Potential of existing and newly designed geothermal heating plants in limiting of low emissions in Poland

Geothermal energy as one of renewable energy sources is an alternative to conventional methods of heat production, and thus contributes to reducing the emission of pollutants into the environment, especially the so-called low emission. Poland is facing the problem of pollutant emissions from combustion processes, in the context to individual households using mainly solid fuels to ensure heat demand for central heating and hot water production. The paper presents the results of calculations of avoided emissions in the context of replacement of conventional individual heating for geothermal heating systems, taking into consideration not only the problem of air pollution in Poland, but also issues of fuel quality (calorific value) and efficiency of used heating devices.


Introduction
Pollution of the environment, that the modern society is struggling with, is mainly related to the energy sector.The right approach to this issue requires taking into account the problem of occurrence so-called low emission.This concept has no authority in Polish legislation, but it has been precisely formulated in the monograph "Low emission.From the reasons for occurrence to the methods of elimination."[1], where it was indicated that it is emission from emitters with a height not exceeding 40 m, and its main source are the processes of heat production for central heating and hot water production in households [2].Therefore, it is natural to look for solutions in the modernization of heat sources, including the development of local heating networks using renewable energy sources.The answer to the need to produce heat in a less invasive way for the environment is geothermal energy, which due to the nature of thermal energy conversion accumulated in geothermal waters for usable energy, can be considered not only as an ecological but also a stable source.

Low emission problem in Poland
Air quality standards in Poland are permanently exceeded, which is evidenced by data published, among others by the Chief Inspectorate for Environmental Protection in Poland (2017), as well as the European Environment Agency (2017).The problem of exceeded standards concerns mainly particulates PM10, PM2.5 and benzo(a)pyrene, and to a lesser extent SOx and NOx. Figure 1 presents the data on concentration of PM10 and PM2.5 in 2015 (annual limit value), in combination with number of days in the Polish agglomerations (EEA, 2017; GIOŚ, 2017).Poland is characterized by one of the highest exceedances of permissible PM10 standards, locally above 50 μg/m 3 .The situation is similar with PM2.5, locally exceeding 30 μg/m 3 .The data presented in the bar graph, which concern PM10 measurements, indicate that by far the worst air quality in Poland is in Krakow, Upper Silesian and Rybnik-Jastrzębie agglomerations.Each agglomeration is characterized by three bars, the first one (dark blue) is responsible for the number of days with exceedances of the permissible level of PM10 recorded at the measuring station, on which the highest number of such days occurred.The second one (blue) means the average number of days with exceedances of the permissible level of PM10 for all measurement stations in the agglomeration.The third (light blue) applies to the number of days with exceeding the permissible level of PM10, recorded at the measuring station where the least number of such days occurred.In addition, the value in brackets indicates the number of measurement stations from which data was acquired.Despite the clearly exceeded standards in almost all of Poland, it should be noted that the positive exception are Szczecin, Tricity and Białystok agglomerations.[2,3]).Dark blue barthe number of days with exceedances of PM10 recorded at the measuring station, on which the highest number of such days occurred.Bluethe average number of days for all measurement stations in the agglomeration.Light bluethe number of days recorded at the station where the least number of such days occurred.Red lineallowed limit of 35 days in a year with exceedances of PM10.
Low emission problem in Poland is primarily caused by heat production for central heating and domestic hot water in households.Evidence for this is the data presented in figure 2 regarding the percentage share of individual sectors in the emissions of PM10, PM2.5 and benzo (a) pyrene.It should be emphasized that for both PM10 and PM2.5, almost 50% of emissions are covered by the so-called combustion processes outside the industry, 48.5% (PM10) and 49.7% (PM2.5)respectively.In the case of benzo(a)pyrene, this value reaches 86%, which taking into account the fact that it is a carcinogen substance, takes on special significance.

