Oil and Gas Firing

Liquid and gaseous fuels are burner fired. Solid fuels when pulverized and mixed with air are also amenable to burner firing; this is covered in detail in Chapter 13. The new duct burners are used to fire light oils and gaseous fuels inside the confines of a duct. This chapter deals with the combustion of liquid and gaseous fuels in burners, whereas the firing of solid fuels in burners is discussed in Chapter 13.

Great strides were made with burner firing of oil and gas, and very large boilers were built in the United States and Europe for both power and process. Although oil is found in several countries, it is most abundant mainly in the Middle East, Venezuela, Nigeria, and Russia, which are politically sensitive. Since the early 1970s, the uncertainties of oil sup­plies and greatly fluctuating prices have slowly moved the markets away from oil. Mean­while, natural gas (NG) has eclipsed even the most dominant fuels for over a decade. However, the use of NG has been in gas turbines (GTs) in combined cycle (CC) mode and not in boilers in conventional steam cycles. As a result, oil and gas firing in boilers is mostly for process and cogeneration (cogen) in refineries and petrochemicals. Some large oil-fired boilers have been built to handle refinery residue and other difficult applications.

• Oil and gas are the most desirable fuels but they are expensive, not produced in sufficient quantities, and riddled with geopolitics.

• Fuel oil (FO) packs the highest amount of heating value in the smallest volume and is the easiest to transport and store.

• Natural gas is excellent for combustion, as its heating value is high and it is the prime fossil fuel that produces the least CO2 and NOx.

• FO and NG are clean (gas more than oil) and easy to burn and exhibit rapid com­bustion characteristics.

• Both installation and O&M are very simple in comparison with solid fuel-fired boilers.

Even the synthetic fuels and waste gases, which may not be as clean or as rich as the fossil-fuel oils and gases, are easier to handle and burn. These features combine to make it possible to produce very compact boilers via shop assembly. Site work is minimal. Even for larger sizes, which cannot be assembled in the shop, because of size or transport considerations, a great deal of modular fabrication simplifies the site assembly. Package and modular boilers, possible mainly with clean and simple fuels such as oil and gas, have

1. Improved the equipment ratings

2. Enhanced the build quality

3. Reduced the delivery times

4. Raised the reliability levels of O&M

Although FO — and NG-fired boilers have shown growth, oil and gas firing is extensively employed in almost every type of boiler.

• Most solid fuels cannot start combustion on their own and need start-up support. Stoker PF, bubbling fluidized bed combustion (BFBC) boilers, and circulating flu — idized bed combustion (CFBC) boilers require oil or gas for ignition.

• All solid fuels have definite load turndown, and below the minimum load, the firing becomes unstable. Oil and gas firing is employed when such low loads are encountered.

• Pulverized fuel boilers have small turndowns of 1:2 or 1:3 at best. Also, with wet coals in monsoons at —50% load, they experience flame instability and oil or gas firing has to be pressed into service.

• With manufactured or waste fuels of limited availability, evaporation shortages and peak loads can only be met by oil and gas firing.

• With seasonal agrofuels, oil and gas are used during off-season unless some other agrofuel is available. Coal firing is not as economical.

• Supplementary firing is quite common to incorporate in gas-based cogen plants to get some additional evaporation or to generate full steam when the GT is down.

Burner firing also has good potential in firing a variety of waste liquid and gaseous fuels and forms the backbone of the waste heat boilers.