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A case study: an environmental and economic analysis of Turin District’s heating system combined with a cogeneration plant

A. Senor, D. Panepinto & G. Genon Turin Polytechnic, Italy

Abstract

The search for technological systems capable of reducing CO2 emissions’ efforts to minimize climate change, show the district heating (DH), combined with cogeneration plants, in the front line. The district’s heating, consisting of the distribution of hot water by means of underground networks for the buildings’ heating and water for sanitary purposes, is an ever-expanding technology that allows the optimization of energy resources, with positive consequences in terms of both economic savings and environmental impacts. The aim of this work is to analyze the existing DH system built in Turin (Piedmont, North Italy). We evaluate in particular, the energetic recovery system analyzing the existing cogeneration system designed to supply DH: from one side, a recovery of electric energy (fed into the national grid) and on the other side, a recovery of thermal energy (heat input into the local DH system). From an environmental standpoint, we use mass and energy balance as a tool to implement pollutant dispersion models in order to obtain results concerning the air quality. Keywords: cogeneration plant, energy recovery, electric energy, thermal energy, environmental compatibility, externality, dispersion models, district heating system.

1 Introduction

www.witpress.com, ISSN 1743-3509 (on-line) WIT Transactions on The Built Environment, Vol 168, © 2015 WIT Press

Sustainable Development, Vol. 2 651

doi:10.2495/SD150572

Turin District’s heating system (Piedmont, Italy) is certainly an interesting example of high technological content because it has established the primacy of the city which has the widest district heating network of Italy, while making it one of the most extensive district heating systems in Europe (Torchio et al. [2];

2 Materials and methods

2.1 Thermoelectric power plant: description

Turin’s integrated district heating system in this study is fed with the heat produced by three modern combined cycle plants designed using the very latest technologies operating in cogeneration mode at the Moncalieri plants (composed of two twin plants, called 2nd GT and 3rd GT with total capacities of 800 MWe and 520 MWt) and Torino Nord (400 MWe and 220 MWt). Needs for supplementary power are covered by back-up plants as well as heat accumulation systems evenly distributed across the area collocated in the plant of Moncalieri, Torino Nord, BIT and the Polytechnic whose overall thermal capacity reaches about 1,100 MW. The cogeneration plants, are made up principally of a thermoelectric, dual shaft, combined-cycle unit (gas turbine and steam turbine), with a gross electrical power in cogeneration mode of about 1200 MW, fed exclusively with natural gas; the supplementary and back-up steam generators, with a total thermal power of 622 MW, fed exclusively with natural gas; the heat accumulation system made up of tanks. Finally, the flue gas treatment consists of selective nitrogen oxide (NOX) catalyst reduction system that drastically reduces the concentrations of oxides in the smoke at the chimney. The combustion chamber has 24 dry low NOX emission burners set in a ring and the combustion area lies around the inlet section of the first turbine stage. Table 1 displays the cogeneration and reserve/integration plant emissions.

Table 1: Plant emissions.

Cogeneration plants [t/y] 2nd GT 3nd GT TO nord CHP Pollutant load NOx 72.31 88.79 23.49 184.59 SO2 0.00 0.00 0.00 0.00 PM10 2.40 2.96 2.82 8.18 CO2 188322.83 231827.56 172380.54 592530.93

Reserve and integration plants [t/y] GT boiler TOnord boiler BIT POLI Back-up Pollutant load NOx 0.32 11.44 0.96 0.60 13.32 SO2 0.00 0.00 0.00 0.00 0.00 PM10 0.01 1.37 0.11 0.07 1.56 CO2 825.87 83909.08 7028.15 4383.38 96601.48


652 Sustainable Development, Vol. 2

Genon et al. [3]; Panepinto et al. [4]). The evaluation may be considered to have a dual purpose: specifically, to establish the compatibility of the actual Turin plant configurations connected with DH, and in general, as a useful decision-making tool for policy-makers.

2.2 The actual Turin District heating network (DHN)

With the technical support of CSI-Piedmont and IREN Energia S.p.A., a spatial and urban analysis was performed by identifying accurately the number of connected dwellings to the district heating network. In agreement with the document “Piano di sviluppo del teleriscaldamento nell’ambito di Torino – PSLRTO”, (Provincia di Torino [1]) we have verified the actual DHN and the results are shown in Table 2.

Table 2: Hypothesis of connection to the district heating network.

