PF Power Plant
Boiler Efficiency
Boiler Types
Mills
Burners
Total pulverised fuel (PF) related costs provide the means to directly compare alternative coals. These costs are directly comparable in that the cost calculated for each coal represents equal level of saleable generation. The components of the fuel related costs calculated for an existing plant are as follows.
  • Annual fuel costs - consist of the FOB mine fuel cost and fuel transportation cost.
  • Maintenance costs- associated with maintenance of the steam generator system, air heaters, fuel preparation and firing system, primary air fans, forced draft fans, induced draft fans, particulate removal system, fly ash system, bottom ash system, FGD additive preparation system, FGD system, waste conditioning system, and coal handling system.
  • Plant operating costs - consist of scrubber additive costs, fixative costs, and waste disposal costs.
  • Replacement power costs - due to unit derates, unavailability and changes in auxiliary power requirements.
For further details on the impact of coal quality on power costs see Conroy & Bennett
Boiler Efficiency
The theoretically achievable maximum efficiency of thermal power generation is limited by a simple relation based only on the lowest and the highest temperature of the cycle.This equation is well known as Carnot’s Law.
The higher steam temperatures of Super-Critical (540°C to 600°C)& Ultra-Super Critical (650°C to 720°C) boilers allows these boilers to obtain higher efficiencies than conventional boilers.  Material suitable for the steam tubes at temperatures of super-critical are now widely available and these boilers are now being built.  Research into suitable materials for ultra-super-critical boilers is currently been done in China, Europe, Japan and USA.
Boiler Types
Three different types of pulverised fuel-firing systems are used in large boilers:
  • Horizontal or slightly angled swirl burners located in the front and/or rear walls of the furnace. In a swirl burner the air, normally secondary air, is given a strong swirl about the axis of the burner. The swirling action increases the mixing of combustion gases, air and fuel and produces a short intense flame.
  • Tangential corner-fired burners introduce the fuel and primary air at a tangent to an imaginary circle in the centre of the combustion chamber. They produce a long low intensity flame that swirls about a vertical axis.
  • Vertical or downshot firing burners are located in the roof of the combustion chamber with the flame projected downwards into the combustion chamber. This firing method is normally used for anthracite.
Mills
In a PF boiler system, coal is pulverised to typically 70–75%, passing 75 micron and entrained in preheated primary air for conveying to the burners. In some small-scale operations indirect firingis used, this is when the pulverised coal is stored in a bin before feeding to the burners.  Normally, Hardgrove Grindability Index (HGI) is used to predict mill performance.
The three types of coal pulveriser are generally identified by the speed of their rotation:
  • Low speed mills are of the ball/tube design with a large steel cylinder and a charge of hardened balls. Coal is ground as it is crushed and abraded between the balls.
  • Medium speed pulverisers are typically vertical spindle mills that grind the coal between rollers or balls and a bowl or race.
  • High speed mills have a high-speed rotor, which impacts on and breaks the coal.
The table below shows the preferred coal properties for each type of pulveriser.

  
Pulveriser Type
 Low
speed
 Medium
speed
 High
speed

Example
 Tube
mill
 Vertical
spindle
mill
 Impact
mill
Coal feed top size
mm
25
40
32
Coal moisture
(as) %
0–10
0–20
0–25
Coal ash
(as) %
1–50
1–30
1–15
Coal quartz
content
(as) %
0–10
0–3
0–1
Hardgrove Grindability Index
 
 		 30–50
80–100
40–60
60–100
Abrasion index
mg/kg
50–100
10–60
5–30
Vertical spindle mills are commonly used in large-scale power plants and to pulverise coal for injection into blast furnaces and can have different configurations as shown below. 
graphic
Burners
Conventional Low NOx burners
Low NOX burners are modified swirl burners that create a fuel-rich combustion zone followed by a leaner burnout zone. The degree of NOx reduction that can be achieved for a given coal is limited by the requirement to produce a stable flame and maintain adequate burnout.
Advanced Low NOx burners
These burners create a hot flame envelope around the fuel rich zone near the burner outlet.  This flame envelope promotes the devolatilisation of the coal which releases a greater amount of the coal nitrogen within this fuel rich region leading to lower NOx formation. As shown in the figure below these burners have improved NOx reduction and better flame stability characteristics than the conventional Low NOx burners. Examples of these burners are the CRIEPI's CI-αburnerand Babcock & Wilcox (B&W) DRB-4Z burner.
graphic
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