Estudio Financiero

Solid fuels fired in industrial and utility boilers include coal (bituminous, anthracite, and lignite or brown coal), paper sludge, biomass (e.g. bagasse, bark, wood), peat, RDF (Refuse Derived Fuel), and municipal waste.

One of the key issues in fuel quality is the heating value. The heating value depends on the fixed carbon content of the fuels.

Solid fuels can be divided into high grade fuel (e.g. bituminous coal) and low grade fuel (e.g. peat and bark). The most typical firing methods of solid fuels are grate firing, cyclone firing, pulverized

STEAM BOILER TECHNOLOGY – Combustion Process Equipment

firing, and fluidized bed firing. Cyclone firing is not common anymore in new boilers due to their high level of NOx formation.

Coal is most widely used fuel in utility boilers. Earlier coal was burned as lumps, but most widely coal is burned as about 0,1 mm particle. Coal quality (LHV) is decreasing as better coal reserves are exhausted. This means that sulfur and ash contents in coal are increasing. With lower grade coals the boiler fouling is becoming more problematic.

Pulverized Coal Firing (PCF)

Coal is mostly burned in pulverized coal firing (PCF), where coal is grinded into a fine particle size and fired in burners, similar to oil and gas burners. Pulverized coal burns like gas and, therefore, fires are easily lighted and controlled. The main advantage of pulverized firing is the high heat release rates and high temperatures that can be achieved. Pulverized firing can be used with very large unit sizes (up to 1000 MWth). The main disadvantage in PCF is that additional units for SOx and NOx control are usually required. Reported NOx emission levels are 100 - 200 ppm for bituminous coal. [4]

Corner PFC burners have rather complicated construction (Figure 6 and Figure 10). Air is inserted through a windbox and the airflow is controlled with dampers. Coal particles are introduced through nozzles with primary air. Air and pulverized coal ports placed sequentially. Typically, oil is used only for the startup.

Figure 10: Variations of the arrangement of corner fired PCF burners. [1]

Wall firing of coal is similar to oil and gas firing. Tertiary or over-fire air (Figure 5) is used in the modern burners to control combustion and lower NOx,

Grate firing

Grate firing is the oldest type of firing and was the main combustion technique up till 1930’s when PFC started to gain hold. In grate or stoker fired boilers, the combustion of solid fuel occurs in a bed at the bottom of the furnace. Primary air is forced through grate and burning bed. Bed burning rate controls combustion process. The benefit of grate firing is that all forms of solid fuel can be fired including crushed coal. Even low grade fuels such as peat and bark can be fired. The main disadvantage of grate firing is the slow change in firing rate. This is because there is relatively large amount of unburned fuel all times at the grate.

Numerous different applications of grate or stoker firing systems exist for burning of different solid fuels. In all cases, the fuel burns on a grate through which some or all the air for combustion passes. The main constructional difference that grates can be divided into is stationary grates and moving (traveling or mechanical) grates.

STEAM BOILER TECHNOLOGY – Combustion Process Equipment

Stationary grates

Stationary grates, such as inclined grates (Figure 11), are more common in small boilers. This was the first grate type. Stationary grates make use of gravity to move the fuel. This requires 30–50° of horizontal inclination [5]. The inclination of the grate depends on the fuel and its ability to flow during combustion. The inclination can change at different locations of the grate. It is typically higher at the upper end of the grate. To complete the burning of fuel, many inclined grates have a small horizontal grate after the inclined section. This section is called the dump grate.

Traveling grate

Instead of gravitation, fuel can be transported by moving belt. This type of grate is called a traveling grate. The traveling grate has solid elements joined to a chain, which moves horizontally and transports fuel. Fuel is commonly fed with a spreader onto the grate. Changing the rate of fuel addition changes the fuel layer thickness. For coal, a suitable thickness is 10–20 cm, and for wood it is 30–90 cm [5]. The speed of the grate is chosen such that the combustion can be completed within the grate. The combustion is often intensified by placing refractory to the walls. The grate is cooled by the primary air. Secondary and tertiary air jets are often employed to control burning.

Mechanical grates

Larger grates (Figure 12) contain moving parts and are equipped with automatic fuel feed and ash removal.

Figure 11: Stationary, inclined grate.

Figure 12: Mechanical grate. [3]

Mechanical grates are almost always inclined. Grate pieces can be mechanically moved horizontally back and forward to facilitate bed movement. A mechanical inclined grate therefore does not have as deep an inclining angle as the stationary grate. A suitable angle is 15° [5]. Regulating the moving speed of fuel on the grate is possible by changing the speed of the grate. The speed can be different at different sections of the grate. A large industrial mechanical grate is seen in Figure 12. The fuel is fed from the right, and the moving grate transports the fuel to the left. The ash ends at a dump grate.

STEAM BOILER TECHNOLOGY – Combustion Process Equipment

A mechanical grate is one of the most typical grates for incinerating municipal waste. Mechanical grates were also used for biomass firing, before fluidized bed boilers became common.

In a step grate, the step construction is made of cast iron grate bars. Air is horizontally introduced between the grate plates. The most famous ‘brand’ of mechanical inclined step grates has been the Kablitz grate.

Spreader design

Spreader firing used to be the most widely used coal burning method. The spreader (Figure 13) consists of a silo from which the fuel is mechanically removed and thrown into the ignited furnace by a mechanical spreader. The spreader stoker has the following parts that regulate the fuel feed (Figure 13):

1. Rotating element for fuel rate setting.

2. Spreader element, which throws the fuel horizontally and with high velocity into the furnace. 3. After being devolatilized and

partially combusted, fuel particles land on the surface of the grate, typically a traveling grate.

Mechanical grate for biofuels

Wärtsilä has a special grate design, patented as the BioGrateTM, which can be described as a rotating conical grate. This grate designed for optimal combustion of biomass fuel with a moisture content as high as 65%. The BioGrate is ideal for burning wet wood residue from sawmills and other wood processing plants. This combustion technology is already in use in 70 plants and saw mills over the world. The output of the BioGrate boiler plants can be designed from 1 MW up to 10 MW In the BioGrate system, the fuel is fed onto the centre of a circular, conical shaped grate from below (Figure 14). The grate is divided into concentric rings with alternate

Figure 13: Spreader design.

Figure 14: BioGrateTM - a rotating conical grate. [6] rings rotating and the rings in between remaining stationary. Alternate rotating rings are pushed hydraulically clockwise or anti-clockwise respectively. This design distributes the fuel evenly over the entire grate with the burning fuel forming an even layer of the required thickness.

STEAM BOILER TECHNOLOGY – Combustion Process Equipment

The water content of the wet fuel in the centre of the grate evaporates rapidly due to the heat of the surrounding burning fuel and thermal radiation from the specially formed brick walls. Gasification and visible combustion of the gases and solid carbon take place as the fuel moves to the periphery of the circular grate. At the edge of the grate ash falls into a water-filled ash basin underneath the grate.

A key issue in highly efficient, low-emission combustion of biofuels is combustion air management. The primary air for combustion and the recirculation flue gas where applicable, are fed from underneath the grate and penetrate the fuel through slots in the concentric rings. Secondary air, and tertiary air if used, is fed above the grate directly into the flame. Air distribution is controlled by dampers and speed-controlled fans to ensure low emissions of NOx and CO with a wide range of different fuels. [6]

Roll grate

Another type of grate is the roll grate (Figure 15). Instead of a stationary surface, the grate consists of large rolls. These mix bed efficiently. Even though roll grates are usually built inclined, they can be built horizontally. Roll grates are used especially in municipal waste incineration.

Figure 15: Roll grate. [3]