Common connected to DH CHP plant operation

Served volume (Mln m3)

Grugliasco, Moncalieri Nichelino, Torino

12 months 16.33 7 months 40.26

TOT 56.59 If we analyze the results that are reported in Table 2 we can see that two types of users were considered: heating only required (7 months) and combined heating/health required (12 months).

2.3 Cogeneration plant: operating conditions

The first step was to define the plant operation considering that the main goal is to satisfy the thermal domain of domestic user; since summer is the least season for the plant’s energy recovery operations, the general stoppage of one month will be required for maintenance in June/July considering each plant at one time. The second step was to determine the amount of electric power that can be introduced into the national network because of thermal power that can be produced to the district heating network.

Table 3: Plant characterization.

Plant Electric capacity Thermal capacity

Pel fu Pth f

u

2nd GT 395 MWel 58% 260 MW

th 89%

3rd GT 383 MWel 58% 260 MW

th 88%

GT integration boilers - 282 MWth

87%

TO nord 400 MWel 58% 220 MW

th 87%

TO nord integration boilers - 340 MWth

87%

BIT integration/reserve boilers - 255 MWth

87%

Polytechnic integration/reserve boilers - 255 MWth

87%



2.4 Trend for exported energy considering requirements

We defined the hypotheses of cogeneration plant operation (shown in Figure 2) by taking into account the number of connected dwellings to the DH network, the hypotheses of plant operation, the ratio of thermal power to electric power and the thermal requirements of the potential users (considering the average trend over the last 10 years), with respect to the cumulate curve of thermal load for the Turin province.

Figure 1: Operative configuration: (a) 2nd GT CHP, (b) 3rd GT CHP, (c) GT integration boilers, (d) TO nord CHP; (e) TO nord integration boilers; (f) BIT integration/reserve boilers; (g) Politecnico integration/reserve boilers.

(g)

(e) (f)

(b)(a)

(c)

(d)



The graph (a), (b) and (d) in Figure 2 reports the net electric power and the net thermal power produced by the thermoelectric power plant that can be introduced into the electrical network and into the district heating network. The graph (c), (e), (f) and (g) included the thermal power generated by the integration and reserve boilers designed to satisfy the requirements during periods of peak heat.

3 Methodology for evaluation of air environmental impact

Since the evaluation of environmental compatibility is considered chiefly with respect to the effects of the plant operation on the air quality, the analysis was performed using two different tools: an environmental balance, in order to define the real introduced and avoided emission fluxes; an implementation of the pollutant dispersion model, to evaluate the real air quality modification consequent to the cogeneration plant activity, and the phasing-out of the substituted energy sources.

3.1 Energy and environmental balance

In order to evaluate the introduced load and the local and global environmental benefits of substitution, it is necessary to compare the emissive fluxes before and after the start-up of the cogeneration plants. The emission factors (concerning the production of thermal and electric energy) used to define the emission avoided are shown in the following tables (Fracastoro et al., 2009; Panepinto and Genon, 2012 b).

Table 4: Emission factors for the production of thermal and electric energy.

EF for thermal energy production (g/GJ) EF for electric energy production (mg/kWh) Methane Combustible oil

SO2 0.83 76.38 600 NOx 42.06 56.23 943 PM10 6.70 5.48 29 CO2 55,500 675,000

Table 5: Average composition of the domestic boiler.

Boilers manufacturing year

Combustible Methane Combustible oil

1980–89 50% 50% 1990–99 75% 25% 2000–09 85% 15% 2010-now 95% 5%

From the urban analysis, we obtained the information that the average composition of the domestic boiler (shown in Table 6) in the analyzed area derived considering the following data:



Table 6: Average composition of the domestic boiler.

Pollutant Equivalent EF [kg/GWh]

SO2 50.45 NOx 159.62 PM10 23.24 CO2 199,800

The formulation of the environmental balance on the local scale is only preparatory for the implementation of the pollutant dispersion model. The aim of the formulation of the environmental balance on the global scale, though, is to evaluate the emission of carbon dioxide.

3.2 Implementation of pollutant dispersion models

In order to evaluate the severity of the local environmental impact produced by the plant, it is necessary to consider the results of the dispersion models. With this approach, it is possible to calculate the real air-quality modifications: the concentrations (annual mean values and maximum hourly values) that will be created by the future plant, and the elimination of concentrations corresponding to the sources that will be avoided (from the elimination of existing domestic boilers). We elaborate concentration maps from the results of the simulation of the atmospheric dispersion of pollutants emitted from all relevant sources, using the Aermod model. It is a Gaussian model, which uses the Gauss function of errors as an analytical solution of the equation of transport in the atmosphere, shown below (U.S. Environmental Protection Agency).

