Pulverised coal combustion
Pulverised coal combustion involves two main sequential, but possibly overlapping, stages. (1) Rapid heating and devolatilisation as a result of pyrolysis followed by (2) combustion of char residue emanating from the devolatilisation stage. The first stage is fast, taking times in the order of 0.1 sec. only. The second stage is slow, requiring times in the order of 1 sec. to 2.5 sec. and therefore, thus has a major effect on the size of the combustion chamber. In practical combustion situations, such as in a large pulverized coal combustor, different particles can undergo concurrently different processes such as pyrolysing, oxidising reactions under different temperature regimes.
In a tangential firing system the coal is pulverized in coal mills and is carried by primary air to the furnace through coal pipes. The mills are usually a constant airflow mill and have a specific output in mass of coal ground depending on coal properties like hardness, moisture, and fineness which affect the mill output. In direct tangential firing systems, the pulverized coal from the coal mills is directly taken to the furnace.
Coal properties such as FC/VM (Fixed Carbon / Volatile Matter), particle size, oxygen, calorific value of the coal, reactivity, and ash content seem to be the most important variables for pulverised coal combustion in tangentially fired boilers, and they are highly inter-related.
The total quantity of coal to be pulverized for a specified size of boiler at a designed efficiency will depend on the calorific value of coal. As the ash content in coal goes up, the calorific value per unit mass of coal comes down. This increases the mass of coal to be prepared, which in turn increases the number of mills or elevations needed in a tangential firing system.
The secondary air required for combustion is sent into the furnace through a windbox housing the coal nozzles, oil guns, and the secondary air nozzles. Behind the coal nozzles there are fuel-air dampers which are used for keeping the flame front away from the coal nozzles by at least one meter from the tip. This is required to prevent the coal nozzle tips from getting burnt due to radiation from coal flame. The flame front is predominantly affected by the volatile matter in coal and the fuel air damper is modulated for controlling the flame front. As the fuel air dampers are opened, more secondary air goes through this damper and physically pushes the flame front away. However, when the flame front is already away from the nozzle tip, the fuel air damper needs to be closed fully.
The total air quantity for combustion in a boiler will depend upon the million kilocalories being fired to generate steam at a specified parameter. This total air is divided into primary air and secondary air in the ratio of 30 % and 70%. As the ash percentage goes up in coal, the amount of mill air flow goes up, as the number of mills to be in operation goes up. This results in an upset between primary and secondary air ratio.
Tuning combustion in high ash pulverized coal fired boilers having direct tangential firing systems should address all the above said factors.
Combustion tuning steps
- Operate the boiler at a constant load and designed steam parameters
- Keep the excess air around 20 to 25%
- Load all the mills equally and keep only the minimum number of mills required
- Adjust the mill fineness to the required level – normally 75% through 200 mesh and less than 2% on 50 mesh sieve
- Keep the mill outlet temperature close to 85 to 90 degrees centigrade
- Adjust the mill air flow to just above the settling velocity
- Note the operating mill air flow reading and reduce the mill air flow in steps of 0.5 to 1 t/hr wait for a minimum of 15 minutes before reducing again
- Watch the furnace draft while doing this, when a small fluctuation starts then stop reducing the air flow and note the reading
- Increase the air flow above that was being maintained before reduction and keep it for 30 minutes – this clears off any settling in the coal pipe
- Now keep the mill air flow at the flow value noted when furnace fluctuation started plus about 1 t/hr, make sure no furnace pressure fluctuation is seen
- Repeat this for all the mills one by one
- This ensures minimum primary air flow being used for transporting the high ash coal powder to the furnace
- Check the flame front if it is one meter away from the coal nozzle tip then close all fuel air dampers. If volatile matter in coal is 20% or less this condition gets satisfied
- Keep the windbox pressure of 80 to 100 mm of water column
- Watch the furnace for the flame conditions like brightness and flickering
The steps carried out will help to achieve combustion at optimal level. Optimum combustion in the boiler will ensure performance of the boiler within a desired limit.