Performance Optimization of Chemical Recovery Boiler
Optimized operation of the chemical recovery boiler means more steam and chemical recovery. Energy savings of major proportions can be realized in recovery boiler operation by focusing attention to the following:
- Develop and maintain a properly tuned process control system
- Better control on input streams such as dry solids concentration, excess air, and salt cake make up
- Optimize the boiler cleaning system steam usage
The black liquor which is input into the boiler should be optimized first for the correct concentration, i.e. the maximum concentration that can be achieved. This reduces the moisture loss in the boiler.
The firing pressure and temperature are to be optimized to minimum value so that liquor is fired as coarsely as the boiler can accept. Normally the pressure is first brought down by using different sizes of black liquor gun tips.
After this, the liquor temperature has to be optimized. A liquor temperature of around 115 Deg C is good to start with.
Once the liquor is burning properly, liquor temperature may be changed in small steps of ½ Deg C with a time interval of 45 minutes to 1 hour between every setting to allow furnace equilibrium to be established. While doing the above, the liquor burning profile has to be watched and brought to the optimum point as this is an important variable governing the reduction efficiency of the furnace.
The bed height can be adjusted easily by slight changes in black liquor temperature entering the furnace. However, this method of control should be avoided, if at any time it forces the use of a fine liquor spray. A better method is to regulate bed height by changing the ratio of primary to secondary air. The peak height of the bed should be about 1000 mm from the primary air port bottom.
The next very important parameter to be optimized is the total air flow, meaning the air flow proportion between primary and secondary, and air temperature. Increasing primary air quantity will tend to increase char bed temperature, increase the burning rate, and lower the reduction efficiency. It is equally important to distribute the primary air around the char bed. The total air flow can be maintained in such a way to get an excess air level around 15% at cascade evaporator inlet. The change in thermal efficiency of the recovery boiler due to excess air changes is low. The air temperature maintained in black liquor fired chemical recovery boilers is around 150 - 200 Deg C, which results in a very good bed stabilization.
It is necessary to avoid fine spray low bed operation of the boiler. Low bed fine spray operation is a safety hazard also. In suspension firing type recovery boilers, operating with a Fine Spray Low Bed (FSLB) leads to a lot of problems and causes high maintenance of the boiler. The reason why FSLB is resorted to by many operators is ease of operation, minimum attention required, and no rodding of the air ports needed. It gets rid of the black liquor easily and creates green liquor.
The various problems caused due to FSLB operation are:
- Increased mechanical carryover of ash
- Increased ash fouling of heat transfer surface
- Increased primary air port fouling rate
- Decreased reduction efficiency
- Increased smelt leak potential
- Decreased burning stability
- Increased smelt water explosion hazard