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Article

Effect of Adding Halloysite to Different Solid Biomass Fuels on Combustion Process in a Small-Scale Domestic Retort Boiler

by
Michał Chabiński
* and
Andrzej Szlęk
*
Department of Thermal Technology, Faculty of Energy and Environmental Engineering, Silesian University of Technology, 44-100 Gliwice, Poland
*
Authors to whom correspondence should be addressed.
Energies 2024, 17(24), 6287; https://doi.org/10.3390/en17246287
Submission received: 28 October 2024 / Revised: 9 December 2024 / Accepted: 11 December 2024 / Published: 13 December 2024
(This article belongs to the Special Issue Combustion of Alternative Fuel Blends)
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Abstract

:
Biomass combustion in small-scale boilers in Eastern Europe has recently become a very popular heating option. Biomass boilers are gradually replacing old, coal-fired installations, especially in the domestic sector. In comparison with coal, biomass contains more phosphorus, chlorine, and potassium, which may cause the corrosion, slagging, and fouling of heating surfaces inside the combustion chamber. Such problems may be reduced by properly controlling the combustion process, as well as adding substances like halloysite to the fuel. This paper presents the results of adding halloysite to wood pellets made of coniferous wood, rape straw, and wood/rape blend in the combustion process of a 25 kW retort boiler. The results demonstrate that adding halloysite to biomass increases the ash sintering temperature, which may, in turn, reduce slagging. The addition of halloysite also reduces the KCl concentration in the ash and the total solid compounds, potentially lowering the risk of corrosion in the boiler. A slight reduction in CO, OGC, and SO2 concentrations was observed for rape straw biomass pellets with the halloysite addition. Moreover, the experimental results indicate that the addition of halloysite to fuel may influence boiler efficiency, especially during the combustion process of agricultural biomass and its blends.

1. Introduction

The combustion of biomass pellets in domestic boilers across Europe offers an interesting and ecologically justified alternative to fossil fuels such as natural gas, oil, and coal. The increase in the number of such installations is primarily driven by the growing need to reduce carbon dioxide (CO2) emissions in the domestic sector. Recently, the most popular devices of this kind have been boilers fueled with biomass pellets. As the number of these boilers increases, more biomass fuel is needed. In some areas, this requires the use of lower-quality biomass as higher-quality sources become scarce. More often, annual crops and bio-wastes of various origins are processed into pellet fuel, despite their negative effects on boiler operation. Pellets made from different types of straw, such as rapeseed or cereal, contain higher levels of chlorine (Cl), potassium (K), and sulfur (S) than wood pellets. These can create in-furnace conditions that promote the corrosion of heating surfaces and slagging and lead to poor boiler performance and energy losses, both in unburned fuel and in flue gases [1,2]. This, in turn, may also increase heating costs or, in the worst case, damage the boiler.
The addition of halloysite, a mineral with high-temperature resistance and the ability to bind potassium, offers a potential solution, mitigating corrosion, slagging, and other performance-related issues during biomass combustion. Halloysite’s chemical composition (Al2Si2O5(OH)4·nH2O) is similar to that of kaolinite but features higher reactivity. As halloysite mainly consists of silica (Si) and aluminum (Al), it can potentially bind with K, reducing potassium chloride (KCl) formation. The addition of halloysite has been reported to reduce slagging in larger combustion installations [3,4].
There are numerous publications focusing on the combustion of various types of biomass pellets in small-scale domestic boilers. Most of these, including [5], note that the combustion of agricultural pellets can also result in lower efficiency and higher carbon monoxide (CO) emissions compared to wood pellets. They also emphasize that using such pellets may require adjustments to the boiler’s controller settings or even modifications to its construction. In Ref. [6], the authors investigated the efficiency, pollutant emissions, and slagging problems in a domestic boiler fueled with different biomass pellets. The study mentions that adding halloysite to fuel can improve boiler performance. However, the article again primarily refers to halloysite addition in large combustion installations, and halloysite itself is not added to the pellets in this specific study. In Ref. [7], biomass pellets from fen paludicultures with the addition of kaolin were burned in a small-capacity boiler, which influenced a reduction in CO and the total solid particle (TSP) emissions in the flue gas. Still, none of these studies have focused directly on the comparison of combustion of pellets with and without halloysite in small-capacity boilers.
In Ref. [4], the effect of adding halloysite to a boiler firing agricultural biomass operation was examined. The study showed that halloysite reduced problems connected to slagging in an OP150 boiler. These large-scale tests have also shown the ability of halloysite to enhance the operating features of furnaces firing various kinds of biomass. The combustion of pellets in small installations differs significantly from that in large-scale boilers. However, the findings on using halloysite in large-scale measurements suggest the possibility of enhancing the operating features of smaller boilers. This led the authors to conclude that such an approach may be innovative.
The aim of this research was to investigate how adding halloysite to biomass fuels, particularly wood and rape straw pellets, affects the combustion process and performance of small-scale biomass boilers. Specifically, it seeks to understand whether halloysite can reduce problems commonly associated with biomass combustion, such as slagging and corrosion caused by K and Cl. This study also evaluates how adding halloysite to pellets impacts flue gas composition, ash properties, and boiler efficiency.

