CC BY
5
Stawomir Stelmacha)*, Katarzyna Matuszeka), Piotr Hryckoa), Pawet Wolnyb), Jiri Horakc), Lenka Kubonovac)
'> Institute of Energy and Fuel Processing Technology / Instytut Technologii Paliw i Energii
b) Faculty of Process Engineering and Environmental Protection, Lodz University of Technology / Wydzial Inzynierii Procesowej i Ochrony Srodowiska, Politechnika Lödzka
c) VSB-Technical University of Ostrava / Wyzsza Szkola Görnicza - Uniwersytet Techniczny w Ostrawie * Corresponding author / Autor korespondencyjny: sstelmach@itpe.pl
Reducing Mercury Emissions from Small-Scale Coal-Fired Boilers Used in Residential Heating
Ograniczenie emisji rt^ci z matych kottow w^glowych wykorzystywanych do ogrzewania budynkow mieszkalnych
ABSTRACT
Purpose: The aim of the research was to demonstrate that the use of low-emission carbon fuels (obtained using the initial thermal conversion of coal feedstock) in residential heating also makes it possible to reduce mercury emissions from small-scale coal-fired boilers.
Project and methods: The publication presents the results of mercury emission tests conducted using five different small-scale coal-fired boilers and five different coal fuels. The research was carried out under laboratory conditions, but also using heating devices of residential users. They covered a wide range of operational parameters, both energy and emission. The flux of coal fuels burned ranged from 2 to 12.2 kg/h, with an equally wide range of boiler efficiencies obtained - 67.6-88.5%.
Results: The test results presented in the article show that the amount of emissions of pollutants limited by the criteria of the PN-EN 303-5+A1:2023-05 standard and the ecodesign, namely carbon monoxide, nitrogen oxides, dust and organic substances, depends mainly on the design and operating conditions of the device in which the specific solid fuel is burned. There is a group of pollutants emitted into the atmosphere, for which the amount of emissions depends primarily on the quality of the fuel burned. These pollutants include sulphur oxides and mercury, whose emissions depend primarily on the combustible sulphur and mercury content of the fuel being burned.
Conclusions: Experimental studies were carried out to verify what portion of Hg contained in coal during its combustion in domestic boilers with manual and automatic fuel feeding remains bound in bottom ash, and what is emitted into the atmosphere. The content of Hg in bottom ash, regardless of the boiler and fuel used, was at a similar low level, reaching a maximum of about 6% of Hg initially contained in the fuel. As studies have shown, more than 94% of mercury contained in coal fuels burned in small-scale coal-fired boilers is emitted into the atmosphere, contributing significantly to the deterioration of the environment. Replacing traditional coal with low-emission carbon fuels (e.g., such as BC fuel) would significantly reduce mercury emissions from small-scale coal-fired boilers, by up to 90% compared to current emissions. Setting a legal requirement for the permissible level of mercury content in coal fuels used in domestic boilers, for example, at a maximum value of 0.05 mg/kg, would reduce mercury emissions from these devices by at least half. Keywords: mercury emission, small-scale coal-fired boilers, low-emission carbon fuel Type of article: original research article
Received: 30.10.2023; Reviewed: 12.12.2023; Accepted: 13.12.2023;
Authors" ORCID IDs: S. Stelmach - 0000-0002-9206-1008; K. Matuszek - 0000-0001-8184-6507; P. Hrycko - 0000-0002-9713-8553; P. Wolny - 0000-0003-2161-4506; J. Horak - 0000-0001-6665-5294; L. Kubonova - 0000-0003-3256-8373; The authors contributed the equally to this article;
Please cite as: SFT Vol. 62 Issue 2, 2023, pp. 50-62, https://doi.org/10.12845/sft.62.2.2023.3;
This is an open access article under the CC BY-SA 4.0 license (https://creativecommons.org/licenses/by-sa/4.0/).
ABSTRAKT
Cel: Celem przeprowadzonych badan bylo wykazanie, ze wykorzystanie niskoemisyjnych paliw wçglowych (uzyskanych w wyniku wstçpnej konwersji termicznej surowca wçglowego) w ogrzewnictwie mieszkaniowym umozliwia röwniez ograniczenie emisji rtçci z malych kotlöw wçglowych. Projekt i metody: W publikacji przedstawiono wyniki badan emisji rtçci przeprowadzonych z wykorzystaniem piçciu röznych maloskalowych kotlöw wçglowych i piçciu röznych paliw wçglowych. Badania przeprowadzono w warunkach laboratoryjnych, ale takze z wykorzystaniem urzqdzen grzewczych uzytkowniköw indywidualnych. Obejmowaly one szeroki zakres parametröw eksploatacyjnych zaröwno energetycznych, jak i emisyjnych. Strumien spalanych paliw wçglowych wahal siç od 2 do 12,2 kg/h, przy röwnie szerokim zakresie uzyskiwanych sprawnosci kotlöw - 67,6-88,5%.
Wyniki: Przedstawione w artykule wyniki badan wskazujq, ze wielkosc emisji zanieczyszczen limitowanych kryteriami normy PN-EN 303-5+A1:2023-05 i ekoprojektu, a mianowicie tlenku wQgla, tlenkow azotu, pylu i substancji organicznych, zalezy glownie od konstrukcji i warunkow pracy urzqdzenia, w ktorym spalane jest okreslone paliwo stale. Istnieje grupa zanieczyszczen emitowanych do atmosfery, dla ktorych ilosc emisji zalezy przede wszyst-kim od jakosci spalanego paliwa. Zanieczyszczenia te obejmujq tlenki siarki i rtQc, ktorych emisje zalezq przede wszystkim od zawartosci siarki palnej i rtQci w spalanym paliwie.
