Diesel standby generators and the environment  

May 30th, 2018, Published in Articles: EE Publishers, Articles: Energize, Articles: Vector

Diesel standby plant has an impact on the environment and is subject to regulations governing noise levels and emissions.

The diesel standby plant (DSP) forms an essential part of the electricity supply network and appears in many forms and in many places. In the past, the DSP was common at high-reliability sites such as telecommunications platforms and hospitals, but it is now much more widespread.

Most manufacturers of large DSPs ensure that their equipment complies with environmental regulations, but it is useful to know what these regulations are and where they apply. Compliance with regulations adds cost, especially with smaller plant. The cost of full compliance for a small DSP may not differ much from that of a much larger plant.

South Africa’s Environmental Conservation Act of 1989 does not refer specifically to DSPs and general rules must therefore be interpreted for the case of DSPs. Countries such as the USA have included DSP in their regulations, applying less strict standards than for other diesel generator plant [2].

Standby plant emissions

The main concern with diesel engines is with NOX and diesel particulate matter (DPM), and there is slightly less concern with sulphur compounds and CO2. The main problem with standby plant is that the plant is required to take load immediately when the required engine speed is reached, and operates below optimum temperature for a significant portion of the cycle. The load on the engine can also depend on the time at which the emergency occurs and can vary from full load to a very small partial load. The DSP is required to meet standards under all these conditions.

There does not appear to be any South African legislation dealing with emissions from stationary engines at the moment . The existing legislation, the National Environmental Management: Air Quality Act provides standards for stationary power generation plant using liquid fuel, but specifically excludes reciprocating engines. Nonetheless, there are a number of national standards of other countries which provide a useful reference.

US Environmental protection agency (EPA)

The EPA has produced a set of standards which have become progressively stricter over the years. Wisely, the EPA has realised the importance of achieving an acceptable level of security of supply at a reasonable cost and has reduced the requirements for standby plant, probably based on the impact of intermittent usage and low duty cycle. Table 1 shows the EPA requirements for different sizes of standby plant.

Table 1:  USA EPA emissions limits [2].
Engine size (kW) NOx (gm/kWh) DPM (gm/kWh)
<18 emergency 7,5 0,4
19 to 55 emergency 7,5 0,3
56 to130 emergency 4,3 0,3
130 to 560 emergency 4,0 0,2
>560 kW emergency 6,0 0,2
130 to 560 non-emergency 0,4 0,02

There is a marked difference between the standards for emergency plant and other stationary engines. EU standards do not take standby operation into account but have similar values to the EPA (see Table 2).

Table 2:  EU stationary diesel engine emission standards [4].
Engine size (kW) NOx (gm/kWh) DPM (gm/kWh)
<18 Not regulated 4,0 Not regulated
19 to 36 7,5 0,6
37 to 74 4,7 0,4
75 to 129 4,0 0,3
130 to 559 4,0 0,2
> 560 Not regulated Not regulated

NOx reduction methods

Nitrogen oxides (NOx) are formed in the combustion process by oxidation of nitrogen (from the atmosphere and fuel) to NO and NO2. the NOX formation rate in an engine is largely temperature driven and consequently a function of the local high-temperature areas and their duration during combustion. To reduce NOX emissions, it is necessary to either prevent their formation in the cylinder (primary method) or to remove the NOX from the exhaust gases in an after-treatment system (secondary method).

Most NOx reduction methods are based on reducing the temperature of combustion. The introduction of water has a positive influence on reducing NOX formation by cutting temperature peaks in the combustion process. Various methods of introducing water have been evaluated and tested, such as water-in-fuel emulsions, humidification of the combustion air and water injection.

Selective catalytic reduction (SCR) is a well-established method of NOX reduction but most systems rely of the exhaust reaching the right temperature. SCR is a proven NOX-reduction technology that uses an ammonia-based solution introduced into the engine’s exhaust system. This generates ammonia gas (NH3), which then reacts with NOX to form N2 and H2O. SCR technology can achieve very low NOX levels while potentially contributing to enhanced generator performance and fuel consumption. [1]. SCR technology relies on proper mixing and decomposition of the solution in the exhaust piping upstream of the catalyst, which needs exhaust conditions such as temperature and flow to be at specified conditions.

As an engine starts up after time at ambient or cold conditions, it takes time for the exhaust to reach the optimum temperature.  Operating the engine and after treatment system at optimum temperatures is required to achieve the lower emission levels desired in the implementation of the after treatment system. NOX emissions won’t be reduced until the optimal temperature for solution injection is reached [1].

DPM reduction

Diesel particulate matter reduction is achieved by diesel oxidation catalysts (DOC) which oxidise the carbon and hydrocarbons in the exhaust gas, and DPM filters which remove particulate matter from the exhaust gas. Diesel oxidation catalysts require the correct temperature for operation and may not be effective on standby plant. The diesel particulate filter is designed to collect diesel particulate matter and must be cleaned fairly often. Regeneration techniques are used and engine manufacturers use heat management systems to increase the exhaust gas temperature.  A special catalytic coating on the filter is used to burn the soot off and oxidise it to CO2 and N2 The regeneration process requires the right temperature and operation for relatively long periods and may not work in standby mode.

