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.