Wnioski: Przeprowadzone badania eksperymentalne mialy na celu sprawdzenie, jaka czqsc Hg zawartej w wQglu podczas jego spalania w domowych kotlach z rQcznym i automatycznym podawaniem paliwa pozostaje zwiqzana w popiele paleniskowym, a jaka jest emitowana do atmosfery. Zawartosc Hg w popiele paleniskowym, niezaleznie od kotla i stosowanego paliwa, byla na podobnym niskim poziomie, osiqgajqc maksymalnie okolo 6% Hg poczqtkowo zawartej w paliwie. Jak wykazaly badania, ponad 94% rtQci zawartej w paliwach wQglowych spalanych w malych kotlach wQglowych jest emitowane do atmosfery, przyczyniajqc siQ znaczqco do pogorszenia stanu srodowiska. Zastqpienie tradycyjnego wQgla niskoemisyjnymi paliwami wQglowymi (np. takimi jak paliwo BC) znacznie zmniejszyloby emisje rtQci z malych kotlow wQglowych, nawet o 90% w porownaniu z obecnymi emisjami. Ustanowie -nie wymogu prawnego dotyczqcego dopuszczalnego poziomu zawartosci rtQci w paliwach wQglowych stosowanych w kotlach domowych, na przyklad na poziomie maksymalnej wartosci 0,05 mg/kg, zmniejszyloby emisje rtQci z tych urzqdzen o co najmniej polowQ. Stowa kluczowe: emisja rtQci, male kotly wQglowe, niskoemisyjne paliwo wQglowe Typ artykutu: oryginalny artykul naukowy
Przyj^ty: 30.10.2023; Zrecenzowany: 12.12.2023; Zaakceptowany: 13.12.2023;
Identyfikatory ORCID autorow: S. Stelmach - 0000-0002-9206-1008; K. Matuszek - 0000-0001-8184-6507; P. Hrycko - 0000-0002-9713-8553; P. Wolny - 0000-0003-2161-4506; J. Horak- 0000-0001-6665-5294; L. Kubonova - 0000-0003-3256-8373; Autorzy wniesli jednakowy wklad merytoryczny w powstanie artykulu;
Prosz<? cytowac: SFT Vol. 62 Issue 2, 2023, pp. 50-62, https://doi.org/10.12845/sft.62.2.2023.3; Artykul udostQpniany na licencji CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/).
Introduction
Mercury is a rare element that is widely dispersed in nature, with an average concentration not exceeding 0.08 ppm. Owing to its properties, it can occur as a contaminant in any state. In the gas phase, mercury can exist in three forms: elemental mercury vapour (Hg0), volatile compounds (such as Hg2+, mainly HgCl2), and volatile organometallic compounds such as dimethylmercury ((CH3)2Hg). Mercury also contaminates soils, primarily as mercury sulphide (HgS) and mercury oxide (HgO), and less commonly as soluble inorganic compounds (HgCl2 and Hg2Cl2) or organometal-lic compounds (e.g. (CH3)2Hg). The increase in Hg concentration in soils is mainly due to geological and anthropogenic factors. Mercury can also be found in aquatic environments. Mercury and its compounds enter the water primarily through atmospheric deposition, anthropogenic activities, and mercury migration from the soil. It is important to emphasize that Hg and its compounds are always present in the natural environment, and their concentrations vary depending on the region, climate, and other external conditions, constituting what is known as background Hg levels. Any values exceeding the background levels are considered environmental pollution [1-2].
The estimated global inventory of the Hg emissions to the atmosphere from human activities in 2015 was 2 220 Mg, which represents approximately 30% of the total annual Hg emissions released into the atmosphere. An additional 60% of the current global mercury emissions are the result of natural environmental processes, primarily involving the recycling of previously deposited anthropogenic mercury in soils and water. The remaining 10% originates from present-day natural sources such as volcanoes. It is important to note that the global inventory for 2015 does not account for sectors that cannot be accurately quantified. These
unquantified sectors may contribute tens to a few hundred tons of Hg to the overall emission inventory [3]. As a result of increasingly intense civilizational development, the emission of Hg from anthropogenic sources has increased over the past decades, causing serious and hazardous contamination of the environment on a global scale, posing a threat to living organisms, including humans. The largest amount of Hg emitted from anthropogenic sources is the result of artisanal gold mining (nearly 40%). The next positions in this ranking are fuel combustion processes (mainly from coal and lignite, approximately 25%), non-ferrous metal smelting (~10%), cement production (~9%), and mercury production processes (~6%) [4].
The structure of mercury emissions from anthropogenic sources in Poland significantly differs from that presented above, primarily because of the practical absence of emissions associated with gold and mercury mining, and the dominant contribution of coal combustion in the domestic energy system, including a substantial share of lignite. Mercury emissions in Poland are estimated to be approximately 11.9 Mg/year in total [5]. The largest share of this emissions comes from the combustion of solid fuels, with an estimated magnitude of approximately 9.1 Mg/year. Emissions from other sectors are primarily attributed to cement production (0.7 Mg/year) and non-ferrous metal smelting (1.1 Mg/year). The Czech Republic reported the total national Hg emissions at the level of 3.48 Mg/year (2010) [6].
In 2022, the Polish electricity production exceeded 175 TWh [7]. Approximately 43% of this production was generated through the combustion of hard coal, whereas approximately 27% was derived from lignite coal [8]. The level of mercury emissions into the atmosphere from coal combustion processes depends on the mercury
content of the burned coal, its chemical composition, the type of boiler, mercury speciation in the flue gases exiting the boiler, the content of combustible parts in fly ash, and the composition of the flue gases. The mercury content of coal varies widely depending on the regionalization of the deposit, age, and species. According to national studies, the average mercury content in coal ranges from 0.02 to 0.52 mg/kg in Poland [9] and from 0.01 to 1.0 mg/kg in the Czech Republic [10]. Since Poland has a long border with the Czech Republic (almost 800 km), transboundary transmission of emissions dependent on weather conditions is also important in this regard.
