Combustion Characteristics of Municipal Solid Waste in a Grate-Fired Solid-Fuel Hot Water Boiler

Abstract

Currently, ecological energy production is one of the most pressing issues in power engineering. In addition, environmental pollution caused by various emissions and the challenge of waste disposal remain significant global concerns. One potential solution to these problems is the conversion of waste into useful energy through combustion. In this study, experimental investigations were carried out on the combustion of municipal solid waste (MSW) in a grate furnace of a 400 kW hot water boiler. The experiments included the combustion of both MSW and traditional brown coal. Data were collected on the concentrations of various substances in the exhaust gases, and thermal imaging was performed to assess heat losses from the boiler surface. When burning waste compared to coal, SO₂ concentrations were significantly lower, ranging from 3.43 to 4.3 ppm, whereas for coal they reached up to 122 ppm. NO_X concentrations during MSW combustion peaked at 106 ppm, while for coal combustion they reached 67.5 ppm. A notable increase in CO concentration was observed during the initial phase of coal combustion, with levels reaching up to 2510 ppm. The thermal efficiency of the boiler plant reached 84.4% when burning waste and 87% when burning brown coal.

1. Introduction

Solid municipal waste represents a significant source of environmental pollution, with per capita waste generation showing consistent annual growth. While the figure stood at 330 kg per person in 2012, projections indicate a 50% increase by 2030 [1]. Currently, over 90% of this waste ends up in unregulated landfills, exacerbating environmental concerns. In response to this challenge, Kazakhstan’s Green Economy Transition Concept advocates for implementing waste-to-energy recycling technologies. The country’s revised Environmental Code now formally recognizes this approach, with the government actively promoting it through incentives such as preferential electricity tariffs and comprehensive support for developing waste processing infrastructure [2]. Given these circumstances, research into municipal solid waste incineration has become particularly relevant for Kazakhstan, as this method presents one of the most practical solutions for sustainable waste management. This technology not only addresses waste disposal needs but also contributes to energy generation, aligning with national environmental and economic objectives. Recent research has extensively examined various aspects of municipal solid waste (MSW) incineration and its environmental impacts. Gao et al. [3] investigated carbon emission reduction in MSW incineration plants, revealing seasonal variations due to changes in waste composition. Their study identified heat recovery as the most promising technology for emission control. Zhu et al. [4] analyzed ash formation during co-incineration of sludge with MSW, while Zhang et al. [5] demonstrated that reducing the excess air coefficient from 1.2 to 0.9 decreases NOx emissions from 398 mg/m³ to 215 mg/m³.

Combustion efficiency has been another key research focus. Wang et al. [6] established that maintaining moisture content below 38.32% enables achieving emission levels of 200 mg/m³. Sun et al. [7] studied multi-component MSW combustion in coal boilers, finding that torrefied waste at 300 °C exhibits H/C and O/C ratios comparable to bituminous coal. Xu et al. [8] reported reduced NOx formation when adding fungi or waste tires to industrial incinerators, whereas Tan et al. [9] observed increased NOx emissions (from 270 mg/m³ to 341 mg/m³) when co-firing coal with 20% sludge in a 100 MW boiler. Fluidized bed combustion studies by Suksankraisorn et al. [10] revealed complex pollutant dynamics: elevated CO concentrations at high excess air ratios (due to furnace cooling slowing CO oxidation), decreased SO₂ emissions, and marginally increased NO levels. Qi et al. [11] documented paradoxical NOx reduction with higher oxygen concentrations during food digestate co-combustion.