Geothermal district heating in Poland
The present state of recognition of geothermal resources in Poland allows to determine prospective areas for geothermal installations of district heating systems.In Poland, four geothermal provinces are distinguished: Polish Lowlands (about 250,000 km 2 -80% of the country), Carpathian Province (6.5%), Carpathian Foredeep (6.5%), Sudety Region.They differ in geological conditions, petrophysical and thermal parameters of rocks, which translates into their usefulness for heating purposes.The most prospective regions from the heating point of view are the Polish Lowlands and the Inner Carpathians.
The Polish Lowlands is formed of sedimentary rocks of the Paleozoic and Mesozoic age with a maximum thickness of approx.8 km.The geothermal gradient in this area is from 2 to over 3°C/100 m, and the heat flux is 35-105 mW/m 2 .Temperatures of geothermal waters (20-130°C), mineralization (1-300 g/l) and flow rate (from a few to above 300 m /h) are also differentiated.However it should be remembered that Polish Lowlands includes 250,000 km 2 area.Geothermal waters reservoirs form sandstones, marls and limestones.The most prospective from the point of view of geothermal heating are Lower Cretaceous and Lower Jurassic reservoirs.Particularly good values of parameters such as temperatures and thickness of reservoir deposits were found in the belt of the Szczecin-Mogilno-Łódź basin [4].Currently, geothermal heat plants in Pyrzyce (6.0 MWt), Stargard Szczeciński (12.6 MWt), Mszczonów (3.7 MWt), Uniejów (3.2 MWt) and Poddębice (10 MWt) operate in the Polish Lowlands [5].
In the case of the Carpathian Province, two areas with a different geological structure are distinguished: Outer Carpathians and Inner Carpathians, which determines large fluctuations in geological, petrophysical and thermal parameters, important from the point of view of district heating.The geothermal gradient for the Carpathian Province is in the range of 2-3.6°C/100 m, and the heat flux is 60-95 mW/m 2 .The reservoirs of the outer Carpathian basin are flysch.This tank is characterized by temperatures of 20-60°C, mineralization of 10-120 g/dm 3 and flow rate with maximum values up to several tens of m 3 /h.From the point of view of geothermal heating systems, these parameters are not sufficient for the independent use of waters with the given parameters, but it is worth considering the use of these resources in systems with heat pumps.The reservoirs of Inner Carpathians Basin are limestones and dolomites of the Middle Triassic, Jurassic sandstones and carbonate forms of the Middle Eocene.The deep temperature of geothermal waters ranges from 20 to even 127°C (at a depth of about 4800 m, Bańska IG-1 borehole), the total area mineralization does not exceed 3 g/l [6][7][8][9], and flow rate reaches a maximum value of 550 m 3 /h.The confirmation of these very good conditions is the heating plant PEC Geotermia Podhalańska SA operating successfully since 1994 (40.7 MWt geothermal power, total 82.6 MWt).
Less-perspective areas from the point of view of district heating, although locally possible to develop, are the Sudety Region and Carpathian Foredeep (relatively poorly recognized area).In the Sudety, geothermal reservoirs form highly crystalline rocks, which results from the occurrence of deep fault zones.The heat flux in this area is in the range of 40-70 mW/m 2 , the temperature is from about 20 to 86°C (at a depth of about 2000 m), and the mineralization does not exceed 1 g/dm 3 .Despite good parameters, this reservoirs are not prospective for district heating applications due to the low flow rate of 10 m 3 /h on average [10] The Carpathian Foredeep deposits are sedimentary rocks of the Oligocene and Miocene age, with a thickness up to 3 km.The heat flux in this area is 60-95 mW/m 2 , and the geothermal gradient is in the range of 1.8-4.5°C/100m.The reservoirs of this area are sandstones, marls and limestones.Geothermal waters are characterized by temperatures from 20 to 100°C (at a depth of about 3 km) and mineralization in the range of 1-100 (locally 150) g/dm 3 .The flow rate from geothermal well is several m 3 /h [11,12].
To sum upgeothermal reservoirs of the greatest importance for use in heating purposes occur within the Carpathian Province as well as Lower Cretaceous and Lower Jurassic reservoirs in the Polish Lowlands.Taking into account the extent of the area and parameters, the effective management of the Polish Lowlands resources is of the greatest importance for Poland.Confirmation of this are the heating plants operating in Podhale, Pyrzyce, Mszczonow, Uniejów, Stargard Szczeciński and in Poddębice.The total installed capacity from geothermal energy in these plants is 76.20 MWt [5] (tab 1).  2 presents the potential locations for geothermal heat plants in Poland, which can be considered as the most prospective, taking into account the reservoir parameters (geothermal water temperature, flow rates) and access to the so-called consumers.However, it should be emphasized that this list does not cover the whole potential of geothermal energy in Poland in the context of heating.