Mercury in coal mainly exists in the form of inorganic compounds, such as HgS, HgO, HgCl2, and others. During coal combustion, these compounds decompose, and at temperatures above 900°C, under the conditions prevailing in the boiler furnace, all mercury from the fuel occurs in the form of elemental mercury vapour (Hg0). Upon cooling the flue gas stream to below 540°C, mercury is partially oxidized to Hg2+, forming volatile compounds such as HgCl2, HgBr2, and others. These compounds exhibit a high affinity for adsorption on the surface of fly ash and unburned coal particles [11].
Mercury emitted from coal combustion is released into the atmosphere in three main forms:
- Hg0(g) - elemental mercury vapour, sparingly soluble in water, capable of persisting in the air for up to two years and capable of long-range transport.
- Hg2+(g) - oxidized form of mercury, forming easily soluble compounds in water that remain in the air for several days to several weeks.
- Hg(p) - mercury bound or adsorbed to fly ash particles, persisting in the atmosphere for several days to several weeks, and spreading only on a local scale [11].
On 30 November 2021, the Commission Implementing Decision (EU) 2021/2326 was published in the Official Journal of the European Union (L 469). This decision establishes the Best Available Techniques (BAT) conclusions for large combustion plants (LCP) in accordance with Directive 2010/75/EU of the European Parliament and of the Council [7]. The BAT conclusions for large fuel combustion installations define the permissible emission levels for various pollutants, including permissible mercury emissions from the combustion of hard coal and lignite. In these installations, mercury emissions can be reduced using different methods, including passive (during flue gas desulfurization in wet scrubbers, selective catalytic reduction (SCR) of nitrogen oxides, flue gas dedusting in electrostatic precipitators, and fabric filters) and active (injection of powdered activated carbon into the flue gas, mercury adsorption on a fixed bed of activated carbon adsorbent, use of impregnated adsorbents, selective coal selection for combustion process, coal enrichment through low-temperature treatment and flotation, and dosing of bromine and iodine salts into the fuel and/or flue gas). However, the new BAT conclusions do not cover small-scale coal and lignite combustion devices, where the aforementioned methods for reducing mercury emissions are not feasible. Such devices are still widely used for heating residential buildings in Poland and the Czech Republic, although their utilization in the Czech Republic is much less common (only about 15% of the heat in households is generated
through coal combustion [13]). Currently, they should meet the requirements of the Commission Regulation (EU) 2015/1189 of 28 April 2015, implementing Directive 2009/125/EC of the European Parliament and of the Council with regard to eco-design requirements for solid fuel boilers and emissions [14]. However, they do not cover mercury emissions, and many of the current criteria are not met by the older-type equipment still in use.
Despite numerous programs supporting the promotion of modern heating methods using renewable energy in Poland, approximately 45% of households still use solid fuel heating devices (most commonly, dual-function central heating boilers used for heat production and water heating) [15]. With these devices (excluding biomass), approximately 10 million tons of various grades of coal are burned annually (lignite is currently a marginal fuel used in small-scale devices). It is easy to calculate that the mercury emissions from these devices in Poland, which are not subject to legislative restrictions or technical measures aimed at reducing them, constitute over a dozen percent of mercury emissions from coal combustion for energy purposes. This fact has inspired preliminary studies, and in this article, the authors present the results of their attempt to determine the magnitude of mercury emissions from coal combustion in small-scale devices and propose a simple method for reducing these emissions other than replacing the existing combustion sources. Thus, the aim of the research carried out was to demonstrate that the use of low-emission carbon fuels (obtained using the initial thermal conversion of the coal feedstock) in residential heating also makes it possible to reduce mercury emissions from small-scale coal-fired boilers.
Materials and Methods
Fuels
The following coal fuel samples were used in the tests conducted:
1. Three different samples of hard coal (C1, C2 and C3).
2. Low-emission carbon fuel produced using the pyrolysis process, specially prepared for test purposes for use in residential heating (BC).
3. A mixture of hard coal (other than C1-C3 samples) and low-emission carbon fuel BC with mass shares respectively 0.85:0.15 (CBC).
C1, C2 and C3 coals were typical coal fuels sold commercially for residential heating use. The other two fuels (BC and CBC), on the other hand, are specially developed low-emission carbon fuels developed by Polish fuel manufacturers with the participation of the Institute of Energy and Fuel Processing Technology. The main benefit of using these fuels is a significant reduction in emissions of polycyclic aromatic hydrocarbons (including benzo(a)pyrene) and particulate matter (including PM10) [16-17]. However, the research presented in this publication focuses on assessing the possibility of reducing mercury emissions through their use in low-power heating devices still in use in Poland and the Czech Republic (but also in many other countries).
A brief explanation is required for the production of low-emission carbon fuel (BC). Hard coal of appropriately selected grain
size (~20^50mm) is subjected to partial degasification in a pyrol-ysis reactor with a moving grate at a temperature of about 850-900°C. Volatile compounds emitted from the processed coal during its heating on the grate are burned in the post-combustion zone of the pyrolysis reactor, and the resulting flue gases are directed to the purification system. The final content of volatile matter in the produced BC fuel is less than 10% m/m (usually about 5% m/m). The heat of the hot flue gases produced in the process is used to produce superheated steam. The resulting BC fuel, after cooling by spraying water, is subjected to sorting. The grain fraction >5mm is bagged and sent for sale.
The results of the physicochemical analyses of the test fuels are shown in Table 1 and Table 2. All the presented analyses were carried out in the accredited laboratories of the Institute of Energy and Fuel Processing Technology in accordance with the European and ISO standards.
Boilers
The combustion of the fuels listed above was carried out using a variety of small-scale coal-fired boilers. The following boilers with different designs and operational characteristics were selected for the planned research:
1. A manually fuelled boiler with a nominal power of 12 kW (boiler no. 1 - B1).
2. A manually fuelled boiler with a nominal power of 15 kW (boiler no. 2 - B2).
3. Boilers with automatic fuel feeding and a nominal power
of 100 kW (boiler no. 3 - B3).