Computational approaches have provided valuable insights into waste combustion dynamics. Tang et al. [12] developed numerical models of MSW combustion, while Hu et al. [13] demonstrated that reducing boiler load from 100% to 50% triples NOx emissions (from 273 mg/m³ to 605 mg/m³), underscoring the importance of maintaining full-load operation. In a related study, Hu et al. [14] examined pollutant formation in a high-capacity incinerator, recording NOx concentrations of 263 mg/m³ and CO₂ levels of 10%, while notably detecting no CO emissions. Lai et al. [15] investigated waste decomposition using different oxidizers, and Chen et al. [16] identified waste sorting combined with anaerobic food waste treatment as the most effective method for minimizing greenhouse gas emissions. Liu et al. [17] reported that flue gas recirculation leads to increased NOx concentrations (from 209 mg/m³ to 307 mg/m³) and elevated oxygen levels (from 4.52% to 8%), revealing important trade-offs in emission control strategies.

Further computational studies by Li et al. [18] focused on optimizing MSW combustion to reduce NOx formation through modeling. Their results indicated that a sludge fraction of 7% yields minimal NOx concentrations, with any further increase leading to higher emissions. Harris et al. [19] conducted a comprehensive analysis of toxic gas emissions from five waste incineration plants, demonstrating that selective catalytic reduction can achieve emissions as low as 4.3 g per ton of incinerated waste.

The literature review highlights a notable gap in empirical studies focused on the combustion of solid waste in hot water boilers equipped with fixed-grate solid fuel systems. Despite the growing interest in waste-to-energy technologies, limited research has explored the operational performance and combustion characteristics of such systems when fueled by heterogeneous solid waste. This study directly addresses this gap by conducting comprehensive experimental investigations, offering a comparative evaluation between solid waste and conventional solid fuel combustion within a grate-based hot water boiler. The significance of this research lies in its novel contribution to both environmental sustainability and energy recovery practices. By generating empirical data on combustion efficiency, emission profiles, and thermal performance, the study provides critical insights for optimizing boiler design, improving fuel utilization, and supporting the integration of waste-derived fuels into existing energy infrastructure. Ultimately, this work advances the understanding of alternative fuel applications in thermal systems, aiding the transition toward more sustainable and resource-efficient energy solutions.

In the experiments conducted, the KSVR-430 boiler with a capacity of 400 kW was studied. Two types of fuel were used—solid waste and brown coal from the Shubarkol city deposit, traditional for these types of hot water boilers.

2. Materials and Methods

The combustion studies were conducted using a mass-burn approach with solid waste combusted in a fixed-bed configuration on the grate of an operational hot water boiler. The experimental setup utilized an existing KSVR-430 hot water boiler installed in an active boiler house facility. The KSVR-430 boiler is manufactured by the KazKotloService LLP company in Almaty, Kazakhstan, based on the patents of the authors [20].

The KSVR-430 represents a typical solid-fuel boiler system with the following technical characteristics: fixed-grate design; vertical, floor-standing monoblock construction; Manual fuel feeding system; water-wall combustion chamber construction (side and upper walls composed of steel-tube panels with welded fins); original design specifications for brown coal combustion. The boiler’s operational configuration and schematic diagram are presented in Figure 1. This experimental setup was selected to provide realistic combustion conditions, while allowing for controlled observation of solid waste combustion characteristics in a conventional boiler system originally designed for coal firing.

The KSVR-430 boiler operates through a systematic combustion process where hot combustion gases exit the firebox through the upper combustion chamber before entering the convective sections. These convective heat exchange elements are arranged in two distinct blocks, with flue gases ultimately exiting at the rear top section of the boiler assembly. The system incorporates multiple design features to ensure stable operation. Precise hydraulic calculations prevent localized overheating of heating surfaces, while dedicated drains and air vents maintain system integrity by removing sludge and trapped air from the piping network. Combustion control is achieved through a forced-air supply system that delivers primary air through grate openings from an underlying air box, with combustion rates regulated by adjusting both the quantity and feed rate of fuel. The boiler handles combustion residues efficiently, collecting ash and slag in the bottom ash pit while directing flue gases through the convective pass before final discharge via the stack. An induced draft fan may be employed when necessary to overcome system flow resistance. This operational configuration, combined with manual fuel feeding and adjustable combustion parameters, makes the unit particularly well suited for research involving solid fuels of variable or uncertain composition. The boiler’s nominal operating parameters are detailed in

Table 1. Nominal parameters of the hot water boiler [21].

