Steam boilers are vital in the process and manufacturing industries, in which fuel plays an integral role for combustion purposes. Various types of steam boilers are ideal for combustion with certain types of fuels such as oil-fired boilers, gas-fired boilers, coal-fired boilers, biomass boilers, etc. Coal is a widely used fuel in India, particularly bituminous and sub-bituminous coal. Coal is classified into three primary types, i.e., anthracite, bituminous, and lignite. Among these, anthracite is the most ancient type of coal whereas lignite was found comparatively later, consisting of volatile matter and moisture content with low fixed carbon. Coal combustion greatly depends on its chemical composition. The properties of coal can be classified as physical properties and chemical properties.
Reactions of Coal Combustion:
The major factor that contributes to releasing heat in steam boilers is the oxidation reaction that converts the elements of coal into their respective oxides. The coal combustion takes place in two stages,
- Evolution of volatile matter in the initial stages of heating along with physical and chemical changes
- Resultant combustion of the residual char
The ignition and combustion of volatile matter result in the oxygen diffusion to the particle surface and ignition of char. Char oxidation occurs in three steps as stated below:
- Oxygen diffusion from bulk gas to char surface
- The reaction between oxygen and char particle surface
- Reaction products diffusion from the char particle surface to the bulk gas
The overall reaction rate is determined by the rate of chemical reaction at low combustion temperature. With an increase in chemical reaction rate with raising the temperature, the carbon-oxygen reaction occurs quickly, leading to difficulty in the diffusion of oxygen to the surface. In such situations, the overall reaction rate is monitored with the diffusion rate of oxygen to the reacting char surface.
Coal Combustion System in Steam Boilers:
Fixed Bed:
In fixed bed combustion, coals sizing between 3 millimeters and 50 millimeters are placed on a grate and preheated with primary air. The air is blown under the bed to burn the fixed carbon along with secondary air that burns the volatiles released from the bed. The coal feeding system can be classified as underfeed, overfeed, spreader, and traveling-grate stoker.
The large-sized coal combustion reduces the heating rate of the particle to about 1°C per second, leading to a combustion time of about 45 to 60 minutes.
Fluidized Bed:
A fluidized bed in steam boilers consists of fluid by an airstream from the bottom of the bed that suspends the material. The bed material usually includes a mixture of coal and sand that acts as a fluid. The crushed coal preheated to about 850 to 950°C and then introduced to the bubbling bed. The coal particles are heated at 1000°C per second and devolatile, which burns the residual in around 20 minutes. Fluidized bed results in effective coal combustion due to a uniform temperature throughout the bed. A fluidized bed requires accessories such as cyclone collectors, separating fines that contain a high amount of combustibles, and recycling them back in the steam boiler system.
Pulverized Coal:
Pulverized coal combustion is highly used in large power stations as it performs fine grounding of coal, in which 70% to 80% by weight passes through a mesh screen. The powder is burned in the combustion chamber by suspending the particles in the combustion air. Due to finely ground coal having more surface area per unit than denser particles, coal combustion takes place quickly. However, this method leads to the excess cost of drying and grinding the coal, fouling and slagging of heat-transfer surfaces, and equipment for collecting fine particles.
Advantages and Disadvantages of Coal Combustion:
Advantages of Coal Combustion:
- Abundant Supply
- Easy Storage
- Low Capital Investment
- Safe to Use
- Simple Burning Process
Disadvantages of Coal Combustion:
- Non-renewable source of energy
- High Carbon Emission
- Harms Natural Habitats
- Coal is potentially radioactive
- Hazardous to the environment by increasing pollution
Alternative for Coal Combustion in Steam Boilers:
Coal is relatively expensive as well as harmful to climate with its carbon emission. Additionally, it is non-renewable and requires careful utilization by ensuring minimal wastage. Non-renewable fossil fuels are expected to deplete in the next three to four decades. Biomass is an excellent alternative as fuel for combustion in the steam boiler system. Biomass fuel includes animal waste, industrial waste, agricultural waste, etc., which is cost-effective and easy to procure, ranking as the fourth energy source globally with approximately 14% of consumption.
Depending on their elemental composition, biomass fuels show different combustion characteristics. It involves chemical reactions wherein carbon is oxidized to carbon dioxide while hydrogen is oxidized into water. Biomass can be ignited with various types of combustion processes like fixed bed combustion, fluidized bed combustion, and pulverized bed combustion. Biomass combustion reduces SO2 by up to 75%, and compared to coal it reduces the CO2 emissions by 93%
Rakhoh Boilers is a leading boiler manufacturer in Pune since 1983. With the expertise of our 500+ members in thermal solutions, we have emerged as a leading name in boiler supplier globally with more than 3000 successful boiler installations in over 26 countries worldwide. We manufacture a range of efficient industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories. To ensure optimal productivity and efficiency of steam boilers, we provide boiler services like solid fuel conversion, energy audit, boiler automation, steam trap assessment, annual maintenance contract, etc.
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