4 Results and discussion

The first tool we used in evaluating environmental compatibility was the environmental balance. In consideration of the quality of the emissions, on both the local and global scales, the analyzed pollutant parameters were: dust, SOx and NOx on the local scale, and CO2 on the global scale.

4.1 Results of environmental balance

For evaluation of the global scale environmental balance, the obtained results are summarized below. From Figure 2 it can be seen that, as a consequence of thermoelectric plant operation, the total emissive flux of the pollutant parameters NOx, SO2 and dust will be decreased. The reason of this decrease arises from a better emission factor for these parameters in the case of the thermoelectric power plant and the integration/reserve plant in comparison with the performances of the national electric energy conventional producers. A strong decrease of CO2 emissions can be also observed as a consequence of the activation of the cogeneration plant. The reason for this emissions saving



Figure 2: Results of global scale environmental balance.

must be sought, firstly in the value of emission factors EF (kg CO2/GWh; EFe = 199,800 << EFth = 675,000) as can be seen from Tables 4 and 6; secondly, for the non-linear thermal and electrical energy produced. Going to the local scale, environmental balance has been used for an evaluation of the real modification of the air quality, by using atmospheric dispersion models to establish the ground-level pollutant concentrations. The results of the local scale balance are presented in Figure 3.

Figure 3: Local scale balance results.

The local-scale results (shown in Figure 4) indicate that the functioning of the plant will lead, merely relative to NOx pollutant, to an increase in the emissive flux while it is possible to recover over 100 tons per year of particulate and near 20 tons per year of SO2.

4.2 Results of the application of the dispersion model

To define the dispersion and transport of pollutants, the Aermod model, which was developed for the EPA (Environmental Protection Agency) by the American Meteorological Society (AMS) as an evolution of the well consolidated Gaussian model ISC3, was used.



An analysis was performed of the parameter NOx and PM10 by considering the low importance of the parameter SO2 by disregarding the chemical transformation and removal phenomena. In order to establish the ground level concentration, within an area extended 40 x 40 km (Figure 4), was utilized methodology as follows: First, a set of maps were constructed of ground-level concentrations consequent upon the activation of the cogeneration plants and integration/reserve plants; Second, barycentre stacks were positioned for each zone of the district heating utilization (home boilers) that will be 56 Mm3. Finally, the result was obtained by calculating the difference between the two results, i.e. added emissions minus avoided emissions (substituted thermal plants).

Figure 4: Study area.

With this data set, we considered the general yearly trend for the DH connection, and the more critical days for the maximum concentrations. On first analysis we considered the mean annual ground-level concentration values of NOx: the maximum concentrations deriving from the cogeneration plant and from the domestic boilers are:

Cogeneration plant: 6.5 µg/Nm3; District heating: 1.6 µg/Nm3.

Comparing the territorial distribution of the annual mean ground level concentrations, referred to Figure 5, it may be observed that, in the case of the substitution, there is a very small advantage for the areas close to the western side of the Turin hill, meaning that the very limited criticality on the more exposed zone remains. In any case, because of the maximum value allowed by National law is 40 µg/Nm3 (D. Lgs. 152/2006), increasing by just 6.5 µg/Nm3 following activation of the plants, the real plant impact

On second analysis we considered the mean annual ground-level concentration values of PM10: the maximum concentrations deriving from the cogeneration plant and from the domestic boilers are:

Cogeneration plant: 0.0095 µg/Nm3; District heating: 0.034 µg/Nm3.



(a)

(b)

(c)

Figure 5: (a) NOx due to the thermoelectric power plants operating (annual averages). (b) NOx effect of the boiler substitution (annual averages). (c) NOx balance effect (annual averages).

Figure 6: PM10 Balance effect (annual averages).

Checking the particulate PM10 ground concentration on annual average (in Figure 6), otherwise the NOx behaviour, it may be observed that, in the case of



the domestic boilers substitution, there is a very important benefit for spread areas close to the whole Turin hill, the entrance and the eastern side of Susa Valley, and Turin flat land. Worse condition are found instead on the mountain peaks of Susa Valley and Monte San Giorgio, nearest the Torino Nord thermoelectric power plant and on small area of South side of Turin Hill. The maximum value of pollutant recovery (PM10), is equal to 0.03µg/m3. The situations that can be observed in the more critical days of the year, the maximum NOx values corresponding to the emissions of the cogeneration plant and of the substituted thermal plants are as follows:

Cogeneration plants: 132.40 µg/Nm3 (corresponds to 26/12); Domestic boilers: 19.40 µg/Nm3 (corresponds to 03/01).

