Steam boilers, process

Recent studies by Thermo Electron Corporation have shown that certain forms of waste heat utilization (e.g., recuperators or process steam boilers) provide a better return on investment than that obtainable from bottoming cycle generators. The optimum choice of heat recovery strategy depends, in part, upon the temperature of exhaust heat available. 

Energy efficiency of the process. If the process requires a furnace or steam boiler to provide a hot utility, then any excessive use of the hot utility will produce excessive utility waste through excessive generation of CO2, NO, SO, particulates, etc. Improved heat recovery will reduce the overall demand for utilities and hence reduce utility waste. 

Reducing products of combustion from furnaces, steam boilers, and gas turbines by making the process more energy efficient through improved heat recovery. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

Process Steam Generation. Steam generated in the process sections of the plant may be at the highest plant pressure level or an intermediate level. Also, the process area may have fired boilers, waste heat boilers, or both. There may be crossties between utility and process generated steam levels. Enough controls must be provided to balance far-ranging steam systems and protect the most critical units in the event of boiler feedwater shortage situations. 

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

Process steam can be supplied at two or more pressures without having to purchase boilers operating at different pressures or having to throttle steam, which is a waste of useful energy. 

The Hartford Steam Boiler Inspection and Insurance Company Center for Chemical Process Safety United States Department of Energy Arthur D. Little, Inc. 

A gas turbine CHP scheme, with a heat recovery steam generator producing process steam, operates at the DOMO plant at Beilen in the Netherlands. The plant, which produces dairy products, originally took its electric power (up to 3.2 MW) from the grid and its heat load was met by two gas-fired boilers with a steam production of 25 t/h at 13 bar. 

Figure 5-6. The Haldor Topsoe and Nippon Kasel process for producing formaldehyde (1) blower, (2) heat exchanger, (3) reactor, (4) steam boiler, (5) absorber, (6,7) coolers, (8) Incinerator, (9) heat recovery, (10) methanol evaporator, (11) boiler feed water.

The choice of boiler steam inlet conditions is usually dictated by the desire to achieve maximum output from the process steam flow. This requires high boiler steam pressure and temperature. However, there are practical considerations to observe. Above 40 bar more exacting feedwater treatment is necessary, and therefore it may be advantageous to maintain pressures below this figure. High steam and temperatures can also influence selection of boiler materials such as alloy steels. The upper limit for industrial applications is around 60 bara and 540°C. 

For watertube boilers it is necessary to maintain low O2 levels, and for this purpose a de-aerator in the feed line is required, which will also provide a degree of feed heating. The steam supply can be taken down from the low-pressure process steam main. 

Having ascertained the process steam flow and developed some ideas on the boiler pressure, the following step is to analyze the power available. Figure 15.23 provides a ready means of determining the approximate relationship between power available and process steam for specific steam conditions. Use of this and similar charts will allow an assessment to be made of the potential of a CHP scheme with a backpressure turbine. The conditions can be changed to give the required balance for heat and power. 

Safety Valve normally used for steam service, but suitable for gases or vapors. When used in steam generation and process steam service the valves conform to the ASME Power Boiler Code as well as the ASME Pressure Vessel... 

Designs often include boilers with economizers and pendant superheaters because many of the largest manufacturing operations require additional mechanical or electrical power to process steam and use combination heating and power services (cogeneration). 

WH boilers are usually packaged WT types of dual-fire design because a supplementary fuel is also required. The thermal energy is recovered and used to produce medium pressure (600-900 psig) process steam. 

In almost all countries today, safety codes and regulations exist for the construction, operation, and inspection of all boilers and associated pressure vessels and boiler systems. Both HW and steam-raising plants are provided with several vital boiler appurtenances (appliances or fittings) and various subsystems containing auxiliaries (accessories) that must be maintained, monitored, and controlled. However, for small HW and LP steam boiler plants the inspection process may be rather cursory with regard to the pressure vessel internals and tends to concentrate primarily on ensuring the proper operation of the various appurtenances that provide for boiler safety. 

Carbon filters find particular application as prefilters for RO and ion-exchange processes in the production of high purity FW. They are also used in clean-steam boilers and other types of steam generators where the steam is ultimately destined for application in food or beverage production, pharmaceuticals, electronics, surgical instrument sterilization, and similar processes. 

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