2. Materials and Methods

The measurements were conducted according to the EN 303-5 standard. This standard is widely used in Europe for testing the performance of solid-fuel domestic boilers. The test area was prepared according to the EN 303-6 standard, which provides guidelines for testing and evaluating the performance of small-capacity solid fuel boilers [8,9]. As for pellet quality and specifications, there are several European standards (EN 14961 [10] and 17225 [11]); none of these standards provide guidelines for pellet production. Therefore, the pellets prepared in this study followed guidelines from the available literature [12,13].

2.1. Fuel Preparation

Wood, rape straw, and wood/rape blend pellets were selected as the fuel for the combustion process in this study. The wood and straw for the pellets were collected from forests and fields in the Silesia region of Poland. After collection, the material was dried to a moisture content of 14–16% and ground into a fine dust suitable for pelletizing. Halloysite was added directly to the dust before loading it into a pelletizer. The mixture of dust and halloysite was prepared in an additional tank where 9.5 kg of biomass dust was mixed with 0.5 kg of halloysite. The 5 wt% halloysite addition was chosen based on preliminary tests and the literature considering large combustion installations and laboratory experiments [14,15].
The pelletizer used for the process was driven by an electrical motor of 7 kW and a maximum production capacity equal to 70 kg/h. As a result of this work, pellets of 6 mm in diameter and 15 to 25 mm in length were received, as shown in Figure 1.
The final pellet samples prepared during the pelletizing process were as follows:
  • One-hundred-percent coniferous wood pellets, labeled as WP.
  • One-hundred-percent rape straw pellets, labeled as RP.
  • Mixed (50 wt%) coniferous/(50% wt%) rape pellet dust, labeled as MIX.
  • One-hundred-percent coniferous wood pellets with halloysite, labeled as WPh.
  • One-hundred-percent rape straw pellets with halloysite, labeled as RPh.
  • Mixed, (50 wt%) coniferous/(50% wt%) rape straw pellets with halloysite, labeled as MIXh.
In the case of color and other physical properties, no differences were observed between pellets with and without the addition of halloysite. Chemical analysis of six types of pellets was conducted by an external laboratory. Standards used during analyses are listed below.
  • EN ISO 18134—Moisture analysis [16].
  • EN ISO 18123—Volatile matter analysis [17].
  • EN ISO 18122—Ash analysis [18].
  • EN ISO 16948—Carbon (C), hydrogen (H), nitrogen (N) [19].
  • EN ISO 16994—S, Cl analysis [20].
  • EN ISO 16967—K, Al, iron (Fe), sodium (Na), magnesium (Mg), calcium (Ca) analysis [21].
  • EN ISO 18125—Higher heating value measurement [22].
The analysis, shown in Table 1, indicates differences between coniferous wood and rape pellets. These differences included higher ash content, lower C content (which also influences the high heating value), significantly higher S content, and higher volatile matter content in the rape pellets compared to the wood pellets.
Rape straw pellets also contained significantly higher amounts of K and Cl compared to wood pellets. Additionally, the Al, Fe, and Mg contents were noticeably higher in rape straw pellets. The data in Table 1 show that adding halloysite to biomass or biomass blends did not significantly alter their physicochemical properties. This was because the pelletizing temperature, which reached a maximum of 100 °C (with the type of pelletizer used), was not high enough to promote a reaction between the chemical elements of the biomass. However, the pelletizing process reduced the moisture content of the biomass pellets by 5 to 7% in comparison with the biomass dust.

