Natural gas engine-driven air compressors offer many potential benefits, including:
Since equipment reliability and emissions are important considerations for all plant owners and operators, it is good to know that natural gas engine-driven compressors are highly reliable. Engine-driven air compressors are reliable because they combine two proven technologiesnatural gas engines and rotary screw compressorsoverlaid with an advanced control system. Typically, these compressors perform at the three 9s (99.9 percent) reliability level. Also, the emissions from these systems meet or exceed all existing and proposed environmental regulations. Engine-driven compressors are operating and meeting regulations in areas with the strictest environmental standards, such as Southern California and New York City.
Natural gas engine-driven air compressors reduce the cost of providing compressed air by achieving the following:
The cost of electricity to commercial customers is based on power demand and electric energy consumption. Natural gas engine-driven air compressors reduce operating cost by reducing power demand and electric energy consumption, and by switching to low-cost natural gas as the primary energy source.
If a manufacturing plant needs compressed air 24 hours per day and uses only electric compressors, it cannot benefit from the time-of-use and real-time electric rates or from the low cost of natural gas. A combination of natural gas engine-driven and electric air compressors increases the capability to deal with the uncertain future energy rates. In such hybrid air compressor systems, natural gas engine-driven air compressors are used as the lead air compressors when electric rates are high. The air compressor sequencing is reversed when electric rates are low. Hybrid plants also provide the capability to maintain level power demand and electric consumption. This capability is beneficial for negotiating the best rate in the deregulated electric energy market. In some plants, even the use of engine-driven air compressors only for peak shaving during the periods of high electric charges could be economically attractive.
Another factor that reduces the cost of compressed air from engine-driven air compressors is the availability of variable speed capacity controls in such systems that allow these air compressors to track the load without an efficiency penalty.
For example, a typical 100-horsepower, air-cooled compressor (110 horsepower at full load with a 3 horsepower fan) operating two shifts with an average load of 71 percent will consume 384,284 kilowatt hours (kWh) of electricity each year. At eight cents per kWh, the annual electricity cost of $30,743 is several thousand dollars more than the first cost of the compressor.
Total cost for providing the needed electric energy (kWh) is more than just the above cost. It also must include the monthly demand charge (based on the maximum kW power load from the system) and any fuel and other riders included in the rate schedule. If monthly demand charge is assumed to be $15/kW, annual demand charge for the 100-horsepower compressor will cost $1,230.
Total cost to operate a natural gas engine-driven compressor at the same loading is only $15, 600, assuming gas cost is $4.00 per million BTU and maintenance cost is 1.5 cents per horsepower-hour over that to maintain an equivalent capacity electric compressor. Therefore, in this particular example, use of engine-driven air compressors saves over $16,000 per year.
Many manufacturing plants need steam or hot water for plant or process heating and cooling needs. Over 70 percent of the natural gas input energy to engine-driven air compressors is available for heat recovery from the engine-jacket coolant and the exhaust of the engines for producing steam or hot water. Therefore, cost of energy for producing steam or hot water is zero if all the natural gas cost is assigned to compressed air cost. If the cost of natural gas is assigned to the production of steam and hot water, the natural gas engine-driven air compressor can be regarded as a boiler that provides compressed air free of charge. Steam or hot water produced from the recovered heat can be used for providing plant building or process heat needs, regenerating desiccant systems (if used for controlling humidity), or energizing absorption chillers for providing plant or process cooling needs.
Even though the equipment and maintenance costs of a natural gas engine-driven air compressor are higher than those of an electric air compressor, the life-cycle cost of an engine-driven air compressor is often much lower because of the significant savings in energy cost over its useful life of more than 20 years.
Natural gas engine-driven air compressors save energy cost by reducing plant power demand and electric energy consumption compared to electric air compressors. If the incremental installed cost of a natural gas engine-driven air compressor, over that for an electric air compressor, is treated as an investment, and the annual savings in its energy cost are treated as the return on that investment, typically this rate of return is very attractive and lies in the range of 20 to 30%.
Most natural gas engine-driven air compressor systems incorporate batteries for the system startup and do not require AC power for that purpose. Therefore, these systems do not require backup power system and reduce the size and cost of emergency backup generator sets needed to ensure continued operation of critical plant equipment during electric power outages. For example, a typical engine-driven 100-horsepower, air-cooled compressor (110 horsepower at full load with a 3 horsepower fan) would reduce backup power capacity needs by 82kW or more than $32,000, assuming backup power generator cost of nearly $400/kW.
Most natural gas engine-driven air compressor systems incorporate batteries for the system startup and do not require AC power for that purpose. Therefore, even during power disruptions these systems can provide a secure capability for supplying compressed air without the cost of a backup power system.