Operational determinants of gaseous air pollutants emissions from coal-fired district heating sources

This study describes the correlation between emission of gaseous pollutants to the atmosphere and the combustion parameters of a coal-fired 25 MW heating capacity water boiler with mechanical grate (boiler type WR-25) in unstable working conditions: start-up, shutdown and loads below the technical minimum. Whereas measurements were made for a specific type and size of coal-fired water boiler with mechanical grate, the measurements and calculations are applicable to WR boilers with a different heating power as well as OR type steam boilers, which have a practically identical design. In sum, there are more than 1,000 coal-fired water and steam boilers of these types in Poland. In addition, the analysis reported in this paper highlights the important role played by boilers operating in unstable conditions in terms of emission of gaseous pollutants to the atmosphere. The conclusions are relevant for other boilers fired with gas, oil or biomass operating under conditions such as start-up, shutdown and loads below the technical minimum. This article fills a gap in air protection engineering practice and the literature with regard to indicators and emission standards, drawing on measurements of pollutant concentrations in the exhaust gases from unstable WR boiler working conditions. The measurements can be used to assess the emission of pollutants to the atmosphere in such boiler working conditions and their impact on air quality. The analyses presented were based on the authors’ own measurements in WR-25 boiler technical installations using portable gas analyser GASMET DX-4000, which uses the FT-IR measurement method for compounds such as SO2, NOx, HCl, HF, NH3, CH4, and CO. Concentrations of CO, NOx and SO2 in exhaust gases were determined with multiple regression with the STATISTICA statistical software and with linear regression complemented by the “smart” package in the MATLAB environment. The study provides computational models to identify pollutant concentrations in the exhaust gases in any working conditions of WR-25 boilers. Operational determinants of gaseous air pollutants emissions from coal-fired district heating sources 109 on outdoor temperature, the heating plant provides heat for space heating purposes and for consumable hot water demand, and reaches nominal power usually at temperatures below minus 20°C. Optimal load and maximum energy efficiency occurs at 80–95% of a boiler’s nominal power. At that point the WR boiler reaches its peak energy efficiency of 82% (boiler no. 2 in heating plant A – 78%). In these conditions heat is generated efficiently with minimum fuel consumption, stable furnace operating parameters and good dust removal. Work at optimal load, with almost perfect and complete combustion, delivers the lowest fuel consumption index and the lowest air pollution emissions index per unit of heat generated. At optimal load, monitoring can be restricted to the emission of gases: SO2, NOx and CO2 alone. There are only trace amounts of VOC, HF, HCl, NH3 and CO, as they are mostly caused by incomplete and imperfect combustion. There is a clear increase in emissions of these substances when combustion takes place in substoichiometric conditions, such as start-up, shutdown and load below the technical minimum. A number of legal regulations determine emission standards relating to fuel combustion. The Environmental Protection Law; Regulation of the Climate Minister; CAFE Directive; IED Directive; MCP Directive all determine emission standards for normal conditions of boiler operation. No documents set standards for unstable conditions, such as start-up, shutdown or loads below the technical minimum of boiler operation. Observations of and experience with the operation of boilers in unstable conditions indicate that they lead to increased emissions of pollutants to the atmosphere. Therefore, it was assumed at the start of this study that a boiler operating under unstable conditions experiences significant changes in the concentration of pollutants in the exhaust gases. The novelty of this paper lies in developing an optimization model using multiple and linear regression that is valid for every make of WR boiler operating in unstable conditions. Review of literature China is currently the largest coal consumer in the world and coal has traditionally been its primary energy source. Although the share of coal in the energy consumption of China has decreased in recent years, it still accounted for 62% of its total energy consumption in 2016 and the total consumption of coal has increased by 14% in the last 10 years (Wang et al. 2019). Searching for effective low carbon solutions for coal-fired DH sources is therefore just as important to China as it is to the world. The heating sector, mostly in the form of DH, is a large coal consumer (Lin and Lin 2017), burning 241 million tons in 2015. Heating generation will continue to depend on coal for