Chain or Traveling Grates with Gravity Feeding

A typical gravity-fed TG is shown in Figure 11.6. These stokers were the forerunners of SS and were popular until 1960s when coals were of good quality and process industry requirements were modest. Although they are suitable for good-quality coals and lignites and especially good for abrasive fuels such as coke breeze, they are not very satisfactory for sized or unsized biofuels because of the way the fuel needs to be admitted.

Design Aspects of Mass Burning

• An MB stoker, with nearly 85% combustion taking place on the grate surface, exhibits slow combustion and modest coal burning rates.

• Performance falls sharply when ash and fines in coal increase above the limits. Coal sizing limits are important.

• Specially shaped front and rear refractory arches help in reradiating the heat and directing the hot flue gases onto the fuel bed to aid ignition.

• Coals should be moderately coking or caking and have the following characteristics:

— Top size 32 mm and fines of <35% through 3 mm (which corresponds to <15% through 1 mm).

— As-fired ash limited to 25%.

— As-fired total moisture limited to 18%.

— Minimum dry ash-free (daf) volatiles to be 25%.

• Nearly 85% of coal ash is retained on the stoker as coarse ash whereas 15% leaves as fly ash. Fly ash is relatively gritty.

• Combustion rates are 160 and 185 kg/m2/h (33 and 38 lb/ft2/h) for arch and arch — less settings of the furnace.

• Ash discharge rate should be <350 kg/m/h (230 lb/ft/h) of ash along the width of stoker to contain the unburnt loss to a reasonable figure.

• Undergrate air pressure is —55 mm w. g. (2 M in. w. g.) as the layer of fuel is thick.

Combustion Process in Mass Burning on CG or TG

The combustion process of coal on the grate fed by gravity follows the following three stages:

1. Drying. The fuel loses its moisture and volatile matter (VM) on receiving radiant heat from furnace.

2. Ignition. The devolatilized fuel attains the necessary temperature to catch fire. This heating of fuel arises from radiant heat from the flames above and also the front refractory arch.

3. Burning. The top surface of the fuel burns completely and gets converted to ash.

Combustion on a grate is broadly described as follows:

• Coal is admitted to the stoker by gravity from the fuel hopper in a layer of —150 mm thickness.

• Depth of the fuel bed is regulated by a guillotine-type gate placed at the entrance to the grate.

• The coal hopper is fed from the bunker by mobile traversing chutes or stationary antisegregation chutes to minimize the separation of fines from the coal so that the fuel layer is uniform.

• Air is provided from below at a pressure of 30-40 mm wg. The quantity is varied along the length of the grate by regulating air to the compartments below with their individual dampers. Maximum air is provided in the burning zone where maximum combustion occurs.

• After traveling for nearly 500 mm and losing the moisture and volatiles, the fuel bed is ignited from the incident radiant heat of the furnace and front arch.

• Fixed carbon (FC) starts burning from the top layer downward. The VM burns in the space above and secondary air (SA) provides air required for combustion.

• Primary air from the bottom of the grate also helps scrub the burning coal to remove the ash layer formed on the outside to sustain the vigor of combustion.

• The approximate residence time of coal on the stoker is 20 min.

• Nearly 150 mm ash layer is generated at the discharge.

Comparison of Mass and Spreader Firing

• Mass firing requires fewer moving parts. Fuel feeders and distributors are not present.

• This results in simpler O&M and negligible erosion.

• The suspension firing is limited to —15% of heat input as opposed to 35-50% in SS for coal. Lower heat rates require a bigger stoker and boiler to achieve the same evaporation.

• This factor limits the range of fuels and the range of coal properties.

• The combustion efficiencies are lower by 5-10%, depending on the ash in the coal.

• Gravity feeding can handle two or even three layers of fuels in a sandwich man­ner. This is a great advantage for burning very low-volatile fuel, which is made to form the bottom layer with a good coal layer on the top to provide ignition and the necessary high temperature. Coke breeze and anthracite can be burnt this way.

With the advent of SS, where nearly twice the combustion rates of MB can be achieved, the MB stokers are confined to

• Smaller boilers that require no fuel feeders and spreaders

• Very erosive fuels such as coke breeze, which require few moving parts

• Low volatility fuels such as coke breeze and anthracite, which need sandwich burning

• High-quality low-ash coals with aggressively erosive nature

RG and PG Grates

• The fuel bed is pushed forward by the reciprocating action of the grate bars and not carried on the grate.

• Airflow is substantially parallel to the fuel bed.

In pushing the bed of fuel, there is a tumbling action continuously exposing fresh fuel to air. The grate is ideally suited for lumpy fuels that are difficult to size and contain high moisture, such as municipal wastes.

Because of the tumbling action of the fuel the tendency to form clinker with coal firing is high. The semifluid state of coal ash provides the base for forming lumps of clinker.

The base quickly grows as the mass is tumbled. Figure 11.7 shows a typical PG/RG used for unsized fuel with medium moisture and calorific value (CV). Ease of fuel admission can be seen.

Combustion Process in RGs

The combustion pattern is similar to the burning of fuel in gravity-fed TGs. Like the gravity-fed TGs, here also there are three zones: (1) drying, (2) ignition, and (3) burning. The front and rear arches play a very important role, particularly with high — moisture fuels, in

• Forming a throat that holds back the gases above the bed for better burning

• Deflecting the hot flue gases from the rear onto the fuel (the rear arch)

• Reradiating the furnace heat to help in drying and devolatilization of the fuel (the front arch)

Secondary air nozzles, strategically located in the the throat and above in the furnace, burn the volatiles. Grate actions of continuously pushing the fuel exposing fresh fuel improve the combustion rate. Grates have a rear slope of 0-15°, depending on the fuel, for easier tumbling of fuel and ash. Unlike the quiescent burning of CGs, the bed is continu­ously tumbled.

Coals with more than 15% ash or low ash fusion temperature end up with clinkers due to continuous agitation as the fuel and soft ash fuse together. Clinkers keep increasing in size, straddling the grate bars. They do not move forward as the grate movement is not positive. But biofuels, with their bulky or fluffy nature and moisture as high as 65%

Burn well in this type of grate, as ash is low and the grate action is conducive to air pen­etration into the thick fuel bed. Bagasse, rice husk, bark, and biological wastes burn well. Although bituminous and sub-bituminous coals are problematic, peat and brown coal can be burnt.

Fuel sizing is not very important as there is no spreading as in SS or pushing as in gravity feeding. Fuel is allowed to gently enter into the hopper from the top and there­after the grate moves the fuel forward. Ratings are similar to the grate ratings of the gravity-fed TG.

Municipal refuse is ideally suited. Three-stage grates are built with drying, burning, and after-burning sections. Grates tend to be long.

Grate bars for biofuels are long and thin (—300 X 25 mm) as shown in Figure 11.8. The rear rests on the grate shaft whereas the front rests on the next bar. Recesses on either side of the grate bar around the ridge portion are built into the casting to create air passages when the bars are arranged on the grate. Normally, the alternate grate sections along the grate width move in opposite direction to provide a scissor-like action to improve the air penetration and dislodge the ash from sticking to the stoker. All the alternate shafts are connected to two link shafts, which are driven by pneumatic power cylinders. These grates have fewer components and no return strand.