Heat recovery boiler

The heat recovery boiler serves to cool the S02-bearing product gas and produce steam for electricity generation. Cooling occurs by feeding low-temperature boiler feed water around the fire tubes (Fig. 3.4). 

The boiler feed water is condensed steam after electricity generation that has been conditioned in a deaerator. The boiler feed water is preheated in economizers prior to entering the heat recovery boiler. Economizers are heat exchangers that transfer heat to boiler feed water while cooling gases entering intermediate and final absorption towers (Chapter 9). 

The saturated steam produced in the heat recovery boiler is superheated prior to entry into a steam turbine generator which makes electricity. Superheating occurs in a superheater. A superheater is another heat exchanger, which cools gas after catalyst bed 1 (Chapter 9) and superheats that steam for power generation. 

The superheated steam goes to the steam turbine generator, produces electricity, and is condensed producing boiler feed water. The steam condenser is usually a shell and tube type heat exchanger supplied with cooling water. Alternatively, air-cooled steam condensers can be used. 

Some of the electricity generated is used in the acid plant. The rest is exported. A modem, 4400 toimes/day sulfur burning acid plant can export approximately 260 kW of electricity per tonne of H2SO4 produced. Some acid plants export their steam instead of turning it into electricity. 

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As discussed in Section 15.2.2, the gas turbine s main disadvantage is its low efficiency of around 25-35 per cent in open cycle. However, this can be significantly improved by the use of a heat-recovery boiler that converts a good proportion of the otherwise waste heat in the turbine exhaust gases to high-pressure superheated steam, which, in turn, drives a conventional steam turbogenerator for supplementary electrical power. This can increase the overall efficiency to 50 per cent for no further heat input as fuel. 

Dual pressure For comparison, a combined cycle scheme with dual pressure is shown in Figure 15.13. In this case, the waste heat recovery boiler also incorporates a low-pressure steam generator, with evaporator and superheater. The LP steam is fed to the turbine at an intermediate stage. As the LP steam boils at a lower temperature than the HP steam, there exists two pinch points between the exhaust gas and the saturated steam temperatures. The addition of the LP circuit gives much higher combined cycle efficiencies with typically 15 per cent more steam turbine output than the single pressure for the same gas turbine. 

If low-cost natural gas is available, a gas turbine can be used to generate power. In this case, the waste heat in the exhaust gas is used to produce steam in a heat recovery boiler (HR boiler). This approach also is used with some gas turbine plants (as in some high-speed navy vessels). Where an HR boiler is employed, if steam demand exceeds power demand, the boiler is fitted with auxiliary burners. 

Where propulsion is provided by diesel or gas turbine, it is common to use an auxiliary boiler in conjunction with a waste heat/heat recovery boiler (WH/HR boiler). The steam-HW mixture, produced by passing the exhaust engine gases over the HR boiler tubes, is piped to the auxiliary boiler steam drum and then mixes with auxiliary BW. All steam utilized by the ship is taken from the auxiliary boiler. 

The term waste heat boiler is also widely used to cover heat recovery boilers (HR boilers), which tend to be direct-fired steam generators, albeit employing low-grade by-product fuels such as bagasse, wood bark, com cobs, peanut shells, blast furnace gas, black liquor, and the like. 

Heat flux density Heat recovery boiler, see Boiler, water tube, heat recovery 219... 

Example 10-13 Production of Cogeneration Steam in a Heat Recovery Boiler (HRB)... 

Given 10,000 Ib/hr of 700°F cycle exhaust gas passing through a heat recovery boiler (HRB) (a) How much 150 psia, 400°F steam can be produced (b) How much heat is transferred from the gas to the steam (c) What is the exhaust temperature of the gas leaving the HRB and (d) Sketch the T-Q (temperature-heat) diagram for the HRB. Assume a gas side mean heat capacity of 0.25 Btu/lb, °F, an evaporator pinch temperature of 30°F, a feedwater temperature of 60°F, and an evaporator drum pressure of 180 psia to allow for pressure losses. 

After 2 hours, it became clear that the openings in the opposite refractory wall were shrinking. These openings permitted the hot flue gas to exit the combustion chamber and flow into the tubes of a heat-recovery boiler. The apparently melting refractory was sagging and restricting these apertures. 

The Oregon installation uses a 25 tire per hour unit manufactured by Nippo in Japan and marketed in the U.S. by Tsurusaki Sealand. The unit has been in operation since 1987 with moderate success, but no U.S. company has yet decided to purchase another one. The draft configuration in the unit allows it to bum at 2,000 degrees Fahrenheit and produce 100 psig process steam. The unit has a Cleaver Brooks waste heat recovery boiler and a bag filter. Whole tires are automatically fed into the unit-both automobile tires and light truck tires. The State of Oregon Department of Environmental Quality has approved the operation of the unit. 

Monolith catalysts are used for the control of carbon monoxide and hydrocarbon (known as volatile organic compounds or VOCs) emissions from chemical plants and cogeneration facilities. In this case, square bricks are stacked on top of one another in a wall perpendicular to the flow of exhaust gases at the appropriate temperature location within the heat recovery boiler. The size of the brick can vary from 6 in (ceramic) to 21 ft (metal). Pt and Pd catalysts are used at operating temperatures between 600 and 1200°F. Cell sizes typically range between 100 and 400 cells per square inch. Typical pressure drop requirements for monoliths are less than 2 in of water. 

R.G.I. Leferink and W.M.M. Huijbregts, Nitrate Stress Corrosion Cracking in Waste-Heat Recovery Boilers, Anti-Corrosion Methods and Materials, 49(2), 118-126 (2002). 

Fig. 12-22 Use of heat pipes to dnVe waste heat recovery boiler. (Courtesy Dr, M.A. Ruch. Q-Dot Corporation, Dallas. Texas.) ...

Heat pipes are particularly useful in energy-conservation equipment. One example is shown in Fig. 12-22 where hot exhaust gases are used to drive a waste heat recovery boiler. The hot gases from a combustion process, which... 

In the power section of the plant the bio-oil is combusted in a gas turbine that is connected to a generator to produce electricity. The hot flue gas from the turbine passes through a heat recovery boiler to produce steam. The steam powers a steam turbine, producing more power, and the low pressure steam from the steam turbine is sent to the sugar mill to meet the mill s steam requirement. The power section for the pyrolysis combined cycle consists of the same design as the power section shown in Figure 4 below for the gasification combined cycle concept. 

The major conclusion is that the mass burning of unprepared municipal solid waste in heat recovery boilers is well established, and can be a technically reliable, environmentally acceptable and economic solution to the problem of disposal of solid wastes. It is not as cheap as is currently available land-filling. However, when the cost is considered of upgrading current landfills and establishing new landfills in accordance with the expected Resource Conservation and Recovery Act (RCRA) provisions, these mass burning waste-to-energy systems are expected to compare more economically with true sanitary landfills. ... 

Heat recovery boiler included in installation. Particulate collection device included in installation. 

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