Steam Generator Furnace Design

The furnace of a modern high-pressure boiler is the most important part of the steam generator and may well be compared to the heart of the human body. The performance of the furnace affects the performance of each and every surface behind it such as superheater, evaporator, economizer, or air heater. When one talks about boiler furnace, the burner also has to be considered as an integral part of it. If combustion is incomplete or poor, NO x and CO formation may increase, adding to the cost of emission control equipment; CO formation also decreases boiler efficiency. Fires are likely in oil- and coal-fired boilers if combustion in the furnace is poor. There have been instances when oil burners in the furnace did not operate properly, resulting in oil droplets being carried over by the flue gases and deposited in the economizer or air heater, leading to fires or explosions. The discussions that follow pertain to package steam generators firing oil or gaseous fuels with

a capacity ranging from 15 to 250 t/h of steam. If the furnace is undersized, the temperature of the flue gas leaving the furnace will be

high, and superheaters located downstream facing the furnace will receive a large amount of direct radiation, resulting in high tube wall temperatures and even in tube failures. If the furnace is oversized, then the exit gas temperature will be lower, and steam temperatures may not be achieved at desired loads. In coal-fired boilers or in heavy oil–fired boilers where ash is present in the fuel, the furnace exit gas temperature must be lower than the ash melting temperature; else, molten ash can deposit on heating surfaces at the furnace exit, and removing them could be a challenge as rock-like substances can form on the screen or superheater tubes blocking the flue gas flow passage, thus affecting the performance of downstream equipment. This adds to the boiler operating as well as mainte- nance costs. Flue gas pressure drop will increase, adding to the cost of operation. Deposits on tubes also cause high-temperature corrosion problems.

In boilers firing solid fuels, the furnace is maintained at near-zero gas pressure so that the products of combustion do not leak to the atmosphere. Forced and induced draft fans are used to achieve this. However, due to the negative pressure in the furnace all the way to the boiler exit, there can be ingress of atmospheric air in poorly designed furnaces, through openings in furnace walls and ducts; this causes heat losses and lowering of flue gas temperatures and steam temperatures. Oil- and gas-fired boilers are always of the pressurized furnace type and use only forced draft fans. Leakage of flue gas from the furnace or boiler to the atmosphere is likely if there are openings that are not sealed properly such as the intersection of a membrane wall section and a refractory-lined wall.

The lesser the refractory usage in a furnace, the better the design. With refractory-lined pressurized furnace (Figure 2.1), it is difficult to maintain a leak-proof enclosure between the refractory walls and the water-cooled tubes as a result of which flue gases can leak to the atmosphere leading to inefficiency as well as corrosion of the casing, particularly when the fuel contains sulfur compounds. The tangent tube design is a slight improvement

42 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Refractory tile Insulation Outer casing

Refractory casing

Inner casing Outer casing

Tangent tube

Insulation Outer casing Membrane wall

FIGURE 2.1

Furnace construction—membrane wall, tangent tube, and refractory. over the refractory casing design, but it has the potential for leakage across the partition

wall. During operation, the tubes are likely to bend or flex due to thermal expansion paving the way for leakage of flue gases from the furnace to the convection bank. This results in higher CO emissions and loss of energy; the difference in gas pressure between the furnace and convection exit can be as high as 250–400 mm wc, which can cause a lot of bypassing of flue gas from the furnace to the convection section, affecting the furnace duty.

If catalysts are used for NO x or CO control, the pressure differential between the furnace and the convection pass can be even higher. The furnace exit gas temperature also may not

be close to that predicted leading to underperformance of the superheater. Hence, present- day designs use completely water-cooled furnace with membrane walls (Figure 2.2).

FIGURE 2.2

Water-cooled membrane wall furnace showing burner openings. (Courtesy of Cleaver Brooks Inc, Engineered Boiler Systems, Thomasville, GA.)

Steam Generator Furnace Design

Furnace dimensions must be such that the flame does not impinge on the walls resulting in overheating. Hence, boiler designers often discuss the furnace geometry with burner suppliers before designing the boiler and ensure that the fuels can be fired safely and efficiently with the least amount of emission of CO and NO x . Plant and process engineers should also be involved in this process of burner selection if the plant is likely to fire a dif- ferent fuel at a later date.

Steam Generator Furnace Design