Combined limitation of noise and polluting emissions from thermal generators

Thermal generators have many advantages when it comes to producing electricity in various contexts (not only in the form of gensets to constitute a constant source of power or to supplement mains failures: sometimes also for cogeneration, i.e. simultaneous electricity and heat production), both with regard to the methods of implementing a facility (e.g. prerequisites, time interval before commissioning, cost, maintenance) and with regard to what concerns operating conditions (e.g. modularity, performance.

The deployment of thermal generators is therefore quite widespread, not only inside buildings (technical rooms, boiler rooms) but also outside (e.g. for the needs of computer data centers, recently) including sometimes - even when an industrial site is concerned - in the immediate vicinity of buildings (residential, offices, in healthcare establishments); these are sometimes motors with a very high power (1 or more mega-watts), and taking into account the limitation of noise and polluting emissions is essential for a successful power plant construction or renovation project.

With regard to noise, energy production facilities (like other Facilities Classified for the Protection of the Environment - FCPE -) are subject to regulatory constraints with, in particular, the following requirements (in France) :

• not exceeding of the admissible limit for emergence: this is the difference between the A-weighted equivalent continuous pressure levels of the ambient noise - considered facility in operation - and of the residual noise - in the absence of noise generated by the considered facility, but measured over its operating period) in zones with regulated emergence (ZRE); depending on the context: 5 or 6 dB(A) during the day and 3 or 4 dB(A) during the night
• not have a tonal component of noise: when, considering an unweighted third-octave spectrum due to the operation of the air handling unit (AHU), the level in a frequency band exceeds by 5 dB or more the average level of the two adjacent frequency bands from 400 Hz to 6300 Hz or 10 dB that of the two adjacent frequency bands from 63 to 315 Hz
• not go over the permissible level at the property boundary, set by prefectural decree - typically 70 dB(A) during the day, 60 dB(A) during the night –

In addition, for workplaces (e.g. in an industrial site), as far as Europe is concerned, the Directive 2003/10/EC of the European Parliament and of the Council, of February 6, 2003, on the minimum health and safety requirements relating to the exposure of workers to the risks due to physical agents (noise) is applicable :

• for the daily noise exposure level L EX, 8h: 80 dB(A) and 85 dB(A) are the thresholds, respectively lower and upper, triggering the action; 87 dB(A) is the authorized limit value
• for the peak sound pressure ρpeak: 112 Pa i.e. 135 dB(C) and 140 Pa i.e. 137 dB(C) are the thresholds, respectively lower and upper, triggering the action; 200 Pa i.e. 140 dB(C) is the permitted limit value

With regard to the pollution of the atmosphere (by exhaust gases),the limitation of emissions of some pollutants into the air from Medium Combustion Plants (MCP) are subject to dedicated regulations^[1][2] in France. Emission limit values (in mg/Nm3) are now fixed:

• depending on the fuel
  • solid: biomass or other
  • liquid: diesel fuel or other
  • gaseous: natural gas or other
• for different pollutants:
  • sulfur dioxide (SO₂) 
  • nitrogen oxides NO_x
  • dust

With regard to nitrogen oxides NO_x:

• for existing engines and gas turbines, these limits are (in Table 3 of part 1 of Annex II of Directive (EU) n° 2015/2193 of 25 November 2015) defined as follows: respectively 190 mg/Nm3 except exceptions^[3] and 200 mg/Nm3 except exception^[4]
• for new engines and new gas turbines, these limits are (in Table 2 of Part 2 of Annex II of Directive (EU) No 2015/2193 of 25 November 2015) defined as follows: respectively 190 mg/Nm3 with exceptions^[5] and 75 mg/Nm3 except exceptions^[6]

 ITS will participate in the combined limitation of noise and polluting emissions of 7 thermal generators with a unit power greater than 3 MW in a computer data center located in Western Europe. The exhaust line of each engine will thus include: 

• an exhaust silencer for generator sets

• This is a hardware allowing noise attenuation:

  • at low and medium frequency (reactive stage): mainly due to the presence of discontinuities (changes in section and/or direction of the gas flow) in relation to chambers connected by tubes (perforated or not) resulting in punctual, cumulative reductions of sound power
  • at high frequency (dissipative stage): by the presence of fibrous materials whose interaction with the gas flow (when it circulates between surfaces thus lined, whether it is the periphery of a duct without any obstacle or it comes to splitters - e.g. baffles - ) causes a decrease in sound power proportional (in first approximation) to the length of the sound absorbing lining
    