Parameter	Unit	Value
Nominal thermal power	kW	400
Fuel consumption	kg/h	36.8
Estimated boiler efficiency	%	85
Boiler furnace volume	m3	25.23
Water consumption (max.)	t/h	4.5

.

The core objective of this investigation focused on achieving stable combustion of solid waste samples in a fixed-bed configuration on the grate of an operational hot water boiler. For comprehensive emissions analysis, researchers employed a Testo-350 portable industrial gas analyzer to monitor exhaust gas composition throughout the trials.

The experimental work successfully accomplished several critical tasks. First, stable combustion conditions were established for solid waste samples in the layer combustion mode on the boiler grate. Simultaneously, continuous monitoring of flue gas composition parameters was conducted through a specially installed sampling port in the chimney. The municipal solid waste samples, sourced from Nur-Sultan city, consisted of two compacted cubes, each with a volume between 1.5 and 2 cubic meters. These prepared samples provided representative material for the combustion tests. The photos of samples are presented in Figure 2.

Technical specifications of the Testo-350 gas analyzer, which served as the primary instrumentation for emissions measurement throughout the study. This experimental approach enabled rigorous evaluation of combustion performance while maintaining conditions representative of actual boiler operation. The methodology proved particularly effective for assessing the combustion characteristics of heterogeneous solid waste materials in a conventional boiler system originally designed for coal firing.

To maintain optimal combustion conditions, operational adjustments were implemented as needed. The smoke exhauster was engaged at 4–5 min intervals when required to stabilize and intensify the combustion process. After achieving steady-state combustion, the fuel bed underwent manual mixing (“stirring”) with periodic addition of fresh MSW batches. Throughout all experiments, standard boiler instrumentation continuously monitored operating parameters while parallel exhaust gas composition measurements were conducted. The study did not include measurements of temperatures in the boiler furnace. The coal specifications and calorific values used in preliminary tests are presented in

Table 2. Brown coal composition [22].

Type	%	LHV, kJ/kg
Moisture, W	16.0
Ash, A	30.2
Sulfur, S	0.8	16,929
Carbon, C	44.7
Hydrogen, H	2.9
Nitrogen, N	0.6
Oxygen, O	4.8

, while measurement equipment specifications and associated error margins appear in

Table 3. Equipment and error margin.

Equipment	Absolute Error	Relative Error σ, %	Measured Values
Flowmeter Vzlyot PRC, Manufacturer: Vzlyot, Saint Petersburg, Russia [23]	±1.5% from 1.0 to 20 m3/s	0.2	0–5.1 m3/h
Laboratory thermometers, Manufacturer: LAB international, Almaty, Kazakhstan [20]	±1.0 °C	0.16	0–100 °C
Thermal imager Testo 880, Manufacturer: Testo SE & Co., Titisee-Neustadt, Germany [24]	<0.1 °C at 30 °C	0.4	0–143 °C
Gas analyzer Testo 350, Manufacturer: Testo SE & Co., Titisee-Neustadt, Germany [25]	NOx: abs. ± 2 ppm with measured values from 0 to 39.9 ppm; from 40 ppm ±5% of measured value	5	NOx: 0–120 ppm; NO: 0–105 ppm; CO2: 0–7.28%; SO2: 0–121 ppm.
	CO: ±10 ppm	1	CO: 0–2510
	Temperature: ±1.0 °C	0.16	Temperature: 20–185 °C

.