4. A boiler with automatic fuel feeding and a nominal power of 24 kW (boiler no. 4 - B4).
5. A boiler with automatic fuel feeding and a nominal power of 150 kW (boiler no. 5 - B5).
Experiments using boilers no. 1 and no. 2 were conducted at the test facility of the Institute of Energy and Fuel Processing Technology. Boilers no. 3-5 were tested at their operating locations. The following is a brief description of the aforementioned boilers.
Boiler no. 1 (B1)
This is a low-temperature steel boiler designed for open systems and adapted for burning hard coal of the pea size assortment. The boiler body is made of steel plates. Inside the body there is an ash chamber, a grate, a combustion chamber, a fuel loading chamber and flue ducts. The boiler is also equipped with an ash door, a fuel loading door and flue ducts cleaning door. The combustion chamber is equipped with a burner nozzle. Air was supplied to the boiler through an adjustable damper located on the loading door. The air passing through the grate participated in the combustion process. Additionally, air was supplied to the combustion chamber by the ash chamber. Periodic ash removal was performed using a scraping mechanism. The tested boiler meets the emission criteria of Class 5 of the EN 303-5:2012 standard and eco-design requirements (except for the energy efficiency criterion) [14]. Its view is presented in Figure 1.
Table 1. Proximate analysis of the tested fuels (as received basis)
Parameter Unit C1 C2 C3 BC CBC
Moisture content [%] 4.010.01 6.61002 13.11008 14.910.09 7.81002
Ash content [%] 6.310.01 11110.02 7.010.01 6.510.01 6.210.01
Volatile matter content [%] 17.6 10.06 29.410.09 32.111110 3.110.01 28.61009
Fixed carbon [%] 72.1 ,0.08 52.91013 47.81019 75.51011 57.41012
Lower calorific value (LCV) [J/g] 29497 25633 24393 26065 27373
Grain size [mm] ~5- 30 ~5-30 ~5-30 ~5-25 ~5-30
Source : Own elaboration.
Table 2. Ultimate analysis of the tested fuels (dry basis)
Parameter Unit C1 C2 C3 BC CBC
Carbon content [%] 83.1104 72.6104 72.7104 88.6104 77.4104
Hydrogen content [%] 3 210.01 4.010.01 4.810.01 1 310.00 4.310.01
Sulfur content [%] 0 910.00 0.310.°° 1 310.00 0.410.00 0.810.°°
Nitrogen content [%] 1 110.00 2 010.00 1 310.00 1.610.00 1.510.00
Oxygen content [%] 5.21042 9 210.43 11 910.42 0.41041 9 310.42
Mercury content [mg/kg] 0.14610015 0.03710.004 0.09810.010 0.00710.001 0.03610.004
Source : Own elaboration.
Figure 1. View and diagram of the boiler no. 1 Source: Own elaboration.
Boiler no. 2 (B2)
This boiler is a representative of a type series of low-temperature, steel water boilers, designed for open systems, adapted for combustion of hard coal of nut type (basic fuel) and hard coal of pea type. In these units, fuel is loaded into a loading chamber terminated with a water grate. The loading chamber is closed by a charging door. A secondary air damper is located on the charging door. Above the charge door there is a cleanout door for access to the heat exchanger. A stream of primary air is fed into the combustion chamber by a blowing fan located on the upper wall of the boiler. The flue gases, after passing through the flue-water heat exchanger, pass through the boiler's flue to the chimney. The boiler is also equipped with a mechanical grate rake. The boiler is insulated with mineral wool, covered with painted steel sheet. The tested device does not meet the emission criteria of Class 5 of the EN 303-5:2012 standard and eco-design [14]. Its view is presented in Figure 2.
Figure 2. View and diagram of the boiler no. 2 Source: Own elaboration.
Boiler no. 3 (B3)
It is a boiler with automatic fuel feeding and retort burner. This boiler belongs to a series of low-temperature, steel water boilers, designed for open systems, adapted for burning hard coal
of the pea-sort. In this unit, fuel is poured into a hopper closed with a steel door, located on the side of the boiler over a screw feeder driven by a gear motor. The feeder moves successive portions of fuel from the hopper to the retort burner, located in the combustion chamber. The retort burner is made of cast iron in the shape of a truncated cone with drilled holes and tabs to the inside forming a screw line. In addition, a cast iron deflector is mounted above the burner. The combustion chamber is lined with ceramic plates and is closed with an insulated door. A stream of air is fed to the burner in the combustion chamber by means of a blowing fan. A feed water muffle is located in the upper rear part of the boiler. The return water muffle is located at the lowest point of the boiler. The boiler has a vertical plate heat exchanger. The flue gases, after passing through the heat exchanger (flue-water), pass through the boiler's flue to the chimney. Flue gas swirlers are installed in the second and third pass of the heat exchanger. Regulation of heat output is carried out by an electronic temperature controller. This regulator controls the operation of the water circulation pump, hot water pump, floor heating pump, circulation pump, air blower (fan) and fuel feeder. The boiler is insulated with mineral wool covered with painted steel sheet. This boiler, when properly operated, meets the criteria of Class 5 of the EN 303-5:2012 standard and eco-design [14]. Its view is presented in Figure 3.
Figure 3. View and diagram of the boiler no. 3 Source: Own elaboration.
Boiler no. 4 (B4)
It is a boiler with automatic fuel feeding and a retort burner with a screw feeder, adapted for burning hard coal of the pea-sort. It is designed for open system water central heating systems. In addition to burning coal in the automatic mode, this boiler also allows burning other fuels in manual loading mode. The tested boiler at the time of the study was already a far out-of-service device. As a new device, it met the criteria of Class 5 of the EN 303-5:2012 standard and eco-design [14]. However, it did not meet these criteria at the time of the presented research. Its view is presented in Figure 4.
