Fuels and Ash

The entire power plant is built around the fuel, and there is perhaps no other parameter more important. A thorough understanding of the fuel—its burning characteristics, trans­portation inside and outside the boiler plant, its ash generation and disposal, the environ­mental aspects, and so on—is necessary for design, selection, operation, and maintenance of the boiler and the power plant. Only those fuels that are primarily used in boilers are discussed in this chapter.

The subject of fuels is vast when aspects such as their availability, mining, treatment, transportation, storage, consumption, by-products, and environmental issues are all con­sidered. The topic becomes more unpredictable and difficult when issues of pricing and geopolitics are considered, particularly for oil and gas. However, this chapter is confined to how a combustion engineer looks at the fuel scenario from the point of burning it in boilers.

For large power plants, the focus is with the prime fuels, as the power costs are to be minimized and the prime fuels are the most competitive. Coal has been the main fuel for power plants in many parts of the world except in areas where oil and gas are available in abundance, such as the Middle East. For some time in the 1960s oil had become abundant, but the oil crisis of the early 1970s has changed this condition permanently, making oil an expensive fuel. Coal reasserted its position since the beginning of this decade as the most dependable relatively low cost fuel; this traditional balance has been disturbed with natu­ral gas (NG) displacing coal as power plant fuel, even in areas where gas was not popular based on the following reasons:

• Greater gas production at more competitive pricing

• Easier availability due to more gas pipes and shipping of liquefied NG (LNG)

• Bigger combined cycle (CC) plants operating at very high overall efficiency

• Power generation in very short periods due to faster deliveries of gas turbines (GTs)

Now coal is again finding favor, as the gas prices climb.

For industry the fuel scene is a little different. Both power and steam are required to be produced competitively, but to a lesser scale as compared with the utilities. Prime fuels are important only when substantial captive or cogenerated power is involved. At other times, for process needs there are various waste or manufactured fuels that can be used. Fuel cost is not always as important to the industry as it is to the utility, as power and steam are only some of the several inputs to the end product.

The fuels can be classified as follows:

1. Fossil fuels such as various grades of coals, oils, and NG

2. Waste fuels such as biofuels, process gases, and spent liquors

3. Manufactured fuels such as producer gas, town gas, and liquefied petroleum gas (LPG)

Fuels can also be classified as solid, liquid, and gaseous. Naturally, the fossil fuels are chiefly used. But in absolute quantity, the use of the other fuels is progressively

Increasing. Manufactured fuels are not used in any appreciable manner for steam gen­

Eration based on cost and availability considerations and hence are not discussed in this chapter.

Fossil fuels are not uniformly available across the globe. The estimates of the fuel reserves and how long they would last have often gone wrong, particularly in the developing countries, where the data are still being updated due to exploration activi­ties. Fuel production, transportation, and pricing are mixed with politics and display extreme volatility. This is particularly a characteristic of crude oil and, to some extent, of NG. It has also not been possible to predict the behavior of the fuel market. The large utili­ties worldwide therefore have found it necessary to diversify fuels and their sources. Also, to gain further economy, multifuel firing is being demanded wherever feasible.

While the fuel decides the type and shape of the boiler, the fuel ash governs the size and disposition of heating surfaces (HSs). The effects of ash are most severe in coal-fired boilers, as coal has the highest ash content compared to all the other fuels. Good coal for burning should also have a benign ash that does not slag, foul, or cause erosion or corro­sion. Ash is as important as any fuel.

Fuels and combustion are closely related. Combustion aspects of fuels are discussed in Chapter 2.

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