The efficiency of a boiler was calculated using the formula [26]

$$Eff=100-\left[\left[{\displaystyle \frac{K_{net}x\left(FT-AT\right)}{{CO}_2}}\right]+\left[{\displaystyle \frac{Xx\left(210+2.1xFT-4.2xAT\right)}{Q_{gr}x1000}}\right]+\left[{\displaystyle \frac{K_{1}x{Q}_{gr}xCO}{Q_{net}x{CO}_2+CO}}\right]\right]$$

(1)

where FT—flue gas temperature; AT—ambient temperature;

• Knet, K₁—fuel specific factors (according to table in [21]);
• X = M + 9H; M, H—fuel specific factors (according to table in [21]);
• Q_gr—LHV of the fuel.

During the experimental studies, the ambient room temperature was maintained at 20 °C. To reduce measurement error and improve repeatability, an analysis of the standard deviation and standard error of the obtained data was conducted. The results of these calculations are presented in

Table 4. Standard deviation.

Parameter	Standard Deviation, σ	Standard Error
O2, %	1.15	±0.01
CO, ppm	228	±1.43
NOx, ppm	18.97	±0.12
CO2, %	1.12	±0.01
SO2, ppm	7.28	±0.05
Temperature, °C	12.51	±0.93

. Additionally, high-precision calibrated instruments were used throughout the experiment to minimize experimental errors.

3. Results

Prior to combustion trials, the municipal solid waste (MSW) samples underwent thorough processing and sorting. Researchers carefully removed non-combustible components and sorting residues from the original compressed cubes, including glass, ceramics, stones, and other incompatible materials. The prepared waste was then divided into uniform batches weighing 10–15 kg each for controlled combustion experiments.

Initial verification tests involved loading a single MSW portion onto a thin burning coal layer to confirm stable combustion characteristics. Following successful verification, subsequent trials proceeded with pure MSW combustion without coal admixture. The complete waste composition analyzed in these tests is detailed in

Table 5. MSW composition.

Type	Mass	%	LHV, kJ/kg
Paper/cardboard	3.91	20.99
Textiles/rags	3,41	18.30
Plastic	4.47	23.99	17,962
Organic waste	4.70	25.23
Ceramics	1.23	6.60
Metal	0.57	3.06
Rubber	0.08	0.43
Wood	0.26	1.40

.

3.1. Concentrations of Toxic Substances in Exhaust Gases

Figure 3a presents the temporal evolution of oxygen concentration in exhaust gases during the initial 800 s of measurement. For MSW combustion, oxygen concentrations remained stable within range of 15.26–18.08%, with a gradual increase observed as waste residues burned out. The maximum recorded oxygen level reached 18.08% at 780–785 s. In contrast, coal combustion showed more dynamic behavior, beginning near 20% oxygen concentration before steadily declining to 14.41% as fuel consumption progressed. The coal profile peaked earlier at 19.01% within the first 5–10 s. These oxygen profiles demonstrate efficient combustion for both fuel types, though with characteristically different temporal patterns.

The sulfur dioxide emission characteristics, shown in Figure 3b, reveal fundamental differences between the fuel sources. Coal combustion produced steadily increasing SO₂ concentrations that inversely correlated with decreasing oxygen levels, reaching maximum values at 800 s. This trend reflects the substantial sulfur content (2–3%) typical in coal [27]. Conversely, MSW combustion maintained consistently low SO₂ levels (14–38 ppm) throughout the measurement period, attributable to the waste’s minimal sulfur content (≤0.66%). The highest sulfur contribution in MSW originates primarily from plastic components, resulting in significantly lower emissions compared to coal—a finding consistent with previous research [27].

Figure 4 illustrates the temporal profiles of NO and NOx emissions during combustion, revealing distinct patterns that vary significantly between fuel types. The concentration of nitrogen oxides exhibits complex dependence on several critical factors including combustion temperature, gas residence time in the high-temperature zone, and fundamental fuel characteristics. These relationships are particularly evident in the comparative analysis of different fuel combustion scenarios. The formation of nitrogen oxides occurs through multiple well-established mechanisms, with thermal NO formation playing a predominant role in these combustion conditions. As documented in reference [28], thermal NO develops through slow high-temperature reactions where molecular nitrogen oxidizes in the combustion zone. This process demonstrates strong temperature dependence, with NOx generation rates increasing substantially as temperatures rise above threshold levels. The experimental data clearly show this relationship, with measured NOx concentrations rising progressively as residence times in the high-temperature zone increase. The comparative analysis between fuel types reveals characteristic differences in emission profiles. Coal combustion typically produces higher initial NOx concentrations that stabilize over time, while MSW combustion shows more gradual NOx formation patterns. These variations reflect fundamental differences in fuel nitrogen content and combustion characteristics. The data further demonstrate how extended residence times lead to increased NOx accumulation, consistent with established combustion theory regarding thermal NO formation mechanisms.

