Abstract. 1. Introduction

Transcription

1 Air quality impact study of the new Gas Combined Heat and Power Plant Zagreb V. Jelavic, H. Su6ic EKONERG - Energy Research and Environmental Protection Institute, Av. Vukovar 37, Zagreb, Croatia Abstract This paper presents the existing air quality situation in the City of Zagreb and the contribution of different groups of sources (public power plants, households and commercial, industry and traffic) to pollution, evaluated by urban air pollution model. The impact of new GCC unit in the TE-TO Zagreb plant on air pollution was superimposed on the existing situation. The calculations show that the contribution of upgraded TE-TO to the average long term concentrations is very low, but the contribution to short term NO2 concentrations could reach recommended limit values if NO% concentration in flue gases of the new unit would be about 100 mg/m^. 1. Introduction Central District Heating Plant Zagreb (TE-TO Zagreb) is situated in the southeast part of Zagreb, about 4.5 km far from the city center. The plant was put in operation in 1963 as the first energy utility in the former Yugoslavia for combined production of thermal and electric power. The construction of the plant was initiated by the extensive development of centralized heating system of Zagreb, as well as by the increase in thermal and electric power consumption during late 50's. The plant is located by the left bank of the river Sava. Today, TE-TO Zagreb is the central utility in the eastern part of the centralized heating system of Zagreb. Numerous industrial companies with a large steam consumption are located in the close vicinity of the plant. In the TE-TO Zagreb there are three units for combined generation of heat and electric power. Their capacity is: 2x32 MW and 1x110 MW with three

2 396 Air Pollution Monitoring, Simulation and Control steam generators of 2x200 t/h and 1x500 t/h. In addition to the mentioned cogeneration units, another two hot water boilers are installed, VK 100 (each of MW), and one steam generator PK 80/64 (55.7 MW), as well as all auxiliary systems necessary for the operation of these units. The subject of this study is a new facility that consists of two gas turbines and two heat recovery steam generators (HRSG) which are to be coupled to existing steam turbines and electric generators of 2x32 MWe. New gas turbines will drive two electric generators of 2x70 MWe. Overall electric capacity of the upgraded unit will be 204 MWe. The results of the study presented here have been elaborated as part of the documentation prepared for the World Bank, Malbasa at. al.[l]. 2. Air quality standards Presently, there are no air quality standards in Croatia. Air quality is in practice usually evaluated on the basis of World Health Organization (WHO) recommendations. (Air Quality Guidelines for Europe, 1987). The adoption of new Croatian Air Pollution Act that will contain Air Quality Standards is expected in The WHO values represent a long-term goal and if pollutant's concentrations remain below that values no harmful effects on human health and vegetation are expected, even in the case of permanent influence. In the further discussion, those values will be referred to "Recommended limit values 3. Existing air quality in Zagreb city Daily concentrations of SO2 and smoke, NOg and suspended particles are continuously measured in the area of Zagreb City, Vadjic at. al.[2]. The amount of deposit substances and analysis of chemical content are measured on monthly samples on 22 locations, Sokoup at. al.[3]. Occasionally, the concentrations of CO and PAH are measured. The results of measurements of sulfur dioxide and smoke indicate that air quality in the center of the Zagreb City is not satisfactory, especially during winter (heating) period. In the narrow center of the city the concentrations of SO2 and smoke usually exceed recommended limit values (RLV=50 pg/w), and during the year often exceed limit values (RLV=120 ^g/m^), too. In wider city center concentrations are lower, but also exceed RLV and occasionally LV. Air pollution modeling shows that about 30% of the population is exposed to SO2 concentrations above RLV. The level of pollution by total suspended particles (TSP) in the city of Zagreb is high, yearly average is above 70jag/m^. Metal content of suspended particles is relatively small and below limit values. For example, the Pb concentrations are below 0.5 and 1.0 Lig/m^. Data on nitrogen dioxide (NC^) concentrations exist, but measurement stations are not

