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1. what is coal?
Coal is a hard black substance that is extracted from the ground and burned as fuel.

2. Types and Ranks of coal
The rank of coal is based on the degree to which the orginal plant material has been transformed into carbon and can be seen as a rough indication of how old the coal is: the older the coal the higher the carbon content (generally). The ranks of coal (from most to least carbon content) are as follows: anthracite, bituminous coal, subbituminous coal, and lignite. The coal with the highest carbon content is the best and cleanest type of coal to use. As you move down the coal rank the heat given out decreases and the dirtyness of the fuel and moisture content increases.
Lignite coal Used almost exclusively for electric power generation lignite is a young type of coal. Lignite is brownish black, has a high moisture content (up to 45 %), and a high suphur content. Lignite is more like soil than a rock and tends to disintegrate when exposed to the weather. Lignite is also called brown coal. Lignite has a colorific value of less than 5 kw/kg approximately.
Subbituminous coal Subbituminous coal is also called black lignite. Subbituminous coal black and contains 20-30 % moisture. Subbituminous coal is used for generating electricity and space heating. Subbitumnious coal has calorific values ranging from 5 - 6.8 kW/kG approximately.
Bituminous coal Bituminous coal is a soft, dense, black coal. Bituminous coal often has bands of bright and dull material in it. Bituminous coal is the most common coal and has a moisture content less than 20 %. Bituminous coal is used for generating electricity, making coke, and space heating. Bituminous coal has calorific values ranging from 6.8 - 9 kW/kG approximately.
Anthracite coal Often referred to as hard coal, anthracite is hard, black and lustrous. Anthracite is low in suplhur and high in carbon. It is the highest rank of coal. moisture content generally is less than 15 %. Anthracite has a calorfific values of around 9 kW/kG or above.

煤
煤是最主要的固体燃料，是可燃性有机岩的一种。它是由一定地质年代生长的繁茂植物，在适宜的地质环境中，逐渐堆积成厚层，并埋没在水底或泥沙中，经过漫长地质年代的天然煤化作用而形成的。煤的含碳量一般为46～97%，呈褐色至黑色，具有暗淡至金属光泽。

煤有褐煤、烟煤、无烟煤、半无烟煤等几种。云南常用的是褐煤、烟煤、无烟煤三种。煤的种类不同，其成分组成与质量不同，发热量也不相同（表4-15）。单位重量燃料燃烧时放出的热量称为发热量，人为规定以每公斤发热量7000千卡的煤作为标准煤，并以此标准折算耗煤量。 （1）褐煤：多为块状，呈黑褐色，光泽暗，质地疏松；含挥发分40%左右，燃点低，容易着火，燃烧时上火快，火焰大，冒黑烟；含碳量与发热量较低（因产地煤级不同，发热量差异很大），燃烧时间短，需经常加煤。 （2）烟煤：一般为粒状、小块状，也有粉状的，多呈黑色而有光泽，质地细致，含挥发分30%以上，燃点不太高，较易点燃；含碳量与发热量较高，燃烧时上火快，火焰长，有大量黑烟，燃烧时间较长；大多数烟煤有粘性，燃烧时易结渣。 （3）无烟煤：有粉状和小块状两种，呈黑色有金属光泽而发亮。杂质少，质地紧密，固定碳含量高，可达80%以上；挥发分含量低，在10%以下，燃点高，不易着火；但发热量高，刚燃烧时上火慢，火上来后比较大，火力强，火焰短，冒烟少，燃烧时间长，粘结性弱，燃烧时不易结渣。应掺入适量煤土烧用，以减轻火力强度。 煤的种类很多，按成煤原始物质的不同，可分为腐植煤、腐泥煤、腐植腐泥煤和残植煤等(见煤化学)。商业上，将商品煤分为原煤、筛选煤和洗选煤等。原煤是从矿井或露天矿采出没有经过加工的煤；筛选煤是原煤经过筛选加工分级，去除部分煤矸石，并根据煤的粒径大小分为大块煤、中块煤、小块煤和粉煤；洗选煤是原煤经过水洗加工，除去煤中大部分矿物杂质的产品。根据洗选后煤中灰分的不同，分为洗精煤、洗中煤、煤泥和尾矿（也称洗矸）。其中洗精煤灰分最低，是质量较好的煤，一般用作炼焦原料；洗中煤灰分较高，多作为电厂燃料；煤泥因粒径很细，含水分和灰分又较高，只能作为民用或一般锅炉燃料；尾矿则废弃不作商品，但有些矿区也作为劣质燃料用。中国南方还有石煤，它是一种灰分很高的腐泥煤，在燃料缺乏的地区，石煤也可就地当作燃料使用。