The experimental results provide valuable quantitative insights into NOx formation dynamics under practical combustion conditions. Of particular significance is the clear temperature dependence observed across all test conditions, confirming the predominance of thermal NO mechanisms in these high-temperature environments. These findings have important implications for both combustion system design and emissions control strategies. The main equations of Thermal NO is as follows:

N₂ + O = NO + N

(2)

N + O₂ = NO + O

(3)

N + OH = NO + H

(4)

Prompt NO, which is based mainly on chemical reactions of carbon fuels. The formation of Prompt NO is based on CHn radicals and N₂ radicals. Moreover, they react in a fairly narrow reaction zone:

CH + N₂ = NCN + H

(5)

NCN + O = NO + CN

(6)

CN + O₂ = NO + CO

(7)

NCO + O = NO + CO

(8)

While multiple NOx formation mechanisms exist beyond those previously discussed, our current analysis focuses on the observed emission patterns. Figure 4a clearly demonstrates significant differences in NOx concentrations between municipal solid waste (MSW) and coal combustion throughout the measurement period. The MSW combustion exhibited consistently elevated NOx levels, maintaining concentrations above 40 ppm at all measurement points. The emissions profile showed dynamic variation, beginning at 40 ppm (0 s), peaking sharply at 106.7 ppm (200–205 s), then gradually declining. In contrast, coal combustion produced substantially lower emissions, with initial concentrations as low as 5.3 ppm that gradually increased to a maximum of 62.3 ppm before stabilizing after approximately 230 s. These findings reveal that MSW combustion generates significantly higher NOx emissions compared to conventional coal fuel. While the precise mechanisms driving this difference require further investigation, we hypothesize two primary contributing factors: First, MSW likely contains higher proportions of nitrogen-containing compounds in its composition. Second, the slower combustion kinetics characteristic of waste materials may prolong nitrogen oxide formation pathways, resulting in elevated emissions. The stabilization of NOx concentrations in coal combustion after 230 s suggests achievement of equilibrium conditions, whereas the more dynamic MSW profile indicates complex, ongoing reactions throughout the combustion process. These observations have important implications for both environmental impact assessments and combustion system optimization when utilizing alternative fuels.

Figure 4b demonstrates that nitric oxide (NO) represents the dominant fraction of total NOx emissions in the exhaust gases. As established earlier, NO formation occurs primarily through thermal and prompt mechanisms, with distinct temporal patterns observed between waste and conventional fuel combustion. For municipal solid waste (MSW), the NO concentration profile exhibits a characteristic trend: concentrations initially rise with increasing combustion time, reach a maximum, and subsequently decline as combustion progresses. This behavior reflects the specific combustion dynamics of heterogeneous waste materials. The initial increase corresponds to rising temperatures that promote thermal NO formation, while the subsequent decrease results from the depletion of combustible material and gradual flame extinction (the study did not include measurements of temperatures in the boiler furnace). In contrast, coal combustion shows a different pattern, with NO concentration stabilizing at a plateau after reaching their maximum value. This plateau indicates that thermal equilibrium has been achieved, where further combustion produces minimal changes in NO emissions. The maximum NO concentration for coal combustion reaches 63 ppm at 800 s, while initial emissions remain low (4–5 ppm). The stable plateau suggests that, unlike MSW, coal combustion maintains consistent conditions without significant influence from additional air injection or fuel replenishment. These differences in emission profiles arise from fundamental variations in fuel properties and combustion behavior. MSW, being a heterogeneous and nitrogen-rich fuel, undergoes more complex combustion kinetics, leading to dynamic NO formation and decline. Coal, with its more uniform composition, achieves steady-state combustion conditions, resulting in stable NO emissions.