The small-scale coal-fired boilers selected for the study represent the variety of designs used for heat and hot water production in residential heating in Poland and the Czech Republic.
Experiments
The tests presented in this publication include coal fuel combustion tests conducted according to the scheme shown in Table 3.
Figure 4. View of the boiler no. 4 Source: Own elaboration.
Boiler no. 5 (B5)
This is a boiler with automatic fuel feeding and retort burner. This boiler belongs to the low-temperature steel water boilers, designed for open systems, adapted for burning hard coal of the pea-sort. In this unit, fuel is poured into a hopper closed with a steel door, located on the side of the boiler over a screw feeder driven by a gear motor. The feeder moves successive portions of fuel from the hopper to the retort burner located in the combustion chamber. The combustion chamber is closed with a door. A circular ceramic deflector is placed above the burner. A stream of air is fed to the burner in the combustion chamber by means of a blowing fan. A feed water muffle is placed in the upper part of the boiler exchanger. The return water muffle is located at the lowest point of the boiler exchanger. The boiler has a plate heat exchanger. The flue gases, after passing through the flue-water heat exchanger, pass through the boiler's flue to the chimney. Regulation of the boiler's heat output is carried out by an electronic temperature controller. This regulator controls the operation of the feeder, blower fan, central heating and hot water pumps. The boiler is insulated with mineral wool covered with painted steel sheet. The tested boiler, when operated properly, meets the criteria of Class 5 of the EN 303-5:2012 standard and eco-design [14]. Its view is presented in Figure 5.
Table 3. Schematic representation of the experiments performed
m
Figure 5. View of the boiler no. 5 Source: Own elaboration.
Fuel
C1 C2 C3 BC CBC
Boiler no. 1 B1/C1 - - - -
Boiler no. 2 - B2/C2 - B2/BC -
Boiler no. 3 - - B3/C3 - B3/CBC
Boiler no. 4 - - B4/C3 - B4/CBC
Boiler no. 5 - - B5/C3 - B5/CBC
Source : Own elaboration.
Laboratory tests
Energy and emission tests of the combustion of hard coal (B1/C1 and B2/C2) and low-emission carbon fuel (B2/BC), and thus with the use of boilers with manual fuel feed, were carried out on a test stand at the Laboratory of Combustion Technologies and Power Generation operating within the structure of ITPE (Zabrze, Poland). These tests included two feeds of portions of fuel into the boiler each time. The weight of the fuel for both intakes was the same and was based on the provisions of the PN-EN 303-5+A1:2023-05 standard [18] and the calorific value of the fuel in question. It shows that the amount of fuel loaded into the boiler must be sufficient for at least 4 h of its operation at nominal power. First, an adequate layer of embers was pregen-erated in the boiler. Then a weighed portion of fuel was poured onto the embers so as to ensure the required test duration, during which the flue gas composition was measured: O2, CO2, CO, NO, SO2, and the content of dust and total organic carbon (TOC). Flue gas collection for the determination of dust and TOC was performed using a system consisting of a probe (installed in the chimney) connected to a heated dust separator, a cooler, a tubing system with sorption material (XAD-2 resin and activated carbon) and a gas aspirator. Collection of flue gases for analysis was conducted based on the laboratory's accredited technical procedure. The dust concentration was determined using the gravimetric (filtration) method in accordance with the internal procedure of the Laboratory of Combustion Technologies and Power Generation Q/LS/02/D:2018 and PN-Z-04030-7 Protection of air purity. Testing of dust content and PN-EN 13284-1 Emissions from stationary sources. Determination of dust mass concentration in the low range. Part V: Manual gravimetric method. In addition, boiler energy parameters such as flue gas temperature, boiler inlet and outlet water temperatures, water flow and chimney draught were recorded during the tests.
A mobile set of SIEMENS analysers was used to measure the composition of the exhaust gas (see Figure 6). The set included ULTRAMAT 23 analysers enabling the measurement of CO in the range of 0-5%, CO2 in the range of 0-25%, SO2 in the range of 0-1000 ppm, and NO in the range of 0-1000 ppm. These analysers allow measurement using the reference NDIR method. O2 concentration in the gas was measured using an analyser of the OXY-MAT 61 type, which operates on the basis of a reference method using the phenomenon of paramagnetism. This analyser has a range of 0-25% O2. Concentrations of O2, CO2, CO, NO, SO2 were determined based on the PN-ISO 10396:2001 standard Emissions from stationary sources. Sampling for automatic measurement of concentrations of gaseous components.
Figure 6. View of the mobile set of SIEMENS analysers used for exhaust gas composition testing
Source: Own elaboration.
After the combustion test of a given fuel was completed, the generated bottom ash was secured and transferred to the laboratory for determination of mercury content. The mercury content of both the fuel and bottom ash samples was determined using the cold vapour atomic absorption amalgamation (CVAAS) method, with an MA-2 analyser from Nippon Instruments Corporation.
ash chamber was cleaned, and a continuous fuel feeding test was performed, which ultimately allowed the determination of the fuel flow fed to the combustion. The boiler was then fired up and the boiler's operating conditions were stabilized over a period of 1.5 to 2 h. The operating conditions of the boiler were set by the local staff each time, without any interference or comments from those in the measurement group.
The actual testing began with the re-cleaning of the boiler's ash chamber. This was followed by an energy-emission test during the combustion of CBC fuel. The combustion test of this fuel was completed by selecting a sample of bottom ash from the boiler's firebox. Then the CBC fuel was selected from the boiler hopper, the remaining fuel was removed from the feeding system and the boiler was carburized with C3 fuel.
After the C3 fuel was fed to the retort burner, the boiler was fired up and the boiler operating conditions were stabilized over a period of 1.5 to 2 h for the same settings at which the CBC fuel combustion was tested, i.e. fuel feed time, fuel feed interval time and fan blowing power percentage setting.
