Power plant performance optimisation is one of the key objectives for power plant owners. Improving boiler efficiency is, therefore, a key area of attention for plant managers, which can help reduce fuel costs, reduce losses and increase compliance with environmental regulations as well as yield higher returns on investment.
Hence, there is an increased focus on reducing efficiency losses which may occur due to the incomplete combustion of fuel; non utilisation of heat energy contained in flue gases that escape into the atmosphere (stack losses); latent heat loss due to the presence of steam/moisture in flue gases; radiating energy loss due to a temperature gradient between the boiler surface and the surroundings; blowdown losses due to excessive levels of dissolved solids (calcium and magnesium ions, chlorides, sulphates and nitrates) in the feed water leading to poor heat transfer and other losses arising due to energy contained in unburned carbon monoxide, slag and flue dust and hydrogen and hydrocarbons.
Efficiency improvement techniques
In theory, to have the most efficient combustion in any combustion process, the quantity of fuel and air must has to be in a perfect ratio to provide perfect combustion with no unused fuel or air. In practice, however, for safety and maintenance needs, additional air beyond the theoretical “perfect ratio” needs to be added to the combustion process – this is referred to as “excess air.” With boiler combustion, if some excess air is not added to the combustion process, unburned fuel, soot, smoke and carbon monoxide exhaust will create additional emissions and surface fouling. From a safety standpoint, properly controlling the excess air reduces flame instability and other boiler hazards. Even though excess air is needed from a practical standpoint, too much excess air can lower boiler efficiency. So a balance must be found between providing the optimal amount of excess air to achieve an ideal combustion and preventing combustion problems associated with too little excess air, while not providing too much excess air to reduce boiler efficiency. In general, most boilers tend to increase excess air requirements as the firing rate of the boiler decreases, leading to a lower efficiency at the lower end of the firing range. To complicate matters, most boilers operate on the lower end of the firing range – so selecting a boiler that has low excess air throughout the firing range is important. Efficiency can be further improved by deploying oxygen trim systems which give feedback to boiler control systems for managing excess air quantity and by regularly monitoring these systems.
There is also a growing use of real-time combustion optimisation technologies to improve combustion and heat rates, while reducing emissions. Newer optimiser systems feature more advanced controls, control more data and are more adaptable to changing conditions. Modern combustion optimisation technologies provide closed-loop optimisation of fuel and air mixing by manipulating fuel and air levels to balance combustion in the furnace. They are software-based solutions that require minor modifications to mechanical equipment and are relatively straightforward when it comes to operations and maintenance training.
Use of economisers, which capture the heat contained in flue gases to preheat the feed water entering the boiler, can reduce flue gas temperature and thus help in improving boiler efficiency. A condensing economiser not only recovers heat from the flue gases but also condenses water vapour present in the flue gases which contain heat energy in it. It is, however, mainly used in only those plants which have a recurring need for low-grade heat produced by the condensing economiser.
Radiation and convection losses which are directly proportional to the surface area of the boiler can be decreased by using proper insulation. New insulation materials tend to insulate better due to a lower heat capacity.
Blowdown losses are a result of hardness of the feed water. Suitable chemical treatment of feed water is carried out to remove the hardness. In some cases, reverse osmosis is carried out for makeup water treatment to reduce the usage of treatment chemicals and save costs. A blowdown heat recovery system with a flash tank and a heat exchanger can help recover the heat energy of the blowdown water which can be used to preheat feed water or combustion air and thus save on fuel cost.
Flue gas monitors which continuously monitor carbon dioxide and oxygen content in flue gases can help in the determination of the optimal amount of excess air to be maintained. Intake airflow monitors help in the detection of leaks in the boiler which can alter the amount of excess air and lead to inefficiency.
Therefore, plant developers and operators have a major role to play in maximising the efficiency of the boiler and thus of the plant, by adopting a rigorous energy efficiency programme emphasising on proper and regular maintenance and integration of the latest technology solutions with production processes in subhead.