European utilities battling greenhouse gas emissions
CO2, SO2 and NOx emission rates are falling rapidly, and the trend is predicted to continue into the next century
By Kevin Dodman, European Editor
Throughout Europe, environmental emissions are falling steadily. Governmental regulations, current and forthcoming, are one impetus; but efficiency and economy are also driving factors. Here, we look at recent trends in four European countries–Poland, Italy, the UK and Spain.
Around 98 percent of Poland`s electricity is generated by coal-fired power plants, with the remainder hydropower based. Of the 98 percent, 55 percent burns hard coal, and 43 percent burns ignite. The power industry, including both electricity and heat production, is widely regarded as the main cause of atmospheric pollution.
The downward trend in emissions of fly ash, SO2 and NOx can be in Table 1. This resulted from a year-on-year decline in the amount of power generated, down from 136.36 TWh in 1990 to 133.66 TWh in 1993, coupled with the use of higher-quality coal, the introduction of flue gas treatment and improved electrostatic precipitators. The quality of hard coal used has improved significantly, while the parameters for lignite have remained relatively constant.
The methods being used for emission reduction in Poland can be divided into four main groups:
1. pre-combustion measures,
2. measures used during combustion,
3. post-combustion measures and
4. improved energy utilization.
In the area of pre-combustion treatment, a number of Polish coal mines have started to install coal-cleaning facilities, as power plants seek lower-sulfur fuels. The sulfur content of Polish hard coal ranges from 0.4 percent to 2 percent, and of lignite it ranges from 0.25 percent to 1.2 percent. It has been estimated that the coal-cleaning technologies being introduced could reduce SO2 emissions by 340,000 tons per annum by the end of 1997. However, there are doubts regarding the extent to which pre-combustion coal cleaning will be used because of its impact on the fuel price.
To reduce emissions during combustion, several approaches are being used. These include fluidized-bed combustion, dry desulfurization and low-NOx burners. Fluidized beds have been chosen for refurbishment of the 10 x 200 MW Turow lignite-fired power plant and the Zeran cogeneration facility, as well as for a number of newly built cogeneration plants, including Lublin, Rzeszow and Kielce. There are also plans to install pressurized fluidized-bed combustion (PFBC) boilers at the Krakow and Pruszkow cogeneration plants.
Regarding dry desulfurization, it has been found that limestone injection can meet the required emission levels, provided low-sulfur coal is used. Systems are in use at Halemba (4 x 50 MW) and Rybnik (8 x 200 MW), and the use of higher-quality coal means that the resultant spare pulverizer capacity can be used to mill the limestone.
Installation costs for dry desulfurization systems are estimated to be around a quarter of those for wet desulfurization units, and their running costs are lower by a factor of between 5 and 6.
However, dry desulfurization is no panacea, because at 35 percent to 40 percent its efficiency is no match for the 95 percent achievable with wet systems. It is a useful option where low-sulfur coal is available, however.
Polish power plants have achieved NOx emission reductions of around 40 percent by using low-NOx burners, and it is anticipated that they will be the primary method of NOx reduction. One exception is the Opole power plant, where an ammonia flue-gas treatment system is likely to be needed in addition to low-NOx burners.
Post-combustion emission reduction measures cover the control of SO2 and fly ash. Wet desulfurization is planned or is under construction at many Polish power plants, including:
Belchatow, 4 x 360 MW;
Opole, 6 x 360 MW;
Siersza, 2 x 130 MW and 4 x 120 MW;
Polaniec, 8 x 200 MW;
Kozienice, 2 x 500 MW and 8 x 200 MW;
Jaworzno, III 6 x 200 MW;
Krakow, 4 x 120 MW and 1,457 MWt; and
Skawina 100 MW.
Apart from its high efficiency, the main advantage of wet desulfurization technology in the Polish context is that the country has abundant limestone resources. Some semi-dry installations are also planned, as shown in Table 2, which indicates the cost of new installed capacity and the percentage required for environmental control equipment.
Fly-ash emissions have been reduced significantly in recent years. Lower overall power production and the use of higher-quality coal have contributed to the reduction, but the main reasons have been that Poland has focused considerable attention in this area for many years.
All new installations now have precipitators of around 99.6 percent to 99.9 percent efficiency, and older units are being upgraded. This is being helped by the fact that electrostatic precipitators are produced in Poland, so considerable local expertise is available. The final emission reduction method being employed is the encouragement of improved energy utilization. Measures include revised pricing structures, demand-side management and incentives for consumers.