Standby plant noise levels

The existing South African environmental regulations governing noise levels do not contain specific reference to standby plant, and reference must be made to the general noise level requirements. This can prove difficult as a specific location may require customised soundproofing because of the site conditions and proximity to occupied premises. Although the industry has adopted general standards for DSP sound levels, special attention may need to be given in specific cases

A standby plant is often located in or close to residential areas, where strict noise control regulations are in place. Most responsible users of standby plant will ensure that the plant is enclosed in a soundproofed container or room, as this ensures protection form the environment as well as soundproofing.

Regulations

The national regulations governing noise levels are contained in the regulations associated with the Environmental Conservation Act of 1989. These regulations stipulate the noise levels measured at particular locations, and not noise levels emitted by the machinery or plant. In other words, the standby plant must not cause the noise level at a particular location, usually an occupied location, to exceed the recommended level.

Recommended maximum noise levels in various environments are contained in SANS 10103 The measurement and rating of environmental noise with respect to annoyance and to speech communication” and this standard is referred to in all legislation. In addition to the national regulations, noise levels are controlled by provincial bylaws and different rules apply for each province. They are nonetheless all based on the national standards. The recommended levels are shown in Table 1. Many of the provincial regulations exempt application of the rules under emergency conditions, but do not go as far as to define what constitutes an emergency.

Table 3: Recommended noise levels (SANS 10103).
Type of district Equivalent continuous rating level (LReq.T) for noise
(
dBa)
Outdoors Indoors with open windows
Day-night Day time Night time Day-night Day time Night time
Residential districts
Rural 45 45 35 35 35 35
Suburban with little road traffic 50 50 40 40 40 30
Urban districts 55 55 45 45 45 35
Non-residential districts
Urban districts with some workshops,  business premises, and with main roads 60 60 50 50 50 40
Central business districts 65 65 55 55 55 45
Industrial districts 70 70 60 60 60 50

Day-time is defined as from 06h00 to 22h00, and night time 22h00 to 06h00.

It can be seen from Table 3 that, for a DSP in an urban residential area, the noise level would have to be as low as 45 dBa. However, the noise level will depend on how close the DSP is to the listener. Noise levels vary according to a square law, e.g. if the distance away from the source is doubled, the noise level falls to a quarter of the previous level, and if the distance to the source is halved, the noise level increases four times.

Fig. 1: Sound proofing.

Sources of generator noise

Noise emitted by an open genset varies from 100 to 120 dBa at a distance of 1 m [1]:

  • Engine noise: This is mainly caused by mechanical and combustion forces and typically ranges from 100 to 121 dBa, measured at one meter, depending on the size of the engine.
  • Cooling fan noise: This results from the sound of air being moved at high speed across the engine and through the radiator. Its level ranges from 100 to 105 dBa.
  • Alternator noise: This is caused by cooling air and brush friction and ranges from approximately 80 to 90 dBa at one meter.
  • Induction noise: This is caused by fluctuations in current in the alternator windings that give rise to mechanical noise that ranges from 80 to 90 dBa at one meter.
  • Engine exhaust: Without an exhaust silencer, this ranges from 120 to 130 dBa or more and is usually reduced by a minimum of 15 dBa with a standard silencer.
  • Structural/mechanical noise: This is caused by mechanical vibration of various structural parts and components, radiated as sound.

Noise reduction methods

Diesel standby plants are usually enclosed in canopies made of sound absorbent material to reduce noise levels. This, however, requires extra ventilation for engine cooling and the canopy will be fitted with a fan for this purpose. Air intake and outlet exhaust openings allow noise to escape and must be equipped with sound absorbent baffles to reduce transmitted noise. The exhaust requires a high attenuation silencer to reduce noise further . Fig. 1shows a typical arrangement.

Fig. 2: Sound level measurement distances.

Sound level measurements

The regulations do not specify noise levels from DSPs, and the industry has adopted two methods of measuring noise levels and associated limits. The first measures noise at 1 m away from the periphery of the DSP, which would be the normal distance a worker would be away from it, and uses a limit of 80 dBa, the industrial noise level. The second method uses points at a distance of 7 m away, which is more likely to represent the effect of the plant on the surrounding area. Levels are taken to be 80 dBa at 1 m, which translates to about 70 dBa at 7 m, the recommended level for industrial districts (see Fig. 2).

References

[1] D Aarberg: Generator set noise solutions: Controlling unwanted noise from on-site power systems”, Cummins power generation, Power topic #7015.
[2] G Johansen: “Choosing the right emission standard and the right technology for emergency standby power systems”, Cummins power generation, Power topic #9021.
[3] Wartsila: “Air emissions legislation review for internal combustion engines”, 2015.
[4] John Deere: Generator drive applications: diesel engine ratings”, John Deere pocket guide.

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