3 Air Pollution Monitoring, Simulation and Control 397 representative for that type of pollution. Calculated N(>2 concentrations show that in the center of Zagreb concentrations are between j,ig/m^. About 40% of population is exposed to values greater than RLV. 4. Contribution of different groups of sources to air pollution Mathematical model LONGZ was used for the calculation of contribution to air pollution by different groups of sources, US EPA[4]. The pollution for 1990 was calculated by using emission inventory of Zagreb City, Jelavic at. al.[5]. Contribution was considered through average annual concentrations and parameter named "population dose" (concentration in the centre of grid quadrant x No. of inhabitants in that quadrant). The results are shown on Fig. 1. Results indicate small contribution of TE-TO Zagreb power plant to total ground concentrations of SO%, NO% and particles (below 10%). Contribution to total population dose is even lower and it is less than 1%. 5. Emission standards Croatian "By-law on Emission Limits From Stationary Sources" is about to be passed in the Parliament by the mid Emission limit values given further ahead are taken from the proposal of the mentioned by-law. Emission limits set for stationary gas turbines of thermal output higher then 100 MWt are given in Table 1. NO% emission limits refer to facilities with net efficiency of fuel transformation to electricity of r^ = 0.3. For a gas turbine with higher net efficiency, NO% emission limits are proportionally increased. NO^ emission limits are defined for the mode of operation with the worst net efficiency considering the operation period of at least 30 days. For the new unit reference period for setting NO% emission limit is the summer, namely summer operational mode when the steam extraction for back-pressure operation is minimum. Net efficiency is then r\ = 0.76, so that emission limit value would be 115 mg/na Table 1. Emission limits for gas turbines of thermal output > 100 MWt (ISO conditions and 15% Q^ ^ Hue gases) p articulates (smoke number) NO% expressed as NO2 CO gaseous fuel 1 50 mg/m^ loomgw liquid fuel 2 loomg/m^ 100mg/m^

4 398 Air Pollution Monitoring, Simulation and Control 6. Emission characteristics of the new unit With regard to the air quality impact, co-generation plant burning natural gas has considerable advantages as compared to other possible technological solutions. This goes for the emission of noxious gases, as well as for the emission of greenhouse gas CC>2 Special attention in this case should be paid to NO% emission. Since this co-generation unit partly substitutes the operation of existing units, the analysis should include the entire environmental impact of the plant, i.e., air impact analysis should include also other harmful substances (gases) such as 882 and particulates. N(\ emission from the new 2x70 MW unit has been calculated assuming NO% concentration in flue gases at the level of the given (proposed) standard, i.e., 115 mg/na CO concentration in flue gases is assumed to be 10 mg/m^(at 15% 62), the value ten times lower than the standard value. It is also assumed that existing units will continue to burn liquid fuel with average sulphur content of 2.5%. Fuel demand of existing and new generation units is calculated on the basis of their engagement in the electric power system (EPS) of Croatia and industrial steam and heating demand of the City of Zagreb as well. New unit 2x70 MW will supply EPS of Croatia with 1681 GWh/yr of electricity in On maximal power load, new unit will have the emission of 12,7 kg/h CO, 138 kg/h NO% and 81 t/h CO2 Small quantities of non-combustible hydrocarbons, volatile organic compounds and particulates are also emitted from gas turbines. Emission of these substances is two times lower than NO% emission and are therefore considered insignificant. Present emission from all units at TE-TO site, together with future emission including new unit and emission of the reference case solution, is given in Fig. 2. The reference case solution in Fig. 2, assumes no upgrading of TE-TO units. New unit will emit 1027 t/yr NO%, 94 t/yr CO and t/yr CO2 It is possible that CO and NO% emission will be even considerably lower if low-emission turbine is provided (i.e., turbine with emission lower than defined by emission standard). The most recent solutions of dry combustion chambers with low NO% emission enable NO% emission concentrations to become as low as 50 mg/m^, the half of the given emission standard. As Fig. 2 shows, the commissioning of the new unit will inconsiderably increase the emission of NO% and CO from the entire TE-TO plant. On the other hand, in the case of new turbine unit with emission lower than 50 rng/m^, the emission of the entire plant will be lower than in the present situation. However, it should be taken into consideration that with the new unit, in the year 2000, about 6 times more electricity will be generated. The increase of CO2 emission is considerable, but it is still much lower than it would be in case of any other conventional solution of fossil-fuel power plant. Total emission ofno% in 1990 in the City of Zagreb was 12,094 t/yr. Thus, the new unit will increase the emission in the Zagreb area by 8.4%.