The actual testing began by cleaning the ash chamber of the device again. This was followed by an energy-emission test during the combustion of C3 fuel. The test was completed by selecting a sample of the bottom ash from the boiler's combustion chamber.
Solid waste samples from the furnace were secured and transferred to the ITPE laboratory for determination of Hg content.
For the field measurements, measuring equipment analogous to that used in the laboratory studies was used. Only TOC emissions were not measured during these experiments. The measuring equipment used during the boiler tests by ITPE employees meets the requirements of PN-EN 303-5 Measuring Instruments and Methods of Measurement.
Results
The basic energy and emission parameters of the conducted experiments are shown in Tables 4 and 5.
Field trials
Energy and emission tests of the combustion of hard coal (B3/C3, B4/C3 and B5/C3) and low-emission carbon fuel (B3/CBC, B4/CBC and B5/CBC), and thus with the use of boilers with automatic fuel feeding, were carried out at individual users in various locations in Poland. At each location, several weeks prior to the start of the tests, the boilers operated continuously burning previously supplied CBC fuel. C3 hard coal was delivered to the site on the day of the measurements, in sufficient quantity for several hours of boiler operation.
The boiler operator at each test site was informed of the test day. Each time, the operator was asked to conduct a thorough cleaning of the boiler on the day before the test day, and then conduct CBC fuel combustion in the boiler in accordance with past practice. Each time the ITPE/VSB measurement group arrived on site, the boiler was extinguished for about 30 minutes. During this time, the measuring apparatus was assembled, the boiler's
Table 4. Results of the measurements carried out using boilers with manual fuel feeding
Parameter Unit B1/C1 B2/C2 B2/BC
Energy parameters
Fuel flow [kg/h] 2.0 3.0 3.8
Lambda - 1.95 2.21 3.30
Boiler efficiency [%] 76.0 72.2 67.6
Boiler power [kW] 12.4 15.4 18.6
Relative thermal load of the boiler [%] 103.4 102.7 124.0
Flue gas parameters
Flue gas temperature [°C] 187.8 256.0 254.5
Chimney draught [Pa] -11.8 -25.2 -31.5
O2 concentration 10.33 11.57 14.57
CO2 concentration [%] 9.40 8.08 5.84
Converted to 10% O2 9.69 9.42 10.00
CO concentration 436.1 4256.1 2755.1
Converted to 10% O2 458.9 4967.4 4712.1
SO2 concentration 967.8 259.8 178.5
Converted to 10% O2 996.7 303.0 305.3
NO concentration 161.6 143.6 47.9
Converted to 10% O2 [mg/Nm'j 255.1 256.6 112.8
Dust concentration 22.3 144.4 31.1
Converted to 10% O2 23.0 166.9 53.2
TOC concentration 31.5 101.2 45.9
Converted to 10% O2 32.5 116.7 78.6
Emissions
CO2 [kg/GJ] 95.8 91.2 93.4
CO [g/GJ] 231.1 2433.2 2229.4
SO2 [g/GJ] 498.2 148.4 144.4
NO [g/GJ] 127.4 125.7 59.4
Dust [g/GJ] 11.5 81.8 25.2
TOC [g/GJ] 16.3 57.1 37.2
Source : Own elaboration.
Table 5. Results of the measurements carried out using boilers with automatic fuel feed
Parameter Unit B3/C3 B3/CBC B4/C3 B4/CBC B5/C3 B5/CBC
Energy parameters
Fuel flow [kg/h] 6.5 6.3 0.9 0.8 12.2 11.7
Lambda - 2.4 2.3 3.8 2.8 4.1 3.9
Boiler efficiency [%] 88.5 87.5 71.4 79.5 70.6 74.3
Boiler power [kW] 39.0 41.9 4.4 4.8 58.4 66.1
Relative thermal load of the boiler [%] 39.0 41.9 18.3 20.0 38.9 44.1
Flue gas parameters
Flue gas temperature [°C] 117.6 118.6 202.3 207.0 172.3 181.6
Chimney draught [Pa] 30.3 30.7 22.0 25.0 20.7 28.9
O2 concentration 12.52 11.81 15.60 13.48 16.00 15.73
CO2 concentration [%] 7.13 7.86 4.48 6.43 4.18 4.45
Converted to 10% O2 9.25 9.41 9.13 9.41 9.20 9.29
CO concentration 654.6 607.9 1507.1 660.9 475.9 462.0
Converted to 10% O2 849.1 727.8 3069.4 967.2 1047.9 964.9
SO2 concentration 1207.7 786.3 570.0 542.8 611.3 396.5
Converted to 10% O2 1566.6 941.3 1160.8 794.4 1346.1 828.1
NO concentration 345.2 342.0 294.9 259.4 187.8 197.4
Converted to 10% O2 447.8 409.3 600.7 379.5 413.0 412.1
Dust concentration 128.3-133 .8 82.7-100.3 138.5-151.3 89.5-105.1 67.6-70.3 55.1-62.0
Converted to 10% O2 166.4-173.6 99.0-120.1 282.1-308.2 131.0-153.8 148.9-154.8 115.1-129.5
Emissions
CO2 [kg/GJ] 89.4 90.4 88.2 90.3 88.9 89.2
CO [g/GJ] 415.1 353.5 1500.7 469.7 511.8 468.5
SO2 [g/GJ] 765.8 457.3 567.5 385.8 657.4 402.1
NO [g/GJ] 218.9 198.9 293.6 184.4 201.9 200.2
Dust [g/GJ] 81.4-84.8 S 48.1-58.3 137.9-150.7 63.6-74.7 72.7-75.6 55.9-62.9
Source : Own elaboration.
Table 6 presents the results of the calculations based on the mer- emissions with volatile combustion products are very strongly
cury analyses performed for the fuels tested and the bottom ash pro- related to its initial content in the fuel fed to the boiler (correlation
duced during the combustion tests. It was assumed that the amount coefficient r = 0.97). Therefore, the initial content of mercury in the
of mercury fed with air into the combustion process is negligible. coal headed for combustion is extremely important.
