Dual Fuel Engines for Emergency Generators

by Starlight Generator dieselgeneratortech

2.3.2.3 Dual Fuel Engines

Dual fuel engines are predominantly fueled by natural gas with a small percentage of diesel oil added.

There are two main configurations for introducing the gaseous fuel in a dual fuel engine. These engines can be purpose built or conversions of diesel engines. Such engines can be switched to 100 percent diesel operation. Dual fuel engines provide a multi-use functionality. Operation on predominantly cheaper and cleaner burning natural gas allows the engine to be used in CHP and peak shaving applications, while operation on 100 percent diesel allows the engine to also meet the onsite fuel requirements of emergency generators. The dual function adds benefit in applications that have specific emergency generator requirements such as in hospitals or in public buildings.

There are three main configurations for introducing the gaseous and pilot diesel fuel: 1) low pressure injection with the intake air, 2) high pressure injection after the intake air has been compressed by the piston, and 3) micropilot prechamber introduction of the diesel fuel. New dual-fuel engines are offered in oil and gas production markets to reduce operating costs. Dual-fuel retrofits of existing diesel engines are also offered as a means to reduce both operating costs and emissions for extending the hours of use for limited duty engines such as emergency and peaking applications. Dual fuel is not widely used for CHP applications.

2.3.2.4 Heat Recovery

The economics of engines in on-site power generation applications often depend on effective use of the thermal energy contained in the exhaust gas and cooling systems, which generally represents 60 to 70 percent of the inlet fuel energy. Most of the waste heat is available in the engine exhaust and jacket coolant, while smaller amounts can be recovered from the lube oil cooler and the turbocharger's intercooler and aftercooler (if so equipped). As shown in the previous table, 45 to 55 percent of the waste heat from engine systems is recovered from jacket cooling water and lube oil cooling systems at a temperature too low to produce steam. This feature is generally less critical in commercial/institutional applications where it is more common to have hot water thermal loads. Steam can be produced from the exhaust heat if required (maximum pressure of 400 psig), but if no hot water is needed, the amount of heat recovered from the engine is reduced and total CHP system efficiency drops accordingly.

Heat in the engine jacket coolant accounts for up to 30 percent of the energy input and is capable of producing 190 to 230 °F hot water. Some engines, such as those with high pressure or ebullient cooling systems, can operate with water jacket temperatures of up to 265°F. Engine exhaust heat represents 30 to 50 percent of the available waste heat. Exhaust temperatures for the example systems range from 720 to 1000°F. By recovering heat in the cooling systems and exhaust, around 80 percent of the fuel's energy can be effectively utilized to produce both power and useful thermal energy.

Closed-loop cooling systems – The most common method of recovering engine heat is the closed-loop cooling system as shown in Figure 2-2. These systems are designed to cool the engine by forced circulation of a coolant through engine passages and an external heat exchanger. An excess heat exchanger transfers engine heat to a cooling tower or a radiator when there is excess heat generated.

Closed-loop water cooling systems can operate at coolant temperatures from 190 to 250°F. Depending on the engine and CHP system’s requirements, the lube oil cooling and turbocharger after-cooling may be either separate or part of the jacket cooling system.

Ebullient Cooling Systems – Ebullient cooling systems cool the engine by natural circulation of a boiling coolant through the engine. This type of cooling system is typically used in conjunction with exhaust heat recovery for production of low-pressure steam. Cooling water is introduced at the bottom of the engine where the transferred heat begins to boil the coolant generating two-phase flow. The formation of bubbles lowers the density of the coolant, causing a natural circulation to the top of the engine.

The coolant at the engine outlet is maintained at saturated steam conditions and is usually limited to 250°F and a maximum of 15 psig. Inlet cooling water is also near saturation conditions and is generally 2 to 3°F below the outlet temperature. The uniform temperature throughout the coolant circuit extends engine life and contributes to improved combustion efficiencies.

Exhaust Heat Recovery – Exhaust heat is typically used to generate hot water of up to about 230°F or steam up to 400 psig. Only a portion of the exhaust heat can be recovered since exhaust gas temperatures are generally kept above temperature thresholds to prevent the corrosive effects of condensation in the exhaust piping. For this reason, most heat recovery units are designed for a 250 to 350°F exhaust outlet temperature.

Exhaust heat recovery can be independent of the engine cooling system or coupled with it. For example, hot water from the engine cooling can be used as feedwater or feedwater preheat to the exhaust recovery unit. In a typical district heating system, jacket cooling, lube oil cooling, single stage aftercooling, and exhaust gas heat recovery are all integrated for steam production.