Natural Gas Engines
Currently two different types of engines are used in air compressor applications: industrial and automotive-derivative.
Industrial engines are typically designed to operate for longer lifetimes than automotive-derivative. These engines operate at speeds up to 1,800 RPM. The time between major overhauls or rebuilds on this type of engine is in excess of 20,000 hours of operation. Industrial engines are designed so that all the parts that experience wear can be replaced. These major overhauls allow the life of an industrial engine to be extended "indefinitely." However, industrial engines are more expensive than automotive-derivative.
Automotive-derivative engines are rated for higher speed (more than 3,000 RPM) operation and are modified versions of those used in automobiles and other mobile applications. These engines are less costly than the industrial grade engines, and therefore can decrease the capital cost of a project. Other benefits of these engines are that they are somewhat smaller and lighter in weight. The cost and availability of parts is generally better than industrial engines. However, the life of these engines is significantly shorter. Automotive-derivative engines are generally replaced at the end of their useful life, and are not rebuilt like the industrial engines. Therefore, the reduced installed cost of this type of engine is countered by the increased operating costs of replacing the engine at regular intervals. Also to be factored in is the downtime to overhaul an industrial engine (2 weeks) versus replacing an auto-derivative engine (2 days).
Analysis to assist in deciding between engine types should consider the following:
Gas engine technologies are available for a variety of applications that require a prime mover. A prime mover simply converts one form of energy (gas or electricity) to mechanical work (turning a shaft) for use in a process. Thus, a prime mover can be an electric motor or a gas engine.
Gas engines perform the same function as an electric motor, they provide shaft work for process applications. Shaft work includes turning a shaft to operate a pump, a compressor, a grinder, a crusher or a generator. In many applications an electric motor can be replaced with a gas engine. The primary advantages of using a gas engine are:
With gas engine applications, customers will be able to achieve a flexibility for processes that werent an option just a short while ago. They will be able to "shape" their energy profile, which will allow them to negotiate much more attractive energy prices than if they were committed to a single technology/energy source.
Gas engines can help customers operate more efficiently, more cost effectively and they can offer customers fuel diversity for their facilities.
Natural Gas Engine basics
When identifying an opportunity for, and selling a customer on a Natural Gas Engine application you should be familiar with the technology. This includes knowing not only the benefits of installing a Natural Gas Engine but being able to inform the customer of some facts about the technology and about the maintenance considerations.
A Natural Gas Engine is similar to the engine in your car except that instead of using liquid gasoline as a fuel source, natural gas is used as the fuel source.
There are two types of Natural Gas Engine technologies available for customers, spark ignited Natural Gas Engines and compression ignition Natural Gas Engines.
Other technical information
I. Low speed Natural Gas Engines (< 1400 RPM):
II. High speed Natural Gas Engines (> 1400 RPM):
III. Automotive derivative Natural Gas Engine:
I. Rich burn engines:
II. Lean burn engines:
Other considerations for Natural Gas Engine systems:
How a Gas Engine Works
A gas engine converts the chemical energy of natural gas to mechanical work. The rate of energy conversion in a gas engine is as follows:
The energy balance above illustrates why heat recovery adds significantly to engine system efficiency. Usable heat can be recovered from both the jacket water and exhaust. Heat from the jacket water can be used to make hot water, heat from the exhaust can be used to make hot water or low pressure steam.
Natural gas engines can be naturally aspirated or turbocharged. In naturally aspirated engines air is drawn into the engine at atmospheric pressure. Turbocharged engines use the exhaust to drive a small turbofan that compresses the intake air. The turbofan compresses the air fuel mixture so more molecules are squeezed into the cylinder. When the mixture is ignited, more energy is released. Thus, a turbocharged engine will provide more shaft work out than a naturally aspirated engine of the same size.
Schematic of a Turbocharger
Turbochargers usually have a heat exchanger (an intercooler or after cooler) located after the compressor fan to remove heat from the compressed air. By cooling the compressed air, the density is increased, so oxygen content for a given volume of air is increased and more fuel can be burned.