The most powerful incentive is pricing, as can be seen from recent trends. During the period from December 1989 to June 1994, the price of industrial hard coal rose by a factor of 62.1 in local currency (equivalent to a factor of 14.3 in US$); the price of electric power for industry rose by a factor of 17.4 (4.1 in US$); and the price of gas for industry rose by a factor of 14.1 (3.3 in US$). At the same time, the price of electricity for domestic consumers climbed by a factor of 57.7 (13.3 in US$) and for gas by 158.6 (36.6 in US$).
In Italy, the state utility ENEL started a test program in the early 1980s to acquire direct experience of desulfurization technologies and to investigate the problems associated with their application in the Italian context.
Because of the lack of suitable disposal sites in Italy for fluidized-gas-desulfurization wastes, ENEL concentrated its development effort on standard nonregenerable-advanced-type and regenerable-type processes for new plants and for retrofits.
The fact that ENEL plants are equipped to burn different fuels led to the elimination of some newer control technologies such as spray dryers, for which efficiency with high-sulfur fuels is not well proven.
A working group examined 10 processes for test at Sulcis in Sardinia, of which three were chosen for detailed evaluation:
1. the limestone-gypsum process, capable of producing commercial-quality gypsum;
2. the Wellman-Lord process, where the pure SO2 separated can be converted to sulfur or sulfuric acid; and
3. the Walther process, which uses ammonia to produce ammonium phosphate, which can be used to manufacture fertilizer.
The construction of three test systems was contracted to the Italian licensees: Idreco for the Bishoff limestone-gypsum process, CIFA for the Davy-McKee Wellman-Lord process and Termokimik for the Walther-Krupp Koppers ammonia-ammonium sulfate process. Ansaldo handled the civil works and the interface to the Sulcis power station.
The test exercise gave ENEL the opportunity to gain experience using the three different systems for a range of different coal parameters and for oil, in addition to the Italian Sulcis coal, which has a sulfur content of up to 8 percent.
The limestone-gypsum system featured a counterflow tower, where sorbent injection took place via nine spray nozzles on nine different levels. The gypsum slurry was extracted from the lower part of the tower and sent to a filtration system.
During 10,000 hours of experimental operation, the following main process tests were made:
– the use of lime instead of limestone,
– the addition of organics to increase SO2 removal efficiency, and
– determination of the desulfurization efficiency and SO2 concentration while varying the absorber liquid/gas ratio from 6 to 20 l/Nm3 and SO2 concentration at the process inlet from 2,000 to 16,000 mg/Nm3, equivalent top a coal sulfur content ranging from 1 percent to 8 percent.
The emissions measured during the tests were:
– particulate emissions of around 10 mg/Nm3, with about 20 mg/Nm3 at the plant inlet–the Italian limit is 50 mg/Nm3;
– chloride emissions of around 1 mg/Nm3 with 50 mg/Nm3 at the inlet–the limit is 100 mg/Nm3; and
– fluoride emissions of 1 mg/Nm3 with 4 mg/Nm3 at the inlet–compared to a limit of 5 mg/Nm3.
When lime was used instead of limestone, a higher removal efficiency was achieved. The gain was around 4 percent with an SO2 concentration of 2,000 mg/Nm3 and more than 10 percent with an SO2 concentration of 15,000 mg/Nm3.
Walther and Wellman-Lord tests
The Sulcis Walther installation was an advanced design, capable of handling combustion gases from coals with sulfur contents ranging from 1 percent to 8 percent, whereas most earlier installations of the technology had been used to treat gas from coal of less than 1-percent sulfur content.
New designs were used for the pre-scrubber, to minimize aerosol formation, and for the wet electrostatic precipitators (ESP) which treat a portion of the whole desulfurized flue gas. This wet ESP was tested for the first time as an alternative to the mechanical filter previously used in commercial plants to remove aerosol mists.
After 3,000 hours of operation, it was shown that the plant was capable of treating raw gases with an SO2 content of up to 4,000 mg/Nm3, corresponding to a sulfur content of 2 percent in the coal, while complying with the Italian regulatory limit of 400 mg/Nm3 for SO2 and achieving aerosol emissions of around 10 mg/Nm3.
It was shown that the wet ESP was an effective way to reduce aerosols to below 10 mg/Nm3 at SO2 concentrations up to 11,000 mg/Nm3, corresponding to a coal sulfur content of about 5 percent.
Chloride and fluoride abatement was around 80 percent and 50 percent respectively, and the ammonium sulfate produced was of commercial grade.
According to ENEL, no problems arose with the scrubbing system during the test period, but there were shutdowns of the ammonium sulfate production unit because of pipe system plugging and plant equipment corrosion. This meant availability was only around 50 percent.