Impact of geothermal heating plants low emission reduction
The specificity of geothermal heat plant's operation means that it does not emit pollutants into the air, or does it in a negligible way.To be precise, it should be clarified that auxiliary equipment used in the construction of a geothermal heating plant, e.g.circulation pumps, security devices or production and heat distribution control centers, consume electricity.However, it is obvious that in the geothermal heat plant's workplace we are not dealing with low emissions only, but also with air pollution taken in general.
The results presented in the paper refer to the comparison of the amount of thermal energy sold by existing geothermal heat plants and the amount of thermal energy possible to be produced in prospective locations, with the emission of combustion products for individual households.Unlike others, the environmental effect was not calculated in relation to the local conventional heating plant, but it was deliberately focused on the source of low emissions, and thus households.In order to calculate the emission of pollutants, the methodology was used in accordance with the guidelines of the National Center for Emissions Management (KOBIZE), on emission factors for fuel combustion in boilers with a nominal thermal capacity of 5 MW [14].The methodology was supplemented with the efficiency of boilers, the minimum and maximum heating values, as well as the sulfur and dust content in particular types of fuels (presented in Tables 3-6).Calculations were made according to the equations: where:

𝐸𝐸 = 𝐵𝐵 • 𝑊𝑊
(2) where: The results obtained for particular groups of devices (biomass boilers, coal and coke boilers, oil boilers and gas boilers) per 1 MJ of heat are presented in Tables 3-6.Based on the above data, average emissions of individual pollutants were calculated for basic groups: biomass boilers, coal and coke boilers, oil fuel boilers, gas boilers.The obtained values have been referred to the total production of thermal energy in existing geothermal heat plants and for prospective locations (where geothermal district heating systems may be developed in the near future).The results are presented in table 7.

Summary and conclusions
The results of calculations presented in the article clearly indicate a very large impact of geothermal heat plants in the context of reducing low emissions.In the case of replacing biomass boilers, the emission limit is over 11 000 SOx tons, 100 000 tons of NOx, 2.5 million tons of CO, 120 million tons of CO2 and 464 000 tons of TSP.In the case of hard coal boilers and coke, these indicators look as follows: over 605 000 SOx of tons, 156 000 tons of NOx, 3.3 million tons of CO, 151 million tons of CO2, 847 000 tons of TSP and 1.02 tons of benzo(a)pyrene.In the case of oil-fired boilers, the limit is: 468 000 tons of SOx, 225 000 tons of NOx, 45.000 tons of CO, 135 million tons of CO2 and 52.000 TSP ton.Most preferably, the presented results are obtained for gas boilers, which is not surprising considering the chemical composition of the fuel: 76 tons of NOx, 15 tons of CO, 100 000 tons of CO2 and 0.02 tons of TSP.In the case of this fuel, the SOx emission is negligible.Regardless of which structure of heat production we adopt in a given commune or village, the ecological effect for investments in a geothermal heating plant will be beneficial.Hence, also this solution should be seen as a chance to reduce low-emission air pollution in Poland.
However, we should remember that the implementation of geothermal heat plant investment affects the natural environment more widely than it is considered in this article, as drilling works or power supply.However, this does not change the fact that among

Fig. 1 .
Fig. 1.Concentration of PM10 and PM2.5 in 2015annual limit value, in combination with number of days with exceedances of the 24-hour level admissible for PM10 in 2015 in Polish agglomerations (based on[2,3]).Dark blue barthe number of days with exceedances of PM10 recorded at the measuring station, on which the highest number of such days occurred.Bluethe average number of days for all measurement stations in the agglomeration.Light bluethe number of days recorded at the station where the least number of such days occurred.Red lineallowed limit of 35 days in a year with exceedances of PM10.

Table 4 .
Pollutants emission for hard coal and coke boilers < 0.5 MW.

Table 7 .
Reduction in emissions of pollutants into the air for existing ones and prospective geothermal heat plants in Poland.