2.2. Experimental Setup

Experiments were conducted using a small-capacity solid multi-fuel boiler with a nominal power of 25 kW (Figure 2), equipped with a screw feeder and retort burner. Both the air and fuel streams delivered to the combustion chamber were controlled by an electronic controller.
The concentrations of combustion products in the flue gases were measured by the following analyzers:
  • Sick-Maihak N710, Maihak AG, Hamburg, Germany—oxygen (O2) concentration in flue gas.
  • NGA2000, Fisher-Rosemount GmbH & Co., Hasselroth, Germany—concentration of CO, nitrogen oxides (NOx) and sulfur dioxide (SO2) in flue gas.
  • Sick-Maihak MODEL 3006, Maihak AG, Hamburg, Germany—concentration of organic compounds (OGC) as C3H6 in flue gas.
  • ZS10, ZAM, Kęty, Poland—concentration of TSP in flue gas.
The experimental setup is shown in Figure 3. The temperatures and heat output of the boiler were measured by a SharkyHeat773 heat meter (Diehl Metering, Nuremberg, Germany) connected to a PC with visualization software. This system recorded the water flow and temperatures of circulating water, allowing for the calculation of the boiler’s power output. Data from the analyzers were recorded by a data logger and stored on the PC. The fuel stream for different biomass pellets was measured before firing the boiler. The fuel loader was operated for one hour, after which, the pellets were collected and weighed using a laboratory scale.

2.3. Measurements

The nominal heat output of the boiler was regulated by adjusting the fuel stream and the cooling water flow in the heat exchanger. The screw loader speed was modified for every type of pellet to achieve the nominal heat output of the boiler. The air excess ratio was set to 1.6 (around 8% of O2 in flue gas) by adjusting the air fan speed. This air excess ratio was optimal in terms of efficiency and emissions, as verified through preliminary measurements. Such an air excess ratio is also recommended in the operation manual for this specific device. Once the boiler reached its nominal heat output and stabilized circulating water temperatures, emissions and heat output were automatically registered every 60 s. Efficiency was calculated using the direct method from the following formula:
η = Q u E c h × 100
where η —efficiency [%]; Qu—heat utilized measured by heat meter [kJ]; Ech—chemical energy supplied, calculated from fuel stream and its calorific value [kJ].
Each test took 6 h. All tests were performed twice (for a single type of biomass pellet) to assess reproducibility. After each test, ash was collected from the ash drawer beneath the burner. After this procedure, the combustion chamber of the boiler was cleaned to remove dust from the walls, which could influence subsequent tests. The KCl content in the bottom ash, as well as the KCl content in the TSP, was analyzed by an external laboratory. Additionally, the sintering temperature of the bottom ash was analyzed, as it may influence slagging intensity according to the literature [23].

3. Results

The composition of flue gases and the main operation parameters of the boiler for different types of biomass pellets are shown in Table 2. The CO concentration in flue gas was highest for rape pellets and a mix of rape and wood pellets, with significantly lower values observed for pellets with halloysite additions. For the four types of pellets (RP, RPh, MIX, and MIXh), higher values of TSP were observed compared to wood pellets (WP and WPh). Additionally, higher concentrations of OGC, NOx, and SO2 were recorded in all cases compared to WP. However, for pellets with 5 wt% halloysite addition, the SO2 concentration was lower than in the case of pellets without halloysite. For WP and WPh, the differences in the concentration of various substances in the flue gas, as well as efficiency, were insignificant. Boiler efficiency was lower for RP, RPh, MIX, and MIXh, but within this group, RPh and MIXh exhibited higher thermal efficiency than the pellets without halloysite.
The parameters of bottom ash and TSP are shown in Table 3. To determine KCl concentration in TSP and bottom ash, the ISO 2050 standard was used [24]. The sintering temperature was determined in accordance with the EN ISO 21404 standard [25].
The sintering temperature was higher for the ash from pellets with halloysite additions. Compared to WP, WPh, and MIXh, KCl content was higher in the case of RP, RPh, and MIX. However, for rape pellets, the KCl content was lower for those with halloysite addition. For the bottom ash of wood pellets and all pellets with halloysite addition, slags were observed only in cases of RPh and MIXh. In these instances, the slags had diameters of 3–8 mm and 1–5 mm, respectively. Larger slags with diameters ranging from 4 to 11 mm were observed in the case of MIX pellets, and even larger slags, ranging from 10 to 27 mm, were found in the case of rape pellets (RPs).