Tests of the Wellman-Lord process showed that the SO2 regulatory limit of 400 mg/Nm3 could be achieved with an inlet concentration of up to 14,000 mg/Nm3, corresponding to a sulfur content of 7 percent in the coal.
Particulate emissions were always below the limit of 50 mg/Nm3, and SO2 removal efficiencies were typically around 70 percent.
The tests carried out on the three pilot systems showed that the limestone- or lime-gypsum system was the one best suited to Italian conditions and environmental regulations, so a standard system of this type was developed.
It is designed to treat combustion gases from coal, oil or mixed fuel and is based on operation with 1-percent sulfur coal or 3-percent sulfur fuel oil. The characteristics of the ENEL power plants with new FGD systems are shown in Table 3.
The standard plant is divided into three areas: absorption, solids handling and the gas area. The absorption area includes two independent absorption modules for each 320- or 660-MWe unit, each sized for 50 percent of the total flow and designed to operate independently.
Each absorption module includes a pre-scrubber to saturate the flue gas and to remove particulates, chlorides and fluorides, together with a counter-current, spray-tower-type scrubber equipped with six spray systems, each fitted with a recirculating pump.
The gas area includes:
ducts and dampers to handle the flue gas,
regenerative heat exchangers equipped with air- and water-washing systems and a scavenging fan to minimize contamination of the treated gas–the 320 and 660 MWe units are equipped with two regenerative heat exchangers and two booster fans for each unit, and
booster fans of sufficient size to overcome the pressure losses in the system.
CO2, SO2 and NOx emissions from UK power stations have all taken a steep drop in the last few years. The Electricity Association–the trade body which represents all the major electricity generation, transmission, distribution and supply companies in the UK–commented “Action by the UK electricity industry is delivering significant reductions in emissions. The 1995 edition of the Department of the Environment`s Digest of Environmental Statistics shows the continuing reduction in total atmospheric emissions from power stations.
“These reductions have been achieved by improvements in fossil fuel generation and in the performance of nuclear plants, the installation of low-NOx burners and flue gas desulfurization, and use of lower-sulfur fuel supplies.”
The industry`s own forecasts suggest that annual CO2 emissions will be 42 million tons of carbon (MtC) in 2000, 12 MtC lower than in 1990. This reduction is greater than the total national savings target of 10 MtC per year by 2000 set by the UK`s commitments under the Climate Change Convention (Table 4).
The Electricity Association stated, “In the light of the success in preventing the rise in CO2 emissions, the government has updated its own emissions forecasts and now expects not just stabilization, but a fall in UK emissions by 2000.”
The government has set the electricity industry annual target reductions for SO2 up to 2003 and for NOx up to 1998, under the European Large Combustion plants Directive. It is interesting to note that according to government figures given in Energy Paper 65 (March 1995), by the year 2000, CO2 emissions are set to rise in every main sector except power stations, so the electricity sector is predicted to more than offset these rises and thereby enable a net reduction in total UK CO2 emissions.
The Spanish utility Endesa is also focusing on CO2 reduction as part of its overall environmental plan. Three areas have been identified for further attention:
1. Improvement of power generation efficiency. Specific actions have included conversion of the As Pontes thermal power plant to use a mixture of lignite and sub-bituminous coal. Endesa is also investigating the use of PFBC and integrated coal-gasification, combined-cycle technologies.
2. Fuel-quality improvement. Lower-ash imported coals are being used, as well as sub-bituminous coal and natural gas.
3. Substitution by non-CO2-producing generation methods. Efforts are under way to increase the use of wind, sun, biomass and mini-hydro technologies.
Progress has been made in reducing CO2 emissions from Endesa`s power plants, as can be seen from Figure 1. This improvement has been achieved mainly by the use of better-quality fuels, resulting in an upward trend in higher heating value, and an associated fall in heat rate. The average net heat rate fell by 138 thousand calories per kWh during the period from 1988 to 1994, leading to a 5-percent reduction in specific emissions.
Bartoli, F., G. Dodero and A. Tosi, ENEL Construction Division, “Flue-
Gas Desulfurization: ENEL Experience and New Projects in Italy,” POWER-GEN Europe `95 Conference, Amsterdam, The Netherlands.
Crespo, P. Martinez, and J. Abadia Ibanez, Endesa, “Endesa`s
Environmental Policy,” POWER-GEN Europe `95 Conference, Amsterdam, The Netherlands.
The Electricity Association, “UK Electricity 95,” London, UK.
Rozewicz, Zygmunt, Energopomiar and Slawomir Pasierb, FEWE,
“Polish Overview of Air Pollution Abatement Technologies,” POWER-GEN Europe `95 Conference, Amsterdam, The Netherlands.