The findings underscore the importance of considering fuel-specific NOx formation mechanisms when evaluating emissions from different combustion sources. The distinct behavior of MSW compared to traditional fuels highlights the need for tailored emission control strategies in waste-to-energy applications.

Figure 5a presents the temporal evolution of CO concentrations during combustion. Both fuel types exhibit generally decreasing CO trends, though with distinct characteristics. Coal combustion produces significantly higher CO emissions, peaking at 2510 ppm within the first 30 s before declining to 342 ppm by 800 s. This pattern reflects suboptimal air-fuel mixing in the grate system, where limited oxygen availability initially promotes incomplete combustion. The gradual decrease correlates with increasing gas residence time, allowing more complete oxidation.

MSW combustion demonstrates more complex behavior, with CO concentrations constrained below 1511 ppm throughout the process. Notably, CO levels increase around 240 s despite rising oxygen concentrations, suggesting combustion challenges specific to waste materials. This phenomenon likely stems from the heterogeneous nature of MSW, where varying material properties create localized zones of incomplete combustion despite adequate overall oxygen supply. The minimum observed CO concentration of 607 ppm at 180 s indicates transient periods of more efficient oxidation.

The CO₂ emission profiles shown in Figure 5b reveal fundamentally different combustion dynamics between fuel types. Coal combustion shows characteristic progression from 1.65% CO₂ at 35 s to a stable plateau, consistent with complete combustion achieved after initial startup. This plateau formation coincides with decreasing oxygen levels and CO oxidation, indicating system equilibrium.

Figure 6a shows the dependence of exhaust gas temperature on time during the combustion of municipal solid waste (MSW) and coal. As can be seen from the figure, the temperature profiles for both fuels generally exhibit similar values, remaining approximately within the range of 140–160 °C. The temperature in the boiler furnace depends on the heat of combustion and the combustion mode. For coal, the maximum temperature is reached during the initial 0–5 s, due to the onset of combustion. The temperature then decreases, reaching a minimum of 146.9 °C at 75 s. Subsequently, it rises steadily, reaching a plateau at around 460 s. In the case of MSW, the temperature gradually decreases from 160 °C to 135 °C over a period of 800 s. In general, oxygen analysis indicates a slow extinction of the flame, which contributes to the decline in exhaust gas temperature. From the analysis of the temperature and other related graphs, it is likely that a slow smoldering of individual waste components is occurring.

Figure 6b illustrates the dependence of boiler efficiency on combustion time, calculated using Formula 1. From the graphs, it is evident that the efficiency of the coal-fired boiler is more stable, with a maximum value of 86.6%. Notably, after about 100 s, the efficiency reaches a plateau, with minimal variation thereafter. During MSW combustion, a gradual decline in efficiency is observed. The maximum efficiency, around 84%, occurs at the initial stage, followed by a smooth decrease over time. This trend is also reflected in the CO concentration and other related graphs. An increase in CO levels indicates incomplete fuel combustion, which in turn leads to a reduction in both temperature and boiler efficiency.

Figure 7 illustrates the dependence of unburned hydrocarbon (CH) concentration on combustion time. As shown in the graphs, coal combustion results in a significant increase in unburned hydrocarbons. This can be attributed to flame ignition and incomplete combustion due to insufficient mixing of the coal layer with air. The maximum CH concentration is observed at 10 s, reaching 1874 ppm. This is followed by a sharp decrease, then a secondary rise to 1175 ppm at 55 s. Afterward, there is a steady decline to a minimum value of 31 ppm at 140 s. Such a pattern is typical during the ignition phase of boiler operation.