4. Discussion

Section 4.1 and Section 4.2 present the flue gas composition and ash composition for biomass pellets and biomass pellet blends, with and without the addition of halloysite.

4.1. Flue Gas Composition

Pellets made from rape straw were the fuel that produced the highest concentration of all measured flue gas components, as seen in Figure 4. NOx and SO2 emissions were higher for RP, RPh, MIX, and MIXh than for WP and WPh.
The concentration of NOx is related to the amount of nitrogen in fuel and the temperature in the combustion chamber (thermal NOx). However, for thermal NOx formation, the temperature must rise above 1400 °C [26], which is rarely achieved in small boilers. That is why, in the case of these measurements, NOx in flue gas was mainly linked to elemental nitrogen. Rape straw pellets and mixed biomass pellets contained the highest levels of nitrogen, which explains the elevated NOx emission from these biomass types. Similarly, SO2 emissions are directly related to sulfur content in the fuel and are formed through sulfur oxidation during the combustion process [27]. Rape straw biomass and its mixes also had the highest sulfur content, which contributed to the higher SO2 emissions.
For NOx, no significant change in emissions was registered for the halloysite addition to biomass. However, for RPh and MIXh pellets, a slight reduction in SO2 concentration was observed compared to the similar pellets without halloysite. The exact cause of this phenomenon is unclear, but under certain conditions, Al and Si in halloysite can capture SO2, leading to the formation of aluminum sulfates or calcium/aluminum/silicate/sulfates. This, in turn, can reduce SO2 concentrations in the flue gas. For example, the formation of aluminum sulfate (Al2(SO4)3) through the reaction of Al2O3 and SO2 can be one such process [28].
TSP emissions are related to combustion conditions, the amount of ash, and its structure. Rape straw and its mixes contained more ash than coniferous wood pellets, which could have contributed to higher TSP emissions. Additionally, rape straw had higher levels of Cl and K that could form KCl during combustion. KCl could also condense into fine particulate matter as the combustion gases cooled. Small changes in TSP concentrations were observed between pellets with and without halloysite addition. The lower concentrations of TSP in WPh, RPh, and MIXh pellets could be the result of Al and Si from halloysite reacting with K, thereby reducing KCl condensation [29].
In the case of CO and OGC, the concentrations of these flue gas components are strongly related to the air excess ratio and the distribution of air within the combustion area [30]. All pellets were burned under similar boiler conditions (heat output and excess air ratio); however, due to slagging in the case of rape straw and its mixes, the proper combustion conditions may have been disrupted. If we relate the slag diameters to CO and OGC concentrations (Figure 5), it can be observed that the more intensive the slagging process, the higher the emissions of CO and OGC were.
For rape pellets and their mixtures with halloysite, the concentration of CO and OGC in the flue gas was lower compared to the same pellets without halloysite. However, the addition of halloysite was not directly linked to the reduced CO and OGC concentrations, as this correlation did not exist in the case of wood pellets. As shown in Section 4.2. halloysite reduced slagging, which in turn improved the air/fuel contact in the combustion chamber and facilitated better CO and OGC oxidation.