Table 6. Results of the calculations based on the mercury analyses performed for the fuels tested and the bottom ashes produced during the combustion tests
Unit B1/C1 B2/C2 B2/BC B3/C3 B3/CBC B4/C3 B4/CBC B5/C3 B5/CBC
Hg flux delivered with fuel mg/kg 0.140 0.035 0.006 0.085 0.033 0.085 0.033 0.085 0.033
[ng/GJ] 4.75 1.37 0.23 3.48 1.21 3.48 1.21 3.48 1.21
Amount of Hg delivered with fuel [%] 100
Amount of Hg remaining in bottom ash [%] 5.76 1.92 2.75 1.83 2.90 0.78 3.05 1.99 3.74
Amount of Hg emitted into the atmosphere [%] 94.24 98.08 97.25 98.17 97.10 99.22 96.95 98.01 96.26
The amount of Hg emitted with flue gas (including dust) per GJ of useful heat generated [mg/GJ] 6 x 10-3 2 x 10-3 3 x 10-4 4 x 10-3 1 x 10-3 5 x 10-3 1 x 10-3 5 x 10-3 2 x 10-3
Source: Own elaboration.
Discussion
Combustion tests of various coal fuels in boilers of different designs covered a wide range of operating parameters, both energy and emissions. The flux of coal fuels burned ranged from 2 to 12.2 kg/h, with an equally wide range of boiler efficiencies obtained - 67.6-88.5%. It should be noted here that the boilers tested in the laboratory (boilers with manual fuel feeding) operated at slightly higher than nominal relative thermal loads, while the boilers tested at individual users operated at very low relative thermal loads - <45% of nominal power.
The test results presented in this article show that the amount of emissions of pollutants limited by the criteria of the PN-EN 303-5+A1:2023-05 standard [18] and the eco-design [14], namely carbon monoxide, nitrogen oxides, dust and organic substances, depends mainly on the design and operating conditions of the device in which the specific solid fuel is burned. A detailed analysis of this issue, is beyond the scope of this publication, although Tables 4 and 5 clearly show the differences in these parameters from one test to another. However, there is a group of pollutants emitted into the atmosphere, for which the amount of emissions depends primarily on the quality of the fuel burned. These pollutants include sulphur oxides and mercury, whose emissions depend primarily on the combustible sulphur and mercury content of the fuel being burned.
Figure 7 shows the dependence of mercury emissions from the tested heating equipment on the mercury content of the fuel being burned. Based on the presented data, it is clear that mercury
/
f'' y ' / p .0 0
o 0
O
iy corneal In fyul [in,.'Ml;. - I..
Figure 7. Mercury emissions depending on its content in the fuel being burned
Source: Own elaboration.
In bituminous coals, mercury occurs primarily in the form of such compounds as HgO, HgS, HgO2, HgCl2, Hg3(SO4)O0 and Hg2SO4. With the exception of HgO, the decomposition temperatures of the aforementioned mercury compounds are in the range below the ignition temperature of coal, which, according to the authors [19], provides potential opportunities for removing mercury from coal before it enters the furnace chamber. This possibility is also supported by the fact that mercury is released from coal practically immediately after the test sample is introduced into the zone of sufficiently high temperatures, which was also demonstrated in the work [19-20]. As presented in the literature review [21], in the case of application of the pre-combustion
method for mercury removal, due to exposure of coal to temperatures up to 330°C, 60 to 80% efficiency of mercury removal from the fuel was found, while in the case of exposure to temperatures up to 410°C, removal of more than 95% of mercury from the converted hard coal was already demonstrated. Pre-treatment of coal with low-temperature pyrolysis makes it possible to obtain coal fuel with significantly better quality parameters from the point of view of emissions to the atmosphere (including mercury emissions) [22], which is usually referred to as low-emission carbon fuel [16-17]. Laboratory studies on obtaining such fuels are presented, for example, in the publication [23]. Other ways of reducing mercury emissions from coal combustion processes are also being studied, such as using coal enrichment to separate any overgrowths with pyrite [24] or using regenerable sorbents based on gold nanoparticles dispersed on activated carbon foam [25]. However, their large-scale application appears to be more difficult than the implementation of large-scale coal processing by low-temperature pyrolysis.
According to data presented in the publication [26], the average Hg content of the hard coals used for energy purposes is 0.070 mg/kg, while the average mercury content of the hard coals used in the study (samples C1, C2 and C3) was 0.094 mg/kg, with one of these coals (C2) having a significantly lower content of this element compared to the other two coal fuels. The other two fuels tested (BC and CBC), were just fuels obtained by pyrolytic pre-treatment of hard coal, in whole and in part, respectively. The BC coal fuel was all pyrolyzed bituminous coal, while the CBC fuel was a mixture of bituminous coal and BC fuel with mass shares of 0.85:0.15. Both fuels were produced to reduce their emissivity in terms of organic pollutants and dust, with no intention of reducing mercury emissions. The conducted studies indicate that low-emission carbon fuels obtained using the pyrolysis process make it possible to significantly reduce mercury emissions from combustion processes in distributed heat and hot water generation systems, which are still widely used in Poland and the Czech Republic. In these countries, there is no obligation to check the Hg content of coal fuels intended for combustion in individual heating, so the use of non-mercury-emitting coal fuels in place of the previously used bituminous coals can bring significant benefits, both from an environmental and health point of view.
It is estimated that the amount of Hg introduced along with hard coal into small-scale coal-fired boilers is about 1.0 Mg per year in Poland. Taking into account the results of the study, it can be concluded that about 0.95% of this mass enters the atmosphere as a result of coal combustion in households. Replacing traditional coal with low-emission carbon fuels (e.g., such as BC fuel) would significantly reduce mercury emissions from small-scale coal-fired boilers, by up to 90% compared to current emissions. Setting a legal requirement for the permissible level of mercury content in coal fuels used in domestic boilers, for example, at a maximum value of 0.05 mg/kg, would reduce mercury emissions from these devices by at least half.
