Heat Recovery Systems

Small heat recovery systems, including required heat exchangers, controls, piping and ductwork, pumps, silencers, diverters, expansion tanks, safety devices, etc., can be supplied as a single package. In some cases, heat recovery is provided as an integral part of a packaged prime mover system. This is particularly common in small reciprocating engine systems. In other cases, the heat recovery system is a separately packaged unit that is interconnected with the prime mover-driven system. Larger systems are entirely field assembled.

Heat recovery systems used in cogeneration applications include the following:

1. Air heating systems. These include systems in which exhaust gas is used directly for process heating or combustion air preheating, as well as systems in which a heat exchanger is used to separate the energy streams. More elaborate heat recovery systems may use jacket water separately, or in combination with exhaust gas, for ventilation air heating or other purposes.

2. Fluid heating systems. In many cases, particularly in smaller packaged reciprocating engine-driven systems, both exhaust gas and engine jacket heat are used to produce hot water. In a typical configuration, water is circulated through engine jacket and exhaust gas heat exchangers in series. Pressurized systems are required for high-temperature applications to prevent boiling. Alternatively, single-phase fluid heating is done by pumping water or some type of organic liquid through a closed-loop heat recovery unit where it is heated by the hot exhaust gas to the required temperature and distributed for process use.

3. Low-pressure steam-generating systems using exhaust gas and/or engine jacket water heat. In mid- to large-capacity engines, low-pressure steam at pressures of up to 15 psig (2 bar) can be produced from engine coolant system rejected heat. This can be accomplished using high-temperature forced circulation systems or ebulliently cooled systems.

In an ebullient system (Figures 8-8 and 8-9), the heat of vaporization removes rejected heat from the engine. As hot water passes through the engine cooling passages, small steam bubbles are formed as the jacket water absorbs heat from the engine. The mixture rises to a steam separator above the engine where the steam is discharged and the water is recirculated back to the engine. No jacket water pump is required, since flow is assured by change in coolant density as it gains heat. The temperature differential available for engine cooling is usually quite low (2 to 3°F or 1.1 to 1.7°C). Two or more engines are often connected to one boiler, usually with separate gas passages to prevent exhaust gas from condensing on a shut-down engine. The boiler must be above the engine to provide sufficient head for recirculation.

Because of aggressive cooling required by major engine components, many modern gas engine designs tend to be incompatible with ebullient cooling. High coolant velocities and the associated small flow areas do not allow for adequate gravity flow. However, some engines are capable of operating with coolant temperatures as high as 260°F (127°C), allowing jacket water cooling systems to be used to generate 15 psig (2 bar) steam. As jacket water outlet temperatures are increased with forced high-temperature systems, the quantity of heat rejection to the jacket water will decrease and the heat rejection to lube oil, ambient air, and exhaust gas will increase.

Whether steam is raised by ebullient cooling or forced circulation cooling, steam pressure available from a reciprocating engine heat recovery system is limited to about 15 psig (2 bar). Where thermal loads require high-pressure steam, it is sometimes cost-effective to employ a steam compressor. In essence, this approach reduces net power generation by the prime mover plant to maximize the use of available rejected heat.

An alternative to providing high-pressure steam (or high-temperature hot water) to an existing process or load could be to redesign the process to operate on lower quality heat. This might involve, for example, changing distribution piping or heat exchange coils.

4. High-pressure steam generation systems using combustion engine exhaust heat. Heat recovery steam generators (HRSGs) are commonly used to recover heat from gas turbine and reciprocating engine exhaust. Elaborate systems use multiple-pressure boilers and can include feedwater heating economizers and superheaters. Steam output can be controlled either by a pressure regulator, which bypasses excess steam to a condenser, or by diverting the exhaust gas around the heat recovery unit. Sometimes a small condenser is included with a bypass valve setup to ensure that exhaust gas leakage around the diverter (which generates small quantities of unwanted steam) does not result in damage from condensation and corrosion.

Reciprocating Engine Coolant System Heat Recovery

Figures 8-4 through 8-8 present alternative heat recovery configurations that may be applied to reciprocating engine cogeneration systems. Note that each of these configurations features a load balancing heat exchanger (or condenser) that must be included in the engine loop, not the load loop. Figure 8-4 illustrates a standard temperature water system with series exhaust heat recovery. A muffler is included in series with the engine system.

Figures 8-5 and 8-6 show standard and high-temperature water system designs. The high-temperature water system employs elevated jacket water temperatures of 210 to 260°F (99 to 127°C). The standard thermostat and bypass are removed and replaced by an external control. A static head must be provided in the engine coolant circuit to assure a pressure of 4 to 5 psig (129 to 136 kPa) above the pressure at which steam forms. The source of this pressure may be a static head imposed by an elevated expansion tank or controlled air pressure in the expansion tank.

Figure 8-7 illustrates a high-temperature water steam system. A circulation pump forces water through the cylinder block to the steam separator. In the steam separator, some of the water flashes to steam at up to 14 psig (2 bar) and the water returns to the engine. Figures 8-8 and 8-9 illustrate ebullient systems. Notice the location of the pressure control valve, excess steam valve, and load balancing condenser in the ebullient systems.

Effective water treatment is essential in all of the above configurations to minimize deposits and avoid corrosion. In many applications, process water (e.g., from space heating systems) is of inferior quality and should not be circulated directly to engines. It is important to carefully follow manufacturers' specifications for water quality, as well as for safety.

Figure 8-10 shows a reciprocating engine heat recovery system featuring intercooler, lube oil, jacket water and exhaust heat exchangers piped in parallel. At full load the engine-generator system produces 1,203 kW and the heat recovery system delivers 5.95 million Btu (6,271 MJ) per hour of recovered heat in the form of hot water. In one circuit, 119 gpm (450 lpm) of incoming water at 85°F (29°C) is heated to 132.8°F (56°C) as it passes through

Air Vent & Deaeration Line

Pressure Cap & Vacuum Breaker Relief e Valve

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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