Coal fired boiler: How to derive optimum efficiency?

Boiler efficiency is based mainly on the quantity of loss suffered by the system. In case of pulverized, high-capacity coal fired boiler, loss amounts to approximately 12-14 percent. 50 percent of losses are governed by different fuel properties such as moisture present in fuel, hydrogen in fuel including ambient air conditions. The remaining 50 percent losses are attributed to dry gas loss and carbon loss.

In boiler, optimum efficiency can be derived if losses faced of the bare minimum. As 50 percent losses are based on ambient condition and fuel, optimum efficiency can be derived by tuning properly the other 50 percent. This is mostly with dry gas loss and carbon loss.

Carbon loss

This is mainly due to carbon that lies unburned in bottom ash and fly ash. The unburned, generally in bottom ash especially in pulverized fuel firing seems to be of a higher-sized particle. It is also of very high specific gravity, with unburned materials such as Fusinite. The latter is rather embedded within the mineral matter while low melting constituents tend to encapsulate reactive maceral. Pulverised coal fired boiler system generates fly ash, which is approximately 80%-90% of total ash removed. General unburned types are carbonaceous clay, cenospheres and inert macerals.

What are the facts that affect carbon loss?

  • Coal Petrographic characteristics
  • Coal quality and rank
  • Low melting inorganics present in coal ash
  • Carbonaceous shale quantum and characteristics
  • Burners number and types
  • Residence time available to combust in furnace
  • Pulverized coal fitness – Coarse particle percentage
  • Primary air control and Milling system types
  • Burner tilt
  • Excess air present at furnace/burner along with air distribution within furnace/burner.
  • Primary air with secondary air ratios.

There are several factors that influence significantly carbon loss. These are coal’s volatile matter content, boiler type, furnace heat loading (furnace volume/heat input), excess air and mass fraction with regards to pulverized coal that is smaller than 75mm.

Dry gas loss

When total losses is concerned, dry gas contributes to around 40 percent. It results from fuel gases carrying away heat from the boiler. These days, several coal fired steam boiler designers have managed to reduce this loss. It is achieved by recovering heat with low temperature superheated through air pre-heaters and large economizer. Designers tend to choose much lower exit gas temperature, which means air pre-heater size increases.

Boiler tuning to derive optimal dry gas

  • Tune coal combustion to optimal condition.
  • Maintain optimal-excess air level, approximately 20 percent in the industrial boiler.
  • Proper combustion air distribution at tertiary, secondary and primary levels.
  • Understand coal getting fired.
  • Reduce air leakage to boiler/
  • Soot blowing through the heat transfer surface especially at optimal frequency.
  • Maintain optimum mill outlet temperature for reducing air that bypasses air pre-heater.
  • Maintain required coal fitness by about 75 percent via 200 mesh. It should be lesser than 2 percent on fifty mesh sieve.

Moreover, economizer feed-water inlet temperature influences significantly dry gas loss along with ambient air temperature. This, in turn, changes air pre-heater and economizer’s heat recovery pattern.

Minor losses

In boiler, there are experienced very few losses that can be tuned. These are carbon monoxide loss and mill reject loss. The latter is based on the pyrite amount present in coal combined with mill operating condition. Mill rejects tend to have heat value, considered as mill reject loss. Proper maintenance of mill clearances along with optimal condition operation helps reduce it. Carbon monoxide loss arises from incomplete combustion. With the losses being small, current environmental condition recommends that such losses are best kept to the bare minimum. Hence, it will be wise to use top quality coal fired steam generator systems.