# On Some New Indicators for the Energo-Ecological  Assessment of Thermo-Power Plants Operation

Authors: Mircea Cârdu; Arina Speranţa Negoiţescu; Dana Sandu
Journal: Research Letters in Ecology
 (2008)

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## Abstract

The authors offer a critical analysis of pollution indicators currently applied for the ecological assessment of the thermo power plant's operation. They forward new “energo-ecological”  indicators to highlight both the qualitative aspect of polluting emissions and their quantitative aspect, by relating their concentration in the flue gases purged into the atmosphere to the energy produced during the same interval. The application of these indicators contributes to the attenuation of the global warming phenomenon and to the protection of the world's resources of fossil fuels.


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## Body

## 1. Introduction

It is known that  the fact that a major part of the consumed primary energy
comes from fossil   fuels. The burning of these fuels result in polluting
emissions purged into the atmosphere, that is, noxious gas emission, toxic for
animal and plants (nitrogen oxides and sulphur oxides, expressed asSO2 [1]), and greenhouse gases (GHGs).
When burning fuels, the latter (GHGs) are the
nitrogen oxides (NOx) and the carbon dioxide (CO2).In the past years, one took into account especially the toxic effect of
polluting emissions and consequently the international norms in this field
referred to the limitation of these emissions (SO2
and NOx)
[2].During the last period, especially after 2000, one has remarked worrying
climatic changes. They are mainly due to the increase of the GHG emissions.
Under these circumstances, one is currently emphasizing the reduction of the
GHG emissions, among which, as we know,CO2 has a high percentage.Considering all the above, it results that in the analyses regarding polluting
gas emissions, besidesNOx and
SO2 (contained in the
present norms for their limitation in the flue gases), we should also consider
the CO2 emissions. On the other hand, in the interest of reducing
the total quantity of such emissions for avoiding the excessive global warming
and for preserving mankind's reserves of classic fuels, it has become important
to consider the polluting emissions related to the energy produces during this
process, in terms of their ratio.We will analyze this issue in detail, with application to the field of thermo
power plants(TPPs).


## 2. The Opportunity of Considering the Net Emission Factor

At present, in the EU countries, for the limitation of emissions of
certain pollutants into the air from large combustion plants, we apply “Directive 2001/80/EC of the European Parliament and of the Council of
23 October 2001” [2]. If we refer to the polluting gas emissions—sulphur dioxide
(SO2) and nitrogen oxides (NOx)—the respective directive
focuses on the concentration of these gases. The dimension of these emissions
will be thus [mg/Nm3].We consider that the limitation of pollutants according to [2] was
appropriate in the past, when the focus was laid on the concentration of
polluting gases in the flue gases, and when we considered as most important the
toxic influence of these gases on animals and plants. But at present, in the
light of the previous chapter (the greenhouse effect of polluting emissions),
we think that the limitations of TPP’s polluting emissions into the air should
also take into account the quantitative aspect of the phenomenon. In other
words, these limitations must lead to the emission of more reduced quantities,
as reduced as possible, of pollutants related to the quantity of useful
electric energy and/or thermal energy the TPP in question produces. These can
be solved by considering the ratio of the concentration of the polluting gases
emissions per energy produced simultaneously with the respective emissions.This approach is not completely unheard of it was encountered in the case
of the norms for the polluting emissions from motor vehicles [3–5].We should also point out thatCO2 is the most important
greenhouse gas (GHG) and consequently the limitations of polluting emissions,
besides NOx and
SO2, will have to refer also to
the carbon dioxide.The following discussion will be focused on the development of the
idea contained in the title of the present chapter.For this purpose, as a first step, we use the notion of“raw emission factor.” We noted it REF and,
for the three polluting
gases, it is given by the following relations:(1)REFc=Vg⋅(CO2)/Qi[kg/kJ],REFn=Vg⋅(NOx)/Qi[kg/kJ],REFs=Vg⋅(SO2)/Qi[kg/kJ].In the above relations,Vg is the volume of flue gases resulted from the
combustion of one kgf of fuels and has the dimension [Nm3/kgf]; kgf
is the measuring unit for the fuel mass (f-fuel). (CO2), (NOx),
and (SO2) represent the concentration of polluting emissions in the
burning gases, and they are described thus by [kg/Nm3]. Qi is the inferior thermal power of the fuel and is measured in [kJ/kgf].The emission factors REFs give indications about the “ecological
characteristics” of a fuel.In order to further analyze the issue we focus on the following. We divided
the conventional fuels into four major categories and we retained for them the
average values of REFs, as listed in Table1. Moreover, the average values of Qi and Vg were also given. Table 1 also shows the values of the Vg/QI ratio.Table 1LignitePit coalFuel oil (withS≤1%)Natural gas106⋅REFc [kg/kJ]105907250106⋅REFn  [kg/kJ]0.240.430.2450.15106⋅REFs  [kg/kJ]3.322.060.459—Qi[kJ/kgf]; [kJ/Nm3]*8000130004260035700*Vg[Nm3/kgf];
[Nm3/Nm3f]**3.3534.41711.3089.983**104⋅Vg/Qi [Nm3/kJ]4.1913.4002.6542.796The last three characteristics will be used below. These values were
taken from the literature. In order to take the second step toward the reaching
of the set goal, we introduce the notions of“net emission factor” NEF, expressed as depending of the REF values
starting from relations (1), and so(2)NEFc=REFc/η=Vg(CO2)/ηQi[kg/kJ],NEFn=REFn/η=Vg(NOx)/ηQi[kg/kJ],NEFs=REFs/η=Vg(SO2)/ηQi[kg/kJ]. In the above
relations, η represents the general efficiency of an TPP, that is, the ratio of
the useful energy delivered by the energetic installation per thermal energy
contained in the fuel.By useful energy
(η⋅Qi), we understand the electric and/or thermal energy delivered
at the terminals of the electric generator or/and at the interface
point of the installation with the system of thermal energy use. We neglected
the internal consumptions of TPPs, such as those for the preparation and
manipulation of fuel up to its introduction into the installation of the
boiler, those for the circulation of the cooling water, and so forth.The consideration of the NEFs indicators fully satisfies the present need
to analyze the TPPs operation in a complete manner, that is, under the aspect
of the atmospheric pollution in correlation with the produced useful energy. Thus,
the NEFs indicators have an energo-ecological character.For an TPP, it is important that during its operation one should obtain
emission indicators as reduced as possible. This can be achieved, on the one hand,
by using high-quality fuel, especially hydrocarbons (see the REFs values in
Table1). On the other hand, the high values of the TPP’s general efficiency
also result in small values of NEF.The emissions of nitrogen oxides and sulphur oxides, that is, (NOx)
and (SO2), may be reduced, in their turn, as shown above by means of
special methods or installations characterized by the efficiency σ. The methods
of CO2 capture are also characterized by their own efficiency. Considering all
this, the NEFs emission factors may be expressed by the following relations:(3)NEFc=3600Vg(1−σc)(CO2)/ηQi[kg/kWh],NEFn=3600Vg(1−σn)(NOx)/ηQi[kg/kWh],NEFs=3600Vg(1−σs)(SO2)/ηQi[kg/kWh].Depending on the system applied for the reduction of theSO2,
NOx, and CO2 emissions into the atmosphere, their efficiency
may have values between 0.5 and 1.0.Compared to the analysis of TPPs depending on the polluting gas emissions
related to the quantity of the flue gases (measured in [kg/Nm3]),
their analysis depending on NEFs factors (measured in [kg/kWh]) has the
following advantages.(i)
One may
highlight the TPP technical level as regards the technology applied for the
process of energy production, the operational state of the technical equipment
as well as the manner of its exploitation.(ii)
One may
identify the elements upon which the TPP owner may intervene, in the sense of
improving the NEFs values.(iii)
By introducing
certain norms of NEFs limitation, one may stimulate the TPPs investors to apply
certain technological diagrams/flow charts and to take some technical measures
of global interest for the entire mankind. Thus, obtaining values of NEFs as
small as possible results in reduced quantities of polluting gases, and thus of
GHG compared to the produced useful energy. This leads to the reduction of the classic
fuels’ consumption. The issue is of great importance also at the global level,
considering that their exhaustion is predicted to happen in a very short
historical time.The values of the general efficiencyη are of remarkable importance for
the values of  NEFs.The next section  will further discuss this issue.


