Coal combustion process a little more complex when compared with oil or gas. Coal combustion process itself is through three phases: drying (drying), the evaporation of volatile (devolatilization) and char combustion (char combustion). For the process of pulverized coal drying, devolatilization and char combustion will take place in sequence with a period of burning char (char burn period) is relatively longer compared to the drying and devolatilization stages
Drying is a process of evaporation / drying moisture in the coal. Moisture in the coal is divided into two types, namely: in the form of free water (free water) is located between the pores in the form of coal and bounded water (bound water) is absorbed in the surface structure of coal. Bituminous coal has a little free water, mostly in the form moisturenya is bounded water. At the moment included in the coal pulverizer, the coal having heating process (heating) by the primary water. Heat transfer process that occurs within and along the pulverized coal pipe is the convection heat transfer by the primary water. Water will evaporate and forced out of the coal particles. Drying time is the time needed to heat the coal particles to the vaporization point (the point of vaporization) and evaporate the moisture content of the existing.
Flame Ignition began to take shape at this point. The higher levels of Volatile Matter then the easier the coal burning and combustion will be more stable.
The final stage of the process of coal combustion is char combustion. When complete devolatilization, the coal is left of carbon char and ash. Carbon char is very porous (porous) so that oxygen can be char berdiffusi penetrate into the outer layer (layer externally) and continued into the char particles. Combustion rate of char depends on the rate of chemical reaction of carbon-oxygen reaction on the char surface and the rate of oxygen in diffusi internal boundary layer (boundary layer). Surface reactions which produce CO outside the particles react to form CO2. The reaction will raise the surface temperature of about 100 char-gas temperature above 200oC. Combustion rate of char is also dependent on oxygen concentration, gas temperature, gas flow Reynolds number, particle size and porosity of char char. For engineering purposes, the approach is to use a global rekasi (global reaction) to calculate the rate of char combustion. Global reaction rate to calculate the mass of char reaction rate per unit area and per unit surface oxygen concentration outside the particle boundary layer. Carbon char reaction with oxygen at the surface to form carbon monoxide (CO) and carbon dioxide (CO2), but its main product is carbon monoxide:
C + ½ O2 CO (a)
Surface will react with carbon korbon dioxide and water vapor:
C + CO2 2CO (b)
C + H2O CO + H2 (c)
Reduction reaction (b) and (c) is generally slower than the oxidation reaction (a) and for combustion usually only reaction (a) being considered.
By setting the sizing coal (70% passes 200 mesh) is to sizing / same diamater then the burnout time of coal will be relatively the same.
So the total time required for burning coal is the perfect summation of the drying time, Pyrolysis time and char burnout time. Coal combustion optimization is achieved when the moisture of coal as fuel into the room only by 1%. The rest have been vaporized (drying) time in the coal pulverizer and pipe. It is intended that the coal will go into the engine has just devolatilization stage, the fire began to form. Remaining 1% moisture is intended as a safe limit to prevent pre-combustion in the pipe. The data show that coal moisture content of Low Rank Coal has a higher moisture so that the total time for the burning of coal is also higher. This means delays will occur in the furnace combustion. Apart from coal burning time, which take into account is the flow rate of coal into the furnace. The faster the flow of coal is the distance from the fire will burn more distant. Low rank has a lower calorific value of coal and thus need more air, automatic high-velocity flow as well. Two factors, time and speed, will determine the residence time of coal (resident time) in the furnace