Most new energy production facilities are constituted by cogeneration installations such as district heating. They are made by the cogenerator true, that in general is an internal combustion engine, or more rarely a gas turbine, and one or more reserve integration boilers.
The problem of noise is clearly present in planning the installation of the cogeneration plants because, for obvious reasons, they are always quite close to the houses that make up the same district heating utilities.
While all precautions are taken to reduce the noise generated by the CHP, we often only partially bring down the noise coming from the boilers, which have a high power of about 6MW to over 15MW.
The noisiest outside elements are the burner, with its combustion air fan and the exhaust duct. The burner and the combustion air fan perform a proper attenuation and it is sufficient to provide the appropriate acoustic baffles in the air intake of the fans and in the ventilation grilles of the building. Another story is the abatement of noise coming out of the exhaust stack.
Here we list a few:
– The noise is characterised by low frequencies
– The fumes are generally high-temperature
– The pressure losses must be very low
– The system layout often does not allow adequate space for the insertion of conventional silencers
Lately, it is quite frequently the case that the cause of the overrun and the audible limit to the most sensitive receptors is not so much due to the CHP, but the integration and reserve boilers and in particular to low-frequency noise that escapes from the chimney of the fumes.
Figure 1 shows the measured spectrum in the third octave inside the flue gas duct of a 14MW boiler. As can be seen, the values are very high and most of the acoustic energy is concentrated at frequencies 40Hz and 50Hz.
In addition to the receptors, characteristics of “pure tone” are often present, which is judged to be particularly bothersome and involves the penalty in case of frequencies below 200Hz and in the night hours of 6dB (A) with respect to noise measured.
It is easy to understand how important it is to break down the low-frequency noise emitted from the duct of the boiler flue gas to prevent the system from being stopped for the exceeding the noise limits. Unfortunately, when this happens you usually adopt “buffer solutions” without warning, which seldom reach a satisfactory result and result in changes to the flues, substantial pressure loss and high costs.
The CTM Ambiente has developed and successfully tested a type of silencer suitable for ducts of boiler fumes that solves such problems, as it concentrates its effectiveness at low frequencies, the pressure losses are negligible and the changes to the layout of the plant are not significant.
The silencer uses the known principles of acoustic resonance, but unlike conventional silencers such as those used for internal combustion engines it does not change the flow of fumes and does not cause turbulence and pressure losses. Moreover, the transverse dimensions of the silencer are slightly higher than that of the flue gas duct itself and may be positioned within the same air shaft or the same metallic support structure of the ducts without substantial changes. The reactive silencer replaces a portion of the smoke flue and is roughly the same size and weight.
Best practice: the case of the cogeneration plant in Rozzano (MI)
In 2010, the silencers were installed in two gas boilers at the cogeneration plant in Rozzano. The boilers were generating a power of 11MW and 14MW, respectively. There was an additional existing system which was composed of two cogenerators and two smaller boilers.
When the new boilers were put in operation, the neighbourhood located in a radius of 100m expressed deep unease because of the noise emitted (it increased by more than 12 dB (A)).
Some interventions acoustic reclamation were performed on the return air and an absorptive silencer on the fumes was added, after which the background noise fell to 6 dB (A). The noise of the two boilers was concentrated at 40Hz and 50Hz frequencies at which the absorption silencers are poorly effective.
CTM Ambiente, therefore, proposed to install a resonance silencer or reactive, which exploits the well-known principle of the Helmholtz resonator (Figure 2). In this case, the resonant volume is formed by a gap between the diameter of the smoke duct and the outer diameter of the silencer.
The resonant effect is expressed through a series of openings that put in communication the interspace with the duct of smoke. Because the fumes do not pass through these openings, the load losses through the silencer are negligible.
CTM Ambiente has developed a calculation programme for determination of the acoustic attenuations, which has already been tested on various applications, mainly on combustion engines, and has been adapted to the needs of the soundproofing of the boilers.
By the use of this programme, the basic parameters for the sound attenuation required were determined.
Three overlapping silencing modules were constructed for each of the boilers – one tuned to 40Hz, another at 50Hz, and the last at 80Hz. A wide sound spectrum was covered and centred on the fundamental frequencies.
The three modules formed a single silencer height of 5m, which replaced a section of the original pipe. The diameter of the silencer was only 200mm bigger than that of the original flue gas duct so as it could be placed in the same support lattice structure: 1.400mm diameter for the boiler 14MW and 1,300mm for the 11MW.
In the picture, you can see the insert operation of one of the silencers in the same structure supporting the existing pipelines that did not need any changes.
The attenuation achieved for the two boilers were in line with theoretical calculations. Figure 3 shows the results of the measures at the mouth of the two chimneys (red and green graphics) before (solid line) and after (dotted line) the acoustic intervention.
As expected, the spectrum of attenuation was particularly high at 40Hz and 50Hz, while it also extends to higher frequencies of up to 250Hz and above.
It is, therefore, impractical to use the default absorption silencers in many applications. The noise level measured with the boilers in operation in the vicinity of housing shows no change compared to background noise, which made it possible to resolve the dispute between the property of the CHP and the surrounding population.
The solution found by the Rozzano cogeneration plant that may be adopted in prevention: its low impact with respect to the boiler operation parameters, which do not permit significant pressure losses and turbulence at the outlet in the conduits, and its limited weight and overall width, which allows you to not change the layout of central and exploit the vertical section of the pipe with the same structure already provided, can prevent the onset of problems related to excessive noise of boiler flue gas in district heating.