5 Air Pollution Monitoring, Simulation and Control 399 The existing and new units operate at variable output, the biggest difference being between summer and winter mode of operation. The highest emission occurs during the coldest winter months. This goes also for the emission from other furnaces in the City area. Monthly variation of NO% emission for the present and future state (including operation of the new unit), are given in Figure Air quality impact of the TE-TO in future Existing units in the TE-TO burn heavy fuel oil or natural gas. The combustion of heavy fuel oil generates substantial SO] and particulates emission. It is assumed that other units at the TE-TO site shall continue to burn fuel oil with average sulfur content in the order of 2.5%. The new unit, in the most unfavorable mode of operation, shall have the efficiency of fuel conversion to electric power in the order of 0.76, which means that the emission of greenhouse gas CO2 from this plant shall be approximately. 2.6 times lower than the CO2 emission from conventional condensing plant burning fuel oil 3.5 times lower than the CO2 emission from conventional condensing plant burning hard coal. The computation of air quality in respect to SO2, NO% and particulates concentrations has been performed by the mathematical model COMPLEX II [3]. The model COMPLEX II simulates hourly dispersion of the plume on a complex terrain. EKONERG has tested this model in a number of research works. Meteorological input data of the Model represent hourly values. A set of data collected in the time-span of two years, i.e., during the period , on the location Zagreb Maksimir has been used in computation. Plant's operation in the year 2000 has been simulated considering variable emissions pursuant to the requirements of electric power and district heating system. The model has been used for the computation of ground level SO^, NO% and particulates concentrations for each quadrant of the 2x2 km grid (quadrant center), encompassing altogether 40x40 km. The concentrations have been computed for each 1x1 km quadrant in the narrow area of 12x12 km (in total, 508 receptor points). Statistical analysis of LANDSAT image shows that the area is 30% covered by forests, 6% is settlement area, 6% traffic and sealed up area, 31% farmland and green areas and 22% meadows. 7.1 Air Quality in View of SO% Concentrations Calculation shows that average annual concentrations of SO2 originating from the operation of the TE-TO in the year 2000 are insignificantly low, except on the slopes of Mount Medvednica where the expected concentration level

6 400 Air Pollution Monitoring, Simulation and Control reaches lojag/m^. Maximum 24-hour concentrations generated by the operation of the TE-TO are mainly below lojug/m^. Maximum 24-hour concentrations exceed 100 jag/m^ on the slopes of Mount Medvednica in the direction NNE at the distance of 12 to 19 km. The highest concentration is 141 (ig/na That concentrations, as regards the protection of human health are on the edge of acceptability (RLV=150 jug/m^). As regards terrestrial vegetation, the concentrations are too high (>100 jig/m^). The computation has been carried out presuming that the TE-TO shall burn fuel oil with 2.5% sulfur content. Based on the results, it could be recommended that the TE-TO fires fuel oil with lower sulfur content, i.e. maximum up to 1.5%. With such fuel oil, concentration will not exceed guideline values for terrestrial vegetation (100 jag/m^). Since the transgression occurs in a relatively short period of time, the fuel with lower sulfur content could be used in particular meteorological conditions. 7.2 Air Quality in View of NO% Concentrations The plant emits the mixture of NO and NO2 gases, in which the share of NO gases is considerably higher ( about 95%). Since the NO gas is significantly less hazardous than the NO2 gas, the observation of ground level concentrations of NO2 is of prime concern. The conversion of NO into NO% influegases depends on meteorological conditions (wind speed, solar radiation) and the presence of other compounds, where VOC is of major importance. In the presence of solar radiation and large quantities of volatile organic compounds, NO% contributes to the formation of ozone. Here, constant relation NO2/NO%=1 is assumed, which is to a certain degree conservative, especially for the distances up to several kilometers far from the plant. In that context, ground level concentrations of NO% computed by a mathematical model are compared with limit values for NOi Yearly average NO% concentrations generated by the TE-TO are very low and in the populated part of the town are below 1 jag/m^. On the slopes of Mount Medvednica, average concentrations could reach 5 jag/m^. Maximum 24-hour average concentrations are below 50 ng/na Besides average daily concentrations and 24-hour concentrations, the concentrations in the time-steps of 1 hour and 4 hours have been computed. The computation shows that 1-hour NO% concentrations are considerably below 400 Lig/m^ (guideline value for health protection) in the entire area. In the most affected point, the 1-hour concentration is 120jag/m^, where the unit 2x70 MW contributes with 40 ig/m^. The calculated concentrations in 4-hour time-steps have been compared with a very stringent criterion for the protection of terrestrial vegetation of 95 j, g/na The computation shows that this margin value could be transgressed on the slopes of Mount Medvednica in the direction NNE. Figure 4 presents maximum values of NO% concentrations in 4 hour intervals with marked area

