Reciprocal Grate Boiler Development and Application

Reciprocal Grate Boiler is another name of reciprocating grate boiler. As a biomass boiler, reciprocal grate boiler is suitable for burning wood dust, straw, bagasse, palm fibre, rice husk. Biomass fuel is a renewable fuel, which has less sulphur and ash, as well as less SO2 and dust emission.
There are many kinds of biomass fuel, including pellet type, briquette type and bulk type. The waste from wood processing plant, such as bark and sawdust, is often used in bulk type. However, the moisture of the waste is 50% or higher, and the calorific value is very low. Thus it is difficult to burn it effectively with conventional biomass boiler. Therefore, we developed a combined reciprocal grate boiler with different inclination angles. The new biomass boiler can adapt to the combustion of such biomass fuel with high moisture and low heating value.
1.Design fuel
This reciprocating grate boiler is specially designed for a wood processing plant. The user requires to burn 200tons wood waste per day to generate 1.25MPa saturated steam for production process. The component analysis result of the wood waste is as follows:
Total moisture: 55%
Carbon: 22.87%
Hydrogen: 2.41%
Oxygen: 17.67%
Nitrogen: 0.95%
Sulfur: 0.09%
Ash: 1.01%
Volatile matter: 76.8%
Lower heating value: 7291kj/kg
After thermal balance calculation, 200tons per day wood waste burning can generate around 20t/h 1.25MPa saturated steam. The wood waste needs pre-treatment, and final size shall not exceed 350*35*35mm.
2.Design parameter
Capacity :20t/h
Rated steam pressure: 1.25MPa
Rated steam temperature: 194℃
Feed water temperature: 104℃
Cold air temperature: 20℃
Design efficiency: 86.1%
Fuel consumption: 7526kg/h
Flue gas temperature: 140℃
3. Overall structure
The reciprocal grate boiler adopts double-drum horizontal natural circulation balanced ventilation structure, and furnace is bottom supported and top hanged.
Considering high moisture and low calorific value, the combustion device adopts combined reciprocating grate with two different inclined angles.
The wood boiler adopts a single-layer layout. The slag remover is below the 0-meter elevation, and the operating layer is at the 0-meter elevation. The system layout is simple, which saves the civil cost to the greatest extent.
4. Design point
4.1 Combustion device
The grate is divided into two sections with different inclined angles. The front part is a preheating and drying section with 32° step grate. The rear part is the main combustion and burn-out section with 10° step grate.
When the fuel enters the furnace from the inlet, it falls to the front of the 32° step grate. Driven by the movable grate, the fuel will roll from top to bottom while moving to the furnace. Thus it is beneficial to the mixing of hot air with fuel. Meanwhile, the fuel is fully radiated by furnace flame while rolling forward, which is beneficial to the precipitation of moisture. Therefore, the fuel can be fully dried in the 32° step grate section. The dried fuel enters the rear 10° step grate. Under the pushing of movable grate, the fuel moves forward continuously and generates relative movement, so that the fuel can be fully mixed with the primary air. The combustion and burn-out process are completed under the continuous radiation of rear arch.
4.2 Feeding device
The front wall has two feeding devices with an inlet section of 1*0.5m. The bottom of the feeding device has a rotatable adjusting plate, where has seeding wind. When changing the angle between adjustment plate and horizontal plane, the dropping point on the grate can be adjusted. A shaftless double spiral feeder is arranged in front of each feeding device, which has no middle shaft, thus avoiding the winding of flexible fuel on the spiral shaft.
4.3 Primary and Secondary Air
Three sets of secondary air are set on the furnace. The secondary air at the outlet of rear arch can promote full mixing of flue gas and air, and push high temperature flue gas to the front arch to facilitate the preheating, drying and ignition of fuel. The secondary air arranged above the feeding port can stir and mix the flue gas from the lower part of the furnace, and provide sufficient air to improve the combustion efficiency. Each secondary air duct has regulating damper, which can adjust the air volume according to the combustion condition. The lower part of grate is divided into several air chambers, providing primary air for fuel and cooling the grate.
4.4 Convective heating surface
The convection tube bundle is in-line arrangement, economizer is bare pipe in-line arrangement, and air preheater is horizontal in-line arrangement. To avoid low-temperature corrosion, air preheater pipe is glass lining pipe. Shock wave soot blowers are installed at each convective heating surface to reduce the ash deposition.
5. Operation effect
Main operating parameters of the reciprocal grate boiler is as follows:
Lower furnace temperature: 801-880℃
Furnace outlet temperature: 723-780℃
Economizer inlet temperature: 298-341℃
Air preheater outlet temperature: 131-146℃
Drum pressure: 1.02-1.21MPa
Evaporation capacity: 18.7-20.2t/h
Feed water temperature: 86-102℃
Oxygen content at outlet: 6.7% ~ 7.9%.



Post time: Mar-02-2020