Water Tube Boilers for Industry and Power

Water tube boilers are made

• From ~5 tph to —4500 tph capacity

• At steam pressures ranging from —3.5 to 320 bar

• At steam temperatures ranging from saturation to —620°C

The lower end of the range up to —400 tph is employed for its process steam, captive power, and cogeneration (cogen) needs. Beyond 400 tph, where reheating of steam becomes viable, boilers are mainly used for utility application. This dividing line is thin and tentative, and exceptions are found on both sides of this arbitrary demarcation. With cogen on the rise and very large plants gaining popularity, what a utility boiler was in the past has become an industrial unit today and this progression is only going to continue.

Characteristic Features of Industrial Boilers

• Variety of configurations are required to meet a wide range of steaming conditions and fuels.

• Fuels burnt include waste and manufactured fuels found in an array of industries and biofuels.

• These boilers are for meeting process steam requirements, captive power, or both (cogen). In cold countries, steam for district heating is another application.

• Steaming conditions are more modest, usually, with maximum continuous rat­ing (MCR) <400 tph, superheater outlet pressure (SOP) <150 bar, and superheater outlet temperature (SOT) <565°C, with no reheat.

• Often multifuel flexibility is required to use the available fuels; optimize the cheapest fuel; use the fuel of the season, in case of biofuels; or increase the reli­ability of the fuel-supply chain.

• When used for generating captive power or in cogen mode, they are expected to integrate with the dynamics of the main plant seamlessly.

• The process leads and the boiler are required to follow. The boiler process control and the dynamics are aligned to match this requirement.

• Availability of the boiler plant is vital for the process industry, even at the cost of fractionally better efficiency. Multiple boilers of smaller size are many times preferred to large-sized single units for the sake of availability, even if this arrange­ment is more expensive.

Besides a steady degeneration of quality of prime fuels available to industry at com­petitive prices, there is an increase in waste fuels from industry, as process indus­tries are becoming larger, more complex, and more varied. With greater attention that biomass firing is receiving, the range of biofuels turned over to boilers for cofiring is also increasing.

Characteristic Features of Utility Boilers

• They are optimized for best performance and highest reliability.

• Their fuel flexibility is rarely the prime consideration.

• They are made to the largest sizes that the grid can bear and to the highest steam­ing conditions that give the best efficiencies, so that the power generation is most reliable and least expensive.

• Their steaming conditions can be as high as MCR —4500 tph, SOP up to —320 bar, and SOT —620°C.

• Plant dynamics are governed by the grid fluctuations.

• Utility boilers come in relatively fewer configurations compared to the industrial boilers.

Large utility boilers based on oil and gas were built in the 1960s. Since then two unfavor­able developments have taken place:

• The oil crisis of the early 1970s has made oil too expensive for power generation.

• The increased reliability of gas turbines (GTs) with exceptionally rapid growth in unit sizes has firmly driven out oil — and gas-based steam cycles in favor of com­bined cycle power plants (CCPPs).

Utility boilers for power today are based on coal when gas-based power is not viable.

Heat recovery steam generators (HRSGs) fall between industrial and utility boilers. They generate steam for driving turbines like any utility boiler, with the main difference in that they do not burn any fuel but use the hot exhaust gases from the GTs. In the largest sizes they have main and reheat steam temperatures of the same range as utility boilers and they even work on sliding pressure mode. Yet their pressures are no more than —170 bar, and in size they are like large industrial boilers. They are like waste heat boilers receiving exhaust gases from GTs, and hence are considered slave units of a process. Large boilermakers, therefore, treat HRSG more as an advanced class industrial boiler for utility application. In fact, the HRSG is the only waste heat boiler of the utility market. Table 9.1 compares the industrial and utility boilers.