(a) (b)

Figure 7: Pollutant NOx. (a) local maximum effect of the boiler substitution (03/01); (b) balance effect (03/01).

Analysing Figure 7, it is clear that there is a sharp air quality worsening in terms of the concentration of NOx close to the Turin hill considering the daily average value. The reasons may be sought in the orographic and weather-climate characterization. However, there are large areas with an improvement in air quality, following the replacement of boilers with district heating, although the maximum values reach 15 µg/Nm3. Taking into account the maximum daily PM10 values corresponding to the emissions of the cogeneration plant and of the substituted thermal plants are as follows:

Cogeneration plants: 0.14 µg/Nm3 (corresponds to 26/12); Domestic boilers: 0.41 µg/Nm3 (corresponds to 03/01).

The daily balance on the ground of PM10, as can be seen from Figure 8 on the right, turns out to be widely beneficial because, especially in the day with maximum savings of pollutants from boilers, allow a recovery to the ground about 0.4 µg/Nm3.



(a) (b)

Figure 8: Pollutant PM10. (a) Local maximum effect of the boiler substitution (03/01); (b) balance effect (03/01).

4.3 Results of the externality assessment

Overall, adding the economic contribution of each pollutant, on local scale, there is a negative economic burden. Moving on to analyse the global externalities, carbon dioxide gives a huge contribution to the budget statement. The monetary value is mostly resulting from the balance between CO2 and PM10 while sulphur oxides and nitrogen play only a secondary role.

5 Conclusions

The results obtained for energy recovery from the Turin thermoelectric power plant in cogenerative configurations, underscore the high-energy efficiency of the combined production of heat and electricity (CHP), and the opportunity to minimize the environmental impact by including cogeneration in a district-heating scheme. To conclude our analysis, the following points may be considered:

It has been possible to establish the environmental compatibility of the plants by evaluating the possibility of obtaining concentration values significantly lower than allowed limits;

The emissive environmental balance leads to definitive information only for CO2;

The ground-level concentration maps establish a fundamental baseline for the local analysis: the mean annual values, for connection to the nearest municipalities (Grugliasco, Moncalieri, Nichelino, Torino), it is possible to achieve a ground-level concentration increases (roughly 6 µg/m3) of NOx, over current level. This worsening is due to the fact that if on the one hand we have only the domestic boilers that are switch-off to be substituted by a district heating service, on the other hand there are three different



thermoelectric power plant that produce not only thermal energy to be put into the DH network, but also electric energy puts into the national grid, therefore it provides no benefit on local scale; another consideration is the strong effect of stack height on emissions. On the contrary, if we consider the most advantageous days in consequence of the district heating (03/01), it is possible to observe important improvements for the ground-level NOx concentrations (recovery from values as high as 15 µg/m3);

The mean annual values of PM10 ground-level concentration are able to decrease the actual pollutant level about 0.03µg/m3 on very large area into the study burden and, considering the most advantageous days in consequence of the substitution of domestic boilers (03/01), there is about 0.30 µg/m3 PM10 regain;

There are important effects on local pollution directed to the inhabitants of the plant’s surrounding area, and these effects can be limited only by using co-generation; the most affected areas are the Turin hills and ridges of the eastern side of the valley of Susa.

Externalities, i.e. costs/environmental benefits related to the environmental impact generated by the operation of the plant to be paid by individual’s target (community), can then be compared with the business plan related to the thermoelectric management. The authority for the regulation of the public local services will determine the final operation scheme of energy recovery.

References

[1] Provincia di Torino. Piano di sviluppo del teleriscaldamento nell’area di Torino, Rapporto finale, February 2009. http://www.provincia.torino.gov.it /ambiente/energia/progetti/piano_sviluppo_TLR. Accessed 02 April 2013

[2] Torchio M. F., Genon G., Poggio A, Poggio M., 2009. Merging of energy and environmental analyses for district heating systems. Elsevier Vol. 34, Issue 3, March 2009, pp. 220–227

[3] Genon G., Torchio M. F., Poggio A., Poggio M., 2009. Energy and environmental assessment of small district heating systems: Global and local effects in two case-studies. Energy Conversion and Management Vol. 50, Issue 3, March 2009, pp. 522–529

[4] Panepinto D., Senor A., Genon G., 2014. Environmental and economic analysis of the Turin incineration plant. WIT Transactions on Ecology and the Environment Vol. 180, 2014, pp. 479–490



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