decades to come, as the share of renewable energy in China is forecast to reach only 30% by 2050. Recently, the Chinese government introduced clean coal technologies into DH, as pilot projects aimed at energy saving and carbon reduction (Wang et al. 2019). The geological resources of coal on the Earth, recalculated as primary energy units, are an estimated 18,000 EJ ‒ two and a half times greater than oil reserves (Wilczyński 2013). The International Energy Agency (IEA) stated that the demand for coal in 2035 in OECD countries would be falling by 1.1% annually, while in China it would be going up by 2.1% each year (Wilk and Bocheńska 2003). Increasing demand for electricity and heat directly contribute to the eventual exhaustion of coal deposits in the world. With the present energy policy of the Chinese state, these deposits will suffice for over 180 years (Miller and Tillman 2008). Moreover, the Indian power sector is playing a key role in global emissions and efforts to mitigate climate change. Lifetime emissions over the next five decades from Indian coal-fired power generation could range from 18 to 39 Pg (Yang and Urpelainen 2019). Global consumption of total energy in 2018 stood at 4,488.2 PJ. This figure differs slightly from the European average – gross inland energy consumption per capita in Poland in 2017 was 115.9 GJ, against the EU average of 137.1 GJ. An increase in global consumption compared to the previous year was observed in the case of hard coal, crude oil, natural gas, renewable energy and other sources, while there was a decrease in brown coal. Charcoal plays a significant role in energy sectors, especially in many developing countries (Marousek et al. 2014). In Poland, the share of the various sources were as follows: hard coal 37.0%, brown coal 9.1%, crude oil 26.3%, natural gas 16.1%, renewables 9.3%, others 2.2%. Domestically sourced primary energy was mostly coal: hard coal with a 56.2% share in 2019, followed by brown coal with 15.2%. The share of natural gas in indigenous production was 5.5%, crude oil 1.5%, and the others, mostly renewables, 18.3% (Statistics Poland, 2019). In 2018, the EU’s consumption of hard coal and brown coal combined reached 596 million Mg, with about 60% of hard coal and more than 90% of brown coal used for electricity and heat production. Between 1990 and 2018 the number of member states of the EU producing hard coal fell from 15 to only 5: Poland, Czech Republic, Germany, United Kingdom and Spain. In 2018, Poland mined 63.4 million Mg of hard coal, 86% of the total EU production (Eurostat, 2019). In Poland, coal is still a strategic source of energy and the vast majority of power plants burn mainly this fuel (EuroHeat and Power 2019, Demirbas 2006, Beom et al. 2018, Wasilewski et al. 2020). This is mostly due to its easy availability and price, as well as the fact that the plants were built in the 1970s and 1980s and designed for coal. Coal consumption in Poland initially rose steeply in the post-war period: it went from 51 million Mg in 1950 to a peak of 164 million Mg in 1980 before trending downward to 63 million Mg in 2018. The significant reduction in coal consumption in Poland is due to the EU’s energy policy, which incentivizes a switch from coal and toward renewable energy sources and, during the transition period, gas and oil combustion. The widely desired decarbonization of the EU economy (in Poland too) is revealing anomalies in its implementation that are throwing an increasingly critical light on the EU’s climate policies and rigorous approach to the combustion of resources such as hard coal and brown coal. The EU itself is an important coal importer. Despite numerous decarbonization programs and a fall in coal mining in EU countries, the EU is actually experiencing rising demand for coal – both hard coal and brown coal. The biggest producer of brown coal in the world is Germany, where extraction of this resource is running at over 170 million Mg 110 R. Zwierzchowski, E. Różycka-Wrońska annually, providing over 30% of the national energy balance. Similar situations can be observed in the economies of some other countries. WR water boilers with movable grate, powered with hard coal culm (coal particles type MI 20–0 mm or MII 10–0 mm) are installed in HOB plants, CHP plants and in industrial plants as heat generating units. A boiler is built in a two-pass system, with one pass in a gas-tight water-wall furnace chamber. Refractory lining is applied to protect front ceiling and sides walls of the furnace in the area of coal combustion. The second pass is a finned convective bundle. The boiler is equipped with installations that control the combustion process, i.e., a secondary air installation over the grate and recirculation of exhaust gases under the grate. The mechanical grille is equipped with a cascade system of fuel supply, offering an option of simultaneous waste co-incineration (Różycka-Wrońska et al. 2014). Refractory lining is applied to protect front ceiling and sides walls of the furna