7 Air Pollution Monitoring, Simulation and Control 401 for which the transgression has been computed. In the most affected point, the concentration amounts 112 jug/m^, where the contribution of the new unit amounts 35 ig/m\ The above stated results point out that, in order to provide full protection of vegetation, the new unit should have lower emission level than the anticipated one. The 4-hour margin value won't be exceeded if the emission of flue gases would be in the order of 50 mgnox/m^, which is more than two times lower than the emission which meets emission standards (115 mg/m^). 8. Conclusions With regard to the air quality impact, co-generation plant burning natural gas has considerable advantages as compared to other possible technological solutions. This goes for the emission of noxious gases, as well as for the emission of the greenhouse gas CO^ As regards NO%, the conclusion could be drawn that the plant having the emission of 115 mg/m^ (Croatian proposed standard) would be satisfactory in view of the protection of human health, but won't meet the requirements regarding a very stringent limit value for the protection of terrestrial vegetation and could exceed it by 20-30%. Due to all previously said, it is advisable that the emissions from the new unit be limited to 50 mg/m^. This could be achieved by the application of state-of-the-art turbine designs. However, limit value of 95 jag/m^ should be examined more thoroughly final EIS since it is valid for the most sensitive vegetation. References: 1. Malbasa N., Jelavic V., Kisic Z. et. al. Draft Environmental Impact isywc/y o/ AW GCC LW m /Ac TTT-TU Zagr^, EKONERG, Zagreb, Vadjic V. at. al./j/r Quality in Zagreb City, Reports , Institute for Medical Research, Zagreb. 3. Sokoup V. at. al. Deposition of Air Pollutants in Zagreb City, Reports , Institute for Health Protection Zagreb, Zagreb. 4. US Environmental Protection Agency. UNAMAP6 - Computer Models Package for A ir Pol Intion A nalysis, Jelavic V., Stojak V., Bakale B, at. ^{.Emission Inventory (SO^ NO^ C/(y, EKONERG, Zagreb, 1994.

8 15% SOx EMISSION NOx 20% TSP t10% O 34% 14% 4% 14% CONCENTRATION SOx + NOx + TSP 38% 8% 6% 2% 64% CHRP TETO CHPP ELTO Small district heating plants Industry Residential & commercial Transport EKONERG 39% DOSE SOx + NOx + TSP o S3 OQ GO I* o CO Q- no I 38% Fig. 1. Contribution to air pollution (%) by different group of sources 52%

9 t/year o -\ (* kt/year for CO2) PRESENT SITUATION OF TE-TO Air Pollution Monitoring, Simulation and Control 403 TE-TO WITH 2x70 MW -Fuel oil 2.5% S -New unit 115 mgnox/nm3 EKONERG TE-TO WITH 2x70 MW -Fuel oil 1% S -New unit 50 mgnox/nm3 Fig.2 Emission levels for the existing TE-TO and after the construction of the new gas units t/year PRESENT SITUATION OF TE-TO TE-TO WITH 2x70 MW -Fuel oil 2.5% S -New unit 115 mgnox/nm3 TE-TO WITH 2x70 MW -Fuel oil 1% S -New unit 50 mgnox/nm3 Fig.3 Emissions of NOx for the existing TE-TO and after the construction of the new gas units, by months

10 LEGEND # 110 pg/m3 en / 7 ou jjg/mo 20 pg/m3 10 pg/m3 5 jkjg/m3 "0 - O OQ GO O Fig. 4 NOx 4-hour maximum concentrations due to TE-TO plant emission - guideline values: 95 ng/m3 - vegetation - max concentration is n o LANDSAT 3M5, 28. JUNE 1993