Utilization and energy density of coal

Utilization of coal

Although the greenhouse gases released by coal combustion are one of the causes of global warming, coal is still used as the preferred energy source. Besides power generation, the most important uses of coal are concentrated in steel production, the cement industry, and the production of liquid fuels. Global coal consumption has continued to increase since 2000. In 2010 alone, the world consumed 6.1 billion tons of hard coal and 1 billion tons of lignite. Different types of coal are used for different purposes. For more knowledge about the classification of coal, you can check the previous article. Thermal coal (also known as thermal coal, which is a hard coal) is primarily used for power and heat production. Coking coal (also known as metallurgical coal, also classified as hard coal) is mainly used in coke (mainly bituminous) and steel production. Alumina refineries, paper industry, chemical and pharmaceutical industries are also important coal users. Coal by-products can also be used to produce a variety of chemicals. Refined coal tar can be used to produce chemicals such as creosote, naphthalene, phenol and benzene. Ammonia gas recovered from coke ovens can be used to make ammonium salts, nitric acid and agricultural fertilizers. Coal or coal by-products are found in thousands of products: soaps, aspirin, solvents, dyes, plastics and fibers such as rayon, nylon, etc. Coal is also an important raw material for the production of some special products, such as activated carbon (for water and air purification filtration and “kidney dialysis machines”, etc.); carbon fiber (a lightweight, strong reinforcement used in construction, mountain bikes, tennis rackets, etc.); silicon metal (used in the production of silicone resins and silanes, which in turn are used in the production of lubricants, water repellants, resins, cosmetics, shampoos and toothpastes, etc.).

Energy density

The energy density (ie calorific value) of coal is about 24MJ/kg, which can also be expressed in units of electrical energy (kWh). 1kWh is equal to 3.6MJ, and the energy density of coal after conversion is equal to 6.67kWh/kg. A typical coal-fired power plant has a thermal efficiency of 30%, so about 2.0kWh/kg (6.67kWh/kg x 30%) of the energy in the coal can be converted into electricity, and the rest is waste heat. Coal-fired power plants can generate close to 2.0kWh of electricity per 1kg of coal burned (Glen Elert).

Example: A 100W incandescent lamp needs 876kWh of electricity for one year (100W×24h/day×365days/year=876000Wh=876kWh). If these electrical energy are provided by coal-fired power generation, the coal consumption is

876kWh/(2.0kWh/kg) =438kg

If the power generation efficiency of coal-fired power plants is improved, the coal consumption to maintain 100W incandescent lamps for one year will be relatively reduced. Among them, the transmission and distribution losses caused by the resistance and thermal effects of the cables should also be considered. This part of the loss accounts for 5% to 10% of the total power consumption, and is related to the distance from the power station and other factors.

1. Carbon intensity

The carbon content of general commercial coal is more than 70%, and the carbon content of the coal with the calorific value of 6.67kWh/kg mentioned above is about 80%. Therefore, the amount of carbon in 1kg of coal is

(0.8kg)/(12kg/kmol) =2/30kmol

In the formula, 1mol is equal to NA atoms, and NA is Avogadro’s constant.

When coal is burned, the carbon in the coal combines with the oxygen in the atmosphere to generate carbon dioxide, and its molecular weight is (12+16×2) kg/kmol=44kg/kmol. Therefore, the mass of carbon dioxide released from burning 1 kg of coal is about

(2/30) kmol × 44kg/kmol = 88/30kg ≈ 2. 93kg

This can be used to calculate CO2 emission factors for coal-fired power plants. Because the effective energy of 6.67kWh/kg of coal is about 31%, burning 1kg of coal can generate 2kWh of electricity, while burning 1kg of coal will emit 2.93kg of CO2, so the direct CO2 emission intensity of coal-fired power plants is about 1.47kg/kWh or 0.407kg/MJ.

2. Calorific value estimation

The calorific value Q of coal is the heat released by its complete combustion in the presence of oxygen. Q is a complex function of the elemental composition of coal. The calorific value Q can be measured with an experimental calorimeter. Dulong gave an approximate formula for the calculation of Q when the oxygen content is less than 10%

Q=337C + 1442 (H – O/8) +93S

In the formula, C is the percentage of carbon in the coal; H is the percentage of hydrogen in the coal; O is the percentage of oxygen in the coal; S is the percentage of sulfur in the coal (all percentages are by mass). The unit of Q in the formula is kJ/g.