In contrast, MSW combustion does not show such a sharp initial increase. The maximum CH concentration is recorded during the first few seconds at 567 ppm, followed by fluctuations over time. The minimum concentration of CH, 0 ppm, is observed at 90 s.

3.2. Thermal Imaging of the Boiler Surface

Figure 8 presents a thermal imaging survey of the boiler surface during operation on waste fuel. The purpose of the survey was to detect thermal anomalies associated with the use of non-standard fuel and to analyze heat losses through the boiler surfaces. Figure 8a shows a thermal image of the fuel loading door. Overall, the surface temperature is within the expected range for this boiler, reaching up to 142 °C in certain areas. The upper section above the door is well-insulated and its temperature is close to that of the surrounding environment. Figure 8b displays a thermal image of the firebox door equipped with a gas analyzer (note: measurement results from the firebox are not provided in the article). The thermal image reveals no significant temperature increase. A localized hot spot on the door reaches 69 °C, while the highest temperature is observed on the gas analyzer tube. Figure 8c shows the thermal image of the boiler’s side. No major heat loss zones were identified in this area. Figure 8d depicts the longitudinal side of the boiler. In this image, notable heating is visible on the side surface, indicating significant heat loss to the surrounding environment.

3.3. Results Discussion

As a result of the experimental studies conducted, temperature graphs were obtained. During waste combustion, the exhaust gas temperature reached 164 °C, while for brown coal, it reached 185 °C. Similar studies were conducted by Zuo et al. [29], in which, at the start of combustion, the temperature for MSW reached 334 °C, and for anthracite 551 °C. The results presented by Shin et al. [30] showed that during the co-combustion of coal and livestock waste fuel, the temperature at the boiler outlet decreased to 140 °C, while the furnace temperature reached 800 °C. The CO₂ concentrations reported by Shin et al. [30] ranged from 8.5% to 10%, depending on the proportion of waste fuel. However, the O₂ concentrations were significantly lower than in the present study. For instance, in this study, the measured O₂ concentration in the exhaust gases ranged from 14% to 20%, whereas in Shin et al. [30], it did not exceed 12%. Similarly, the NO concentrations obtained in this study were 106 ppm for MSW and 67 ppm for brown coal. In contrast, Shin et al. [30] reported concentrations exceeding 250 ppm at the maximum trash-to-coal ratio. Muthuraman et al. [31] conducted experiments by adding MSW to Indonesian coal, and the exhaust gas temperatures in their incinerator were in the range of 250–300 °C.

The analysis of boiler efficiency showed that during MSW combustion, the efficiency averaged 77%, while for coal it was 83%. In the study by Zhou et al. [32], efficiencies ranged from 50% to 73%. The SO₂ concentrations during coal combustion ranged from 0 to 122 ppm, with an average of 91 ppm, whereas for MSW combustion, the average concentration was 39 ppm. In the work of Shim et al. [33], SO₂ concentrations during coal combustion reached 180 ppm. CO₂ concentrations in the flue gases reached 5.82% for MSW and 7.28% for brown coal in this study. By comparison, in the work of Wu et al. [34], the CO₂ concentration reached 19.9% during the combustion of sewage sludge mixed with MSW.

4. Summary

In the present study, the combustion of municipal solid waste (MSW) in a grate furnace of a 400 kW hot water boiler was investigated. The waste used for combustion was pre-sorted to remove various non-combustible materials.

The analysis of oxygen concentration during combustion shows that the maximum O₂ levels during municipal solid waste (MSW) combustion ranged from 18.3% to 18.5%, while for coal combustion, the maximum was approximately 17.9%. This indicates generally higher oxygen consumption during coal combustion. Experimental data show that during coal combustion, the oxygen concentration drops to 14.92% at 110 s and then gradually decreases further. This behavior is attributed to the burnout of fuel in the combustion layer. In contrast, the oxygen concentration during MSW combustion remains relatively stable, between 15.09% and 15.48%, up to 155–160 s, after which it begins to increase. This suggests that oxygen is poorly mixed with the MSW layer.