Turbocharger with Intercooler
The advantage of a turbocharged engine is that about 35% more work can be done by a turbocharged engine as compared to a naturally aspirated engine of the same size.
Gas engine with turbocharger
For more information:
Interested in learning about the parts of a gas engine? Want to understand how a gas engine works? Go here: http://www.howstuffworks.com/engine.htm
Gas Engine Maintenance
Gas engines require regular maintenance just like the engine in your automobile. In fact, the maintenance activities are similar to the maintenance activities you do on your car. Customers with Natural Gas Engine systems can sign a maintenance agreement with their vendor at a reasonable cost.
A good rule of thumb for annualized maintenance costs is $.011 (one and one tenth cent) per horsepower-hour. This amount will vary depending on how the engine is used but is a very good ball park figure to use to determine annualized maintenance costs.
Example: If you have a 200 Hp engine that operates for 2500 hours a year the annualized maintenance costs would be:
200 Hp x 2500 hrs/yr. x $.011/Hp-hr. = $5500
Maintenance is scheduled by the run hours on the engine. A good guide for gas engine maintenance is in the table below:
Top end overhaul:
Engine oil analysis:
Note: regarding maintenance, to convert maintenance hours to something your customer would understand, simply multiply the hours by 50 or 60 to get a mileage equivalent.
For example, the Mfr. recommends changing the water pump at 6000 hours of operation. If you drove your car for 6000 hours at 50 mph, you would accumulate approximately 300,000 miles on your odometer! This gives the customer a better feel for the maintenance requirements of a gas engine.
Air Quality for Gas Engines:
Natural Gas engines may require Air Quality permitting if they are installed. The exception to the permitting requirement is for Agricultural customers (farmers) who use them for Agricultural water pumping for crops or to supply water to livestock.
Most Air Quality Districts have similar emissions limits. For the South Coast Air Quality Management District the limits are:
Gas engine emissions are very easy to control. For rich burn engines, a non-selective catalyst with an air-fuel ratio controller can be used to keep the emissions below the limits. A non-selective catalyst is similar to the catalytic converter on your car.
Lean burn engines require a selective catalytic reduction system, typically ammonia injection, to clean emissions. Selective catalytic reduction systems are more costly to buy, operate and maintain.
Gas engine Features and Benefits:
On the next several links we will review benefits of gas engines for specific applications.
Natural Gas Engines offer customers many options for their business and also offer many benefits to customers that choose to make a Gas Engine an integral part of their process.
Superior part-load performance: Gas engines operate much more efficiently at part load than electric motors. The energy consumption will closely follow the load on a gas engine. Electric motors have very, very poor part load efficiency. They consume about 95 - 98% of full load power even at low loads.
More stable energy prices: Gas engines often have better operating economics than an electric motor. This is particularly true during warm weather when heavy demand charge penalties kick in.
Gas engines are high-tech: New gas engine packages have advanced microprocessor controls that continuously monitor engine conditions and alert operators of any items that may need attention. They can be used in systems that have centralized controls.
Higher potential total efficiency: Gas engine systems can incorporate heat recovery into the process that raises the overall efficiency to well over 50%. Traditional electric plants have an efficiency of 30%. By including heat recovery in a process savings are achieved on the shaft work and in the recovered heat.
Natural gas has an Octane rating of 130. Octane rating is a measure of the antiknock property of a fuel, it can be thought of as resistance to detonation. High compression engines require high-octane fuels to operate properly and natural gas fits the bill.
Natural gas is free from liquid hydrocarbons. Liquid hydrocarbons, such as Gasoline, Ethanol and Diesel, wash the lubricant from the cylinder walls and dilute engine lubricants. They also leave gum and varnish deposits. Natural gas does not dilute lubricants or leave gum and varnish deposits. Some customers have experienced longer times between oil and filter changes because Natural Gas burns so clean.
Natural gas burns cooler and cleaner than other liquid fuels such as diesel and propane. There is less contamination of the lubricating oil with carbon and nitrates, which helps keep the equipment operating at top performance.