    

• a Selective Catalytic Reduction Device SCR - catalytic converter

  This is a hardware that makes possible chemical reactions involving nitrogen compounds (e.g. urea CO(NH₂)₂ in aqueous solution), using precious alloy cassettes (e.g. tungsten W or vanadium V on a support with base of titanium oxide TiO2), and the result of which is to obtain compounds as harmless as nitrogen (N₂) and water (H₂O) instead of the undesirable gases that are NOx. Such a process is accompanied by noise attenuation at high frequencies, completing the performance of a silencer. The combined limitation of noise and polluting emissions from thermal generators exhaust will be based on the series assembly of these hardware, as a whole.

The high-performing subsets mentioned above will make it possible to obtain a multifunctional system (presenting itself in the form of a set on a chassis, that can be handled with a crane, ready for connection and almost immediate use - as per plug & play concept -). In the present context, the design shall take into account the various technical constraints inherent to the project, among which severe limitations of space and access will be added to the imperatives (ordinary, but always requiring complex dimensioning) of a total pressure loss compatible with optimal operation of heat engines.

With regard to the limitation of noise and polluting emissions from thermal generators exhaust, the following performance indicators are guaranteed (in order not to expose the neighborhood to nuisances and also the environment closest to high-power industrial engines):

• a sound pressure level at 1 m equal to 85 dB(A) in the worst case
• NOx emissions equal to 0.5 mg/Nm3 in the worst case

The raw materials (e.g. for the manufacture of the burnt gases exhaust silencers, the mechanically welded frame) will be of first choice (mostly: stainless steel), and the welds will be carried out by highly qualified personnel, using proven construction technologies; the quality of the other components - e.g. Selective Catalytic Reduction (SCR) module with continuous urea supply, and dosage managed by means of a PLC and sensors - will be subject to rigorous selection, while the assembly and quality control will be meticulous, so as to deliver, within the allotted time, hardware whose robustness and durability will be, in addition to efficiency, one of the strong points (in the context of outdoor installation). 

This project to limit noise and polluting emissions from high-power thermal generators will once again be an opportunity for ITS and its business partners to implement high-performance solutions, based on advanced- technologies e.g. in the field of acoustics and fluid mechanics, for the preservation of the environment.

^[1] This applies in particular:

• to engines: gas, diesel or dual fuel, defined (in the directive) as follows:
  • "'gas engine"': internal combustion engine operating according to the Otto cycle and using spark ignition to burn fuel
  • "'diesel engine": an internal combustion engine operating on the diesel cycle and using compression ignition to burn fuel
  • "'dual fuel engine": an internal combustion engine using compression ignition and operating according to the diesel cycle to burn liquid fuels and according to the Otto cycle to burn gaseous fuels
• to gas (combustion) turbines defined (in the directive) as follows:
  • any rotating device which converts thermal energy into mechanical work and consists mainly of a compressor, a thermal device for oxidizing the fuel so as to heat the working fluid and a turbine; this definition includes open circuit gas turbines and combined cycle gas turbines, as well as gas turbines in cogeneration mode, whether or not equipped with an additional burner in each case

^[2] The following are concerned: new installations with a capacity of more than 1 megawatt (MW) from 20 December 2018, existing installations with a capacity of more than 5 MW from 1 January 2025 and existing installations with a power between 1 and 5 MW from January 1, 2030

^[3] 250 mg/Nm3 to 1850 mg/Nm3 depending on the considered combination of variables which are the type of engine (and its fuel), its thermal power, its date of construction

^[4] 150 mg/Nm3 if the fuel is natural gas

^[5] 225 mg/Nm3 to 1850 mg/Nm3 depending on the considered combination of variables which are the type of engine (and its fuel), its thermal power, its number of hours of use

^[6] 50 mg/Nm3 if the fuel is natural gas, 550 mg/Nm3 (until January 1, 2025) for facilties that are part of isolated small networks or micro-grids within the meaning of Article 2 of Directive 2009/72/EC of the European Parliament and of the Council

^[7] the emission limit values are valid for a temperature of 273.15 K, a pressure of 101.3 kPa and after correction according to the water vapor content of the waste gases, and for a standardized content of O₂ (15% in the case of gas engines and turbines)