## 3. The General Efficiency of TPPS

It is known that the general efficiencyη occurring in the relations for
NEFs in Section 2 depends on a series of constructive and operational factors.All this considered and keeping in mind the stringent necessity of
intensive reduction, in the near future, of the polluting gases emissions of
TPPs; we show in Table2 the minimum values of the efficiencies η that can be
considered in the next period for the four categories of fuels.Table 2FuelηminLignite0.44–0.75Pit
coal0.45–0.80Fuel
oil0.48–0.85Natural
gas0.48–0.85The minimum values of the efficiencies presented in Table2 are specific
to some simple thermodynamic cycles (the Rankine cycle with steam condensing
turbine) and to some ordinary parameters of fluids (steam, gas) at the turbine
entrance. Increased values of the efficiency are obtained in the case of the
thermodynamic cycles that are superior from a technical point of view (with
cogeneration, combined gas and steam cycles) and respective of some very high
parameters of those fluids (i.e., ultrasupracritical parameters in steam case).Based on the above elements, we consider that it is high time the
provisions of [2] be replaced by others, based on the net emission factors—NEFs). Only
then TPPs’ emissions of polluting gases can be monitored under the aspect of
concentrations (toxic effect) but also from the perspective of the total
quantity of such gases (especially GHG).This last aspect is extremely important, considering the high risk of
major climactic changes as well as the rapid exhaustion of classic fuels.


## 4. Conclusions

From all the above we may draw the following important conclusions.


## 4.1. The Necessity of New Regulations for TPPS Atmospheric Emissions

The concentration (expressed in [mg/Nm3]) of the atmospheric
emissions of polluting gases resulted from the operation of thermo power plants (TPPs) is no longer an indicator
enabling us to monitor two major challenges facing mankind today: global
warming and rapid exhaustion of classic fuel reserves. This has become
stringent as the present regulations regarding the respective concentrations [2]
refer only to NOx and SO2 and not to CO2 which
has an essential role in the creation of the greenhouse effect.


## 4.2. The Importance of the Net Emission Factors

The net emission factors (NEFs) expressed in [kg/kWh] analyzed in
the present paper solve the acute problem of monitoring the emissions of all
polluting gases—NOx,
SO2, and CO2—in relation with
the electric and/or thermal energy produced by the TPPs along with the
respective emissions. In these indicators, a very important part is played by
the general efficiency of the analyzed TPP. In order to get better values of
these indicators (as reduced as possible NEFs), the owners of TPPs will be
forced to apply technologies leading to smaller concentrations of the polluting
emissions, but also to the obtaining of electric and/or thermal energy at the
highest possible efficiency.Consequently, one can thus automatically reach the global interest
desiderata of reducing the total quantity of polluting gases (especially
greenhouse gases) and of a more and more rational use of the world’s classic
fuels resources.



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*Source: 236907-2008-05-12.xml*