Conclusions
Experimental studies were carried out to verify what portion of Hg contained in coal during its combustion in domestic boilers with manual and automatic fuel feeding remains bound in bottom ash, and what is emitted into the atmosphere. The content of Hg in bottom ash, regardless of the boiler and fuel used, was at a similar low level, reaching a maximum of about 6% of the Hg initially contained in the fuel. As studies have shown, more than 94% of the mercury contained in coal fuels burned in small-scale coal-fired boilers is emitted into the atmosphere, contributing significantly to the deterioration of the environment. The problem of mercury emissions from individual heating devices used in Poland and the Czech Republic (but also in many other countries around the world) can be minimized through the use of low-emission carbon fuels in place of the previously used coals. They are characterized by much better emission parameters in comparison with coal - lower emissions of organic substances and dust, but also mercury (as shown in studies conducted). Also extremely important is the issue of modernization of the current dispersed boiler infrastructure, i.e. replacement of old equipment with highly efficient automatic units, as well as a systematic energy transition aimed at replacing coal-fired equipment with modern devices producing heat and hot water (e.g. heat pumps).
The limited coverage in the literature regarding the reduction of mercury emissions from small-scale coal-fired boilers can be attributed to the regional nature of these heating systems. The findings presented here represent the initial endeavour in our country to evaluate the potential for lowering mercury emissions from small-scale coal-fired boilers through the utilization of low-emission carbon fuels.
If the current EU environmental policy is maintained in the future, the issue of using small-scale coal-fired boilers burning solid fuels will steadily decline in importance. However, the use of low-emission carbon fuel for residential heating may continue for another 20 years or so. With the planned start of large-scale production of CBC-type fuel in our country, there will be an opportunity to test the environmental effects (including reduction of mercury emissions) with a wider group of users. It is also important to consider the distribution of mercury between carbon conversion products during the production of low-emission carbon fuel (BC), which will be the subject of future research.
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StAWOMIR STELMACH, D.SC. - currently serves as scientific secretary of the Institute of Energy and Fuel Processing Technology (formerly Institute for Chemical Processing of Coal) in Zabrze. Involved primarily in research, development and commercialisation of thermal conversion of solid fuels using pyrolysis, gasification and combustion. He has a publication output in national and international journals, monographs and conference proceedings (more than 130 items). He is the author and co-author of several patented technical solutions. He has participated in the implementation of many national and international scientific and R&D projects.
KATARZYNA MATUSZEK, PH.D. - graduated in 2000 from the Faculty of Environmental Engineering and Power Engineering at the Silesian University of Technology in Gliwice, where she also obtained her doctoral degree in 2007. Since January 2011, she is the head of the Laboratory of Combustion Technologies and Power Generation and deputy head of the Department of Air Protection at the Institute of Energy and Fuel Processing Technology in Zabrze. She is the author or co-author of dozen scientific and specialist papers on pollutant emissions and fuel combustion in residential heating.
PIOTR HRYCKO, M.SC., ENG. - graduated in 2002 from the Faculty of Environmental Engineering and Power Engineering of the Silesian University of Technology. Since January 2011, he is the deputy head of the Laboratory of Combustion Technologies and Power Generation at the Institute of Energy and Fuel Processing Technology in Zabrze. In his work he deals with energy and emission tests of heating devices for a specific type of fuel with determination of heat output, energy efficiency, concentrations of dust, OGC, CO2, CO, NOx, SO2 in exhaust gases, evaluation of solid fuels, including smokeless fuels for individual heating. He is the author or co-author of dozen scientific and specialist papers on pollutant emissions and fuel combustion in residential heating.
JIRl' HORAK, PH.D. - head of the research group Modern Heating Systems at the Energy Research Centre, Centre for Energy and Environmental Technologies at the VSB-TU Ostrava, Czech Republic. He graduated his master degree in the study program Mechanical Engineering Technology at the VSB-TU Ostrava, Czech Republic and his doctoral degree in the study program Environmental Protection in Industry at the VSB-TU Ostrava, Czech Republic. His research and professional interests include modern heating systems of households, quality of solid fuels combustion, efficiency of combustion devices, pollutant emissions, dimensional characteristics of dust particles - nanoparticles and education of operators of small boilers and stoves. He is known as Smokeman with his educational show presented in the Czech Republic and abroad. He is the author or co-author of more than one hundred of manuscripts in international and national journals on combustion technologies and pollutant emissions from solid fuels combustion in households.
LENKA KUBONOVA, PH.D. - researcher at the Energy Research Centre, Centre for Energy and Environmental Technologies at the VSB-TU Ostrava, Czech Republic. She graduated her master degree in the study program Chemical Engineering at the VSB-TU Ostrava, Czech Republic and her doctoral degree in the study program Nanotechnology at the VSB-TU Ostrava, Czech Republic. During her studies, she had several international experiences and she has focused on cooperation with international partners during her professional life as well. Her current research and professional interests include pollutant emissions, secondary cleaning of flue gases, nanoparticles and fuel analysis. She is the author or co-author of 40 manuscripts in international journals on catalysis, adsorption, combustion technologies, pollutant emissions, secondary cleaning of flue gases and fuel analysis.
PAWEL WOLNY, PH.D. - research and teaching assistant professor at The Faculty of Process and Environmental Engineering. He graduated from the Main School of Fire Service, the Finance Academy in Warsaw and Central Institute for Labour Protection - National Research Institute. His research and professional interests include safety, fire protection, technical fire protection systems (mostly modern active system like mist extinguishing systems). He is the author or co-author of several scientific and specialist papers on safety, fire protection, modern technologies in safety and risk management.
