COAL COMBUSTION PROCESS PLTU

Coal combustion process a little more complex when compared to 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 the period of burning char (char burn period) are relatively much longer when compared to the drying and devolatilization stage

Drying.

Drying is the process of evaporation / drying of 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 of coal and in the form of bounded water (bound water) is absorbed in the surface structure of coal. Bituminous coal has little free water, mostly in the form moisturenya is bounded water. At the time put into PULVERIZER coal, coal has a heating process (heating) by the primary water. The process of heat transfer that occurs within and along the pulverized coal pipe is heat transfer by convection by the primary water. Water will evaporate and be forced out of the coal particles. Drying time is the time required to heat the coal particles to the vaporization point (the point of evaporation) and vaporize all the existing moisture content. The time taken by particles of coal in the process of convection heat transfer to evaporate the water is :

with:
MWI: mass of water, kg
cw: energy in the water, kJ / kg
mdf: mass of dry fuel, kg
cdf: energy in dry fuel, kJ / kg
m: mass of water flow rate, kg / sec
hfg: latent heat of evaporation of water, kJ / kg
h: primary water convection coefficient, W/m2 OK
Ap: coal particle surface area, m2
Ta: primary water temperature, OK
Tp: temperature of coal particles, OK

So the use of coal with a higher moisture values ​​will cause the drying time is also higher.

Devolatilization

When the drying process has been completed then the particles of coal began to change with the release of volatile composition. Volatile is the content of gases in coal. During volatile exit from the pores of coal, external oxygen can not enter into the particle penetration. Devolatilization process known as pyrolysis stage. The rate of devolatilization and pyrolysis products depends on the temperature and fuel type. In the pyrolysis process will be the release of carbon monoxide, hydrocarbons, and soot. Along with the release of volatile, there will be diffuse oxygen so that the pyrolysis products began to burn. Flame happened to be the more heat will cause the particles and the faster the rate of devolatilization. The rate of devolatilization can be calculated using the equation:

with:
mv: volatile mass, kg
kpyr: constant pyrolysis
k0, PYR: constant pyrolysis at temperatures of reference
Epyr: Energy pyrolysis
R: ideal gas constant
Tp: temperature of particles

Integration of the equation above:

Flame Ignition began to take shape at this stage. The higher levels of Volatile Matter then the coal will burn and the easier it will be more stable combustion.

Char Combustion

The final stage of the coal combustion process is char combustion. When complete devolatilization, coal is the remains of the carbon char and ash. Carbon is very porous char (porous) so that oxygen can penetrate the char berdiffusi into the outer layer (layer externally) and continues into the char particle. The rate of burning of the char depends on the rate of chemical reaction of carbon-oxygen reaction on the char surface and internal diffusi rate of oxygen in the boundary layer (boundary layer). Surface reactions produce CO which then reacts to form CO2 out of the particles. Reaction will raise the surface temperature of about 100-200oC char above the gas temperature. The rate of burning of the char is also dependent on the concentration of oxygen, the gas temperature, gas flow Reynolds number, particle size and porosity of the char char. For engineering purposes, the approach used is to use the global reaction (global reaction) to calculate the rate of char combustion. Global reaction rate will calculate the mass of char reaction rate per unit area and per unit of surface oxygen concentration outside the particle boundary layer. Carbon char reaction with oxygen on the surface will form carbon monoxide (CO) and carbon dioxide (CO2), but its main products are carbon monoxide:
C CO + ½ O2 (a)
The surface will react with carbon dioxide and water vapor korbon:
C + CO2 2CO (b)
C + H2O CO + H2 (c)
Reduction reactions (b) and (c) is generally slower than the oxidation reaction (a) and for combustion is usually only the reaction (a) are considered. Char burnout time obtained by the equation:

with:
mC: the mass of carbon, kg
d: diameter of char particles, kg
to: effectively constant firing rate, g / (cm2.s.atm O2)
: Density of oxygen, kg/m3

By adjusting the sizing of coal (70% passes 200 mesh) then for sizing / same diamater then the burnout time of coal will also be relatively the same.

So the total time required for burning coal is the perfect time is the sum of the drying, pyrolysis time and char burnout time. Optimization of coal combustion is achieved when the moisture coal into the combustion chamber while only 1%. The rest must have evaporated (drying) time in the PULVERIZER and coal pipe. It is intended that coal now going into the combustion chamber has been right on the devolatilization stage, which began to form a fire. The remaining 1% moisture is intended as a safe limit to prevent pre-combustion in the pipe. From the data shows that coal moisture content of low rank coal has a higher moisture so the total time for combustion of coal is also higher. That is going to happen delay combustion in the furnace. In addition to the burning of coal, which take into account is the flow rate of coal into the furnace. The faster the flow of coal combustion is the distance from the fire will be more distant. Low rank has a lower calorific value of coal and thus need more air, automatic flow rate is also high. Two factors, time and speed, will determine the residence time of coal (residence time) in the furnace.