Pulverized fuel (PF) firing is the most dominant type because of its great versatility and scalability. Since coal and lignites are the most competitive fuels in large parts of the world, the growth and dominance of PF firing for the last eight decades has almost been unassail­able. The bulk of power generation has been and continues to be by PF firing. Only since the early 1990s, gas turbines (GTs) and circulating fluidized bed (CFB) boilers have gained market share. As the environmental friendliness has improved in along with the lowering of costs of milling, firing, and gas cleaning equipment, PF firing is now competing with the newer technologies with renewed vigor.

When a solid fuel such as coal is reduced to the consistency of talcum powder and fired in an open furnace, the resulting combustion is akin to oil or gas firing—in speed, controllability, and heat release. A coal particle burns out between 1 and 2 s, depend­ing on its volatile content, similar to oil, and the combustion is most complete at >99% carbon burnup. For high-volatile matter (VM) coals and lignites, the efficiency can be as high as 99.7%.

This feature has made the PF firing rule the utility market for over eight decades. The roots for PF firing can be traced to its use in the cement industry —200 years ago. The steel industry was the next to use and develop PF firing. The early PF boilers were different from the present-day units, inferior, smaller, and unreliable in milling, firing, controls, and safety, but the main process has remained unchanged. Starting from about 15 MWe size, boilers reached 1300 MWe by the early 1970s. A furious pace of development was witnessed in the 1950s and 1960s, and the world’s largest boiler of —4000 tph was com­missioned in 1973. Since then steady consolidation and development of technology have taken place, greatly improving the flexibility, reliability, environmental friendliness, and controllability. This improvement has also spread to all the auxiliary plants.

Main Features

• Modern direct-fired PF firing involves

— Size reduction and drying in mills

— Transportation from mills to burners

— Combustion in burners and furnaces

Depending on the moisture of fuel and the desired mill outlet temperature, the airheater (AH) is sized or the hot gas inlet chosen (for lignites).

• As the fuel is in powder form, the combustion speeds are high, almost as high as that of oil. With about 5 min or more of fuel storage in mills, there is an ade­quate flow of fuel and air to provide, at times of sudden load demand, a very good response to load variation, almost matching the oil firing.

• Any fuel that is not fibrous or stringy and can be powdered to the desired level, has adequate heating value (HV) to sustain autocombustion, and has enough VM to provide ignition of fuel can be burnt in the PF mode. Coals of all varieties, from anthracites to peat, have been successfully burnt.

• Although PF firing is a very versatile technology, the boiler is fuel specific. For a particular heat duty and fuel, the numbers of mills and burners are selected and an appropriate AH is designed to provide air at the right temperature. This combination of the AH, milling, and firing equipment addresses a certain lim­ited variation in fuel properties that usually exists between the worst and the best fuel.

• Since PF boilers are burner-fired, the ability to fire a variety of oil and gaseous fuels alone or in combination with PF is exceptionally good.

• A modern PF boiler is

— Invariably top supported.

— Built with membrane panels in furnace.

— Provided with platens, division walls, and wing walls to achieve a balance between the effective projected radiant surface (EPRS) and the furnace vol­ume in achieving furnace exit gas temperature (FEGT) low enough to avoid fouling in the SH.

— Built entirely with plain tube heating surfaces (HSs) to prevent erosion with the exception of economizers (ECONs) in which extended surface or gilled tubes are sometimes employed.

— Equipped with tubular or rotary AHs, as elaborated in Section 13.5.

• In terms of fuel handling and firing, a PF boiler is

— Usually matched with the minimum number of the biggest mills that meet the turndown requirements adequately.

— Equipped with usually one spare mill over the minimum number required for the worst fuel, reduced to zero in some advanced countries and increased to two for use with very poor fuels.

— Wall fired or tangentially fired for normal coals and down-shot fired for low- volatile coals and anthracites; usually corner fired for brown coals.

— Provided with belt-type or chain-type volumetric feeders. For brown coals, the plate-type feeder deals with large volumes of fuel.

• The pollution control aspects of PF firing are as follows:

— Low-NOx burners with staged (and hence delayed) combustion techniques with overfire airports (OFAs), together with larger furnace dimensions to accommo­date longer flames and consequently reduced heat flux in plan, have worked in tandem to lower the emissions to acceptable levels. Separate deNOx plants are added only when this effort is inadequate.

— Flue gas desulfurization (FGD) or deSOx plants in the form of gas scrubbers in second pass are installed when high-S fuels are burnt. In situ desulfurization is not feasible.

— Electrostatic precipitators (ESP) are popular for normal — and high-ash coals for achieving —50 mg/N/m3 outlet dust levels, but fabric filters are needed for even lower particulate emissions.

Advantages and Limitations

Advantages

• A maximum combustion efficiency of 99% or higher is achievable.

• With improved modern mills, the fineness is better and power consumption lower.

• Load response is very fast and comparable with oil — and gas-fired boilers.

• All types of coals, from anthracite to lignite and peat, have been burnt efficiently.

• Low-NOx burners, deSOx units, and deNOx units, all working in combination, have made PF boilers environmentally compliant.

PF firing has been the preferred method for solid fuel firing in large quantities for utilities. However, in the last few years, circulating fluidized bed combustion (CFBC) boilers have begun to disturb this equilibrium by offering reliable solutions in the areas not served well by PF firing.

Limitations

The limitations relate to the inability of PF to

• Deal with variation in fuel without risking fouling and slagging of boilers

• Provide multifuel firing

• Fire fuels with >40% total moisture unless there is enough VM such as in brown coals

• Burn very low-volatile fuels such as petcoke

• Combust very poor fuels with

— Gross calorific value (GCV) <2000 kcal/kg

— A burden >65% (ash and H2O)

— Very high S

• Have stepless load turndown of more than —70 to 100%

These limitations of PF boilers were recognized decades ago. In the absence of a better alternative, they have been accepted. With the advent of the CFBC technology, remedial measures have since been found to some extent. Circulating fluidized bed combustion boilers

• Can address all the above issues

• Are nearly as efficient for conventional fuels

• Are more operator friendly, with very few moving parts and controls

• Have lower O&M costs if erosion issues are not encountered

• Offer better environmental friendliness and ensure against emerging requirements

• Present little danger of explosion

For these reasons, they have practically displaced PF boilers from the industrial segment. Now they have begun to push PF boilers away even in the utility segment up to 150 MWe, depending on the pollution norms to be met. A comparison of CFBC and PF boilers is provided in Table 12.5.