The analysis of sulfur dioxide (SO₂) concentrations in the exhaust gases revealed significantly higher levels during coal combustion compared to municipal solid waste (MSW) combustion. While SO₂ concentrations during MSW combustion peaked at 39 ppm, coal combustion produced levels as high as 117 ppm. This difference is primarily attributed to the higher sulfur content in coal. Experimental data showed that during coal combustion, SO₂ concentrations did not exceed 5 ppm in the first 45 s, followed by a sharp increase, eventually reaching 122 ppm over the course of the combustion process. In contrast, during MSW combustion, the SO₂ concentration peaked between 40 and 50 s and then gradually decreased. This behavior is explained by the burnout of sulfur-containing components, particularly plastics and rubber. Due to the relatively small amount of these materials in MSW, the resulting SO₂ concentrations remain comparatively low.

The analysis of nitrogen oxides (NOx and NO) showed that MSW combustion generates considerably higher levels of nitrogen oxides compared to coal combustion. The maximum NOx concentration during coal combustion was 67.5 ppm, while MSW combustion reached 106.7 ppm. These levels are relatively high from an emissions standpoint. Further analysis indicated that nitric oxide (NO) is the dominant component of total NOx emissions. The data show that during MSW combustion, the initial stage is characterized by higher temperatures, which contribute to increased NOx formation via the thermal mechanism. As the waste burns out, the temperature gradually decreases, resulting in a corresponding decline in both NOx and NO concentrations. In contrast, during the combustion of conventional fuel—brown coal—the temperature remains relatively stable. As a result, the concentrations of NOx and NO also remain nearly constant throughout the entire measurement period.

Carbon monoxide (CO) concentrations in the exhaust gases were significantly higher during the early stages of coal combustion, likely due to insufficient mixing of coal with air on the grate. The maximum CO concentration during coal combustion reached 2510 ppm at 30 s and gradually decreased to 342 ppm by 800 s. In contrast, during MSW combustion, CO concentrations remained relatively low and showed a slight increase around 240 s. Interestingly, even as the oxygen concentration increased, the CO concentration also rose, suggesting a more complex and less efficient combustion process. It is well known that CO concentrations are largely influenced by the completeness of combustion. According to the data, the initial stage of MSW combustion is characterized by higher temperatures, which promote more complete combustion of waste components. This results in lower CO levels and higher CO₂ concentrations in the exhaust gases. In the case of coal combustion, the initial temperature is lower, indicating incomplete combustion. This leads to elevated CO concentrations and reduced CO₂ levels. However, after approximately 150–200 s, the combustion process becomes more complete, resulting in a decline in CO concentrations and a corresponding increase in CO₂ levels in the exhaust gases.

Temperature analysis of the exhaust gases showed minor differences between the two fuel types, as the system was monitored and adjusted to maintain a target temperature. The maximum exhaust gas temperature recorded was 160 °C. Over time, the temperature decreased during MSW combustion, while it gradually increased during coal combustion. Measurements indicate that the temperature during the initial stages of MSW combustion is higher than that observed during brown coal combustion. This difference influences the overall combustion dynamics. The decrease in temperature during MSW combustion is explained by the early burnout of the more combustible components of the waste. In contrast, during coal combustion, the temperature rises gradually as the fuel burns more steadily over time.

The efficiency of the boiler depends on several factors, with the temperature of the exhaust gases and heat losses to the environment being particularly significant. Based on the analysis of the obtained data, the efficiency of the boiler during MSW (garbage) combustion decreases as the waste burns out. At the initial stage (0–100 s), the efficiency ranges from 83.2% to 83.7%, but it gradually declines, reaching 66% by 770 s. In contrast, during brown coal combustion, the boiler efficiency increases in the early stage to 84–86% and remains nearly constant throughout the combustion process.