Engine-driven compressor systems are designed to provide automatic operation with unattended service. Most systems have microprocessor-based controls with an operator interface to permit access to controls and set points for adjustment. Engine-driven compressors are equipped to start and stop automatically based on air system demand. Part load operation is automatically controlled to provide optimal performance by varying engine speed and adjusting internal compressor capacity control mechanisms.
For heat recovery applications involving steam generation, verification that the heat recovery unit and steam separator are filled with water must be performed prior to engine start-up. Low- and high-level alarms and safety relief valves should also be tested at start-up. The engine should be operated at part load until steam production begins.
The information on the operation of engine-driven air compressors is presented here in the following sections:
The ability to vary engine speed to control compressor output is the predominant advantage when considering the part load operation of an engine-driven compressor in comparison to an electric motor-driven unit. From an efficiency perspective, reducing engine speed to achieve better part load performance is highly advantageous both for the engine and the compressor. Engine efficiency increases as shaft speed decreases. Similarly, compressor efficiency remains high when part load conditions can be attained without resorting to control with slide valves (screw machines), cylinder unloading (reciprocating machines), or inlet guide vanes (centrifugal machines). Efficiencies for a throttled compressor inlet and compressor with a variable speed engine-drive are shown in the figure below. Note that for the variable speed case, once the engine has slowed down to a minimum level (approximately 60% full load in this case), the throttling of the inlet then occurs for lower load levels.
The figure below illustrates the savings that can be achieved for compressor operation at 60% of full load for a compressor of 1,000 Hp peak capacity using the curves shown in the figure above. The electric unit requires 206 BHp at 60% load, the gas engine unit requires only 141 BHp. This equates to an energy savings of 31.5%.
Compressor Energy Comparison at Part Load
Since engine design as well as the compressor selection affects efficiency, the part load characteristics of both the engine and the compressor operating as a packaged unit must be considered when performing analyses such as those illustrated in this example.
Regulations governing the allowable quantity of pollutants which an engine can discharge vary from region to region depending on local ambient air quality conditions. Regions with poor air quality have tighter restrictions on exhaust emissions than the areas where the air quality is good. For this reason the local regulations must be considered when engines are installed.
Air pollutants regulated by national standards are NOx (oxides of nitrogen), CO (carbon monoxide), HC (hydrocarbons), SOx (oxides of sulfur), PM10 (particulate matter 10 microns and smaller), and PM2.5 (particulate matter 2.5 microns and smaller). This group of regulated pollutants are known as “Criteria Pollutants”. When considering natural gas engine emissions the most important criteria pollutant emissions are NOx and PM2.5. CO is present in engine exhaust gas, but in quantities that generally are below the level of concern where additional controls might be required. HC, SOx, and PM10 emissions are typically negligible from natural gas combustion.
NOx is a generic name for nitrogen oxide molecules that are formed when nitrogen and oxygen react in high temperature conditions, as occur in the combustion chamber as the fuel is burned. The predominant molecules in the grouping are NO (nitric oxide) and NO2 (nitrogen dioxide). While NOx is a harmful pollutant in its own right, its most damaging role is as a precursor to ozone formation. NOx reacts with other pollutants to form ozone (O3) when exposed to sunlight in the lower atmosphere. Ozone is a major component of smog, the haze that hangs over many urban areas. Ozone damages plants and synthetics and is an irritant to humans and animals.
PM2.5 was added to the criteria pollutant list in 1997. Actual engine emission levels of PM2.5 are largely unknown at this time. In addition, source limits have not yet been set. However, particles of this size are largely formed from gaseous combustion precursors. Therefore, natural gas combustion in engines is likely to be an important source of PM2.5 emissions. Particulate matter is associated with premature mortality from respiratory diseases, asthma, and heart attacks.
Particulate matter (PM10) is formed during combustion of liquid fuels and engine lubricating oil. Particulate levels from natural gas are insignificant when compared to diesel engines.
Carbon monoxide (CO) is formed by incomplete fuel combustion. Complete combustion produces carbon dioxide (CO2). Incomplete combustion occurs when oxygen concentrations near the fuel molecule are insufficient for complete combustion, or when combustion is quenched near a cold surface in the combustion chamber. CO is a poisonous gas which causes nausea, headache, fatigue, and, at high concentration, death.
A small percentage of hydrocarbons (HC) found in natural gas will pass through the combustion chamber without reacting. These hydrocarbons will be found intact, or modified but incompletely oxidized, in the exhaust. These hydrocarbon emissions are often defined in three categories:
- THC (Total Hydrocarbons) or TOC (Total Organic Compounds)
- NMHC (Non-Methane Hydrocarbons), VOC (Volatile Organic Compounds), ROG (Reactive Organic Gases), or ROC (Reactive Organic Compounds)
- NMHC-NEHC (Non-Methane, Non-Ethane Hydrocarbons)
Oxides of Sulfur (SOx) are formed when sulfur compounds from the fuel or lube oil are oxidized in the combustion chamber. In the atmosphere SOx combines with water to form sulfurous and sulfuric acids which contribute to acid rain. Emissions from natural gas-fired engines are negligible.
Besides Criteria Pollutants, another class of air pollutants, called Hazardous Air Pollutants (HAP) are regulated at both a local and national level. The national list of HAPs includes 189 different pollutants. The list includes organic chemicals, metals, acids, bases pesticides and other miscellaneous materials. Natural gas burning engines may have significant emissions of certain aldehydes (CHO) on the HAPs list. These aldehydes are principally formed during the combustion of liquid fuels and lube oils. However, when compared with liquid fueled engines, aldehyde levels from natural gas-fueled engines are extremely low and are normally not regulated for natural gas engines.
Current practice for the control of exhaust gas emissions involves two options: lean burn engines and catalysts.
Currently, emissions control of most requirements, e.g. those subject to RACT (Reasonably Available Control Technology) provisions, will be met with lean burn engines. In some cases, particularly in California, catalyst control is required to reach desired levels.
Lean burn engines were developed in the 1980’s in response to the need for cleaner burning engines and reduced airborne pollutants. Lean burn engines consume 50% to 100% excess air to reduce temperatures in the combustion chamber and limit the creation of NOx, CO, and NMHC.
Emissions from an engine which is not lean burn can be reduced by chemically converting these pollutants into harmless, naturally occurring compounds. The most common method for achieving this is through the use of a catalytic converter. A catalyst is a substance which promotes a chemical reaction without being chemically changed itself. In a catalytic converter, the catalyst will either oxidize (oxidation catalyst) a CO or fuel molecule, or it will reduce (reduction catalyst) a NOx molecule
A 3-way catalyst contains both reduction and oxidation catalyst materials and will convert NOx, CO, and NMHC (non-methane hydrocarbons) to N2, CO2, and H2O. A catalyst process which causes reactions of several pollutant components is referred to as a Non Selective Catalyst Reduction (NSCR). The efficiency of a 3-way catalyst is highly dependent on the percentages of the pollutants in the exhaust stream.
Catalytic converters should be installed as close to the engine as possible to retain high temperatures in the system. For the same reason, the catalytic converter should be installed upstream of any exhaust heat recovery equipment. Flexible connections should be used to connect the catalytic converter to the exhaust gas piping for thermal expansion compensation in addition to minimizing transmission of vibration.
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