not exceeding 28%)
* Washery Grade IV (Exceeding 28% but not exceeding 35%)<ref>{{Cite web |url=http://www.coal.nic.in/content/coal-grades |title=Coal Grades |archive-url=https://web.archive.org/web/20160201122923/http://coal.nic.in/content/coal-grades |archive-date=1 February 2016 |work=Ministry of Coal}}</ref>