4.2. Ash Composition

The slagging of fuel and ash in the combustion chamber is influenced by several factors, including fuel properties, boiler construction, and combustion process parameters. One of the most important factors related to fuel properties is the sintering temperature of ash (St). For biomass fuels like rape straw and wheat straw, this temperature is significantly lower than for wood-based fuels. Rape straw typically has higher K and Na contents compared to wood. During the combustion process, these elements can form compounds such as K2CO3 and Na2CO3, which have relatively low melting points (around 760 °C for potassium carbonate) [31]. Since the temperatures near the burner can reach 800–1000 °C, this can lead to sintering and slagging, which are undesirable phenomena in the biomass combustion process [32].
As shown in Figure 6, adding halloysite to the biomass increased the sintering temperature, and, in the case of RP and MIX pellets, it reduced the sintering of ash. Similar observations have been made in large fluidized bed combustion installations [33]. This is because halloysite provides a source of Al and Si, which could react with alkali metals like K and Na in the ash. During the combustion, alkali metals in the biomass could react with Si from the halloysite to form potassium aluminum silicates. These reactions led to the formation of high-melting-point compounds, such as alkali aluminosilicates and potassium aluminosilicate glasses, which have higher melting points, often above 1000 °C. This, in turn, helped prevent the formation of low-melting-point ash compounds that typically cause sintering and slagging [34]. That is why sintering temperature influenced the maximum slag diameter in the bottom ash.
In terms of ash composition, the amount of KCl in TSP and bottom ash was also examined. This is because KCl is known to be a key factor responsible for slagging, sintering, the formation of particulate matter, and ultimately the corrosion of boiler components [35]. Figure 7 presents the amounts of KCl in ash for different biomass pellets, along with the K and Cl concentrations in the fuel. Rape pellets contained higher levels of K and Cl, which directly led to greater KCl formation. Additionally, even after pelletizing, rape straw had a more porous structure and a higher surface area compared to wood. This structure enhanced the release of potassium and chlorine into the ash and gas phase during combustion [36]. The high amounts of KCl in the ash indicated that KCl levels in the flue gases were also elevated. This relationship existed due to the way KCl was formed and volatilized (through high-temperature direct reactions) and recondensed (as the flue gases cooled).
Studying the corrosion process of materials in the combustion chamber of a boiler can be time-consuming, often taking months or even years. Therefore, the KCl concentration in TSP and bottom ash can serve as a useful indicator for assessing the suitability of different solid fuels for specific types of installations. Rape straw biomass pellets may pose a corrosion risk for small domestic boilers. However, as shown in Figure 7, mixing rape straw with wood biomass or adding additives like halloysite to rape biomass can decrease KCl concentrations in ash, TSP, and flue gases.
The reduced KCl content in both the TSP and bottom ash when halloysite was added to rape biomass pellets was likely due to the formation of stable potassium compounds such as potassium aluminosilicate (KAlSi3O8), potassium silicate (K2SiO3), potassium sulfate (K2SO4), and potassium carbonate (K2CO3). These compounds are less volatile than KCl and are more likely to remain in the solid phase, reducing KCl emissions and improving the combustion process [37]. This reduction may help prevent the corrosion of surfaces and the burner in the combustion chamber, ultimately protecting the boiler from damage.
The type of pellets used during the combustion process also influenced the boiler’s efficiency, which is closely linked to the fuel and air parameters within the combustion chamber. As shown in Figure 8, the highest efficiencies were achieved with wood pellets, while the lowest efficiencies were observed with rape pellets.
The explanation for this is that the HHV of RP, RPh, MIX, and MIXh was lower compared to WP and WPh. The measurements in the biomass boiler were conducted according to standard 303-5, with the nominal heat output of the boiler. As a result, the fuel stream for rape straw pellets had to be higher to achieve the same heat output as for pellets made from wood. This, in turn, led to lower efficiency. Furthermore, as mentioned in Ref. [38], slagging and fouling reduce the operational efficiency of coal-fired boilers by forming insulating layers on heat transfer surfaces, leading to increased furnace exit gas temperatures and decreased heat absorption. In Ref. [39], it is highlighted that certain biomass fuels, particularly those with high silicon and alkali metal contents, are more prone to slagging. This increased slagging tendency can lead to operational challenges, including reduced heat transfer efficiency and potential boiler shutdowns for cleaning and maintenance.
However, when halloysite was added, the boiler’s efficiency was higher for RPh and MIXh than for RP and MIX. This improvement in efficiency is again due to reduced slagging when halloysite is added to rape pellets. By limiting slagging in rape pellet mixes with halloysite, better air/fuel contact is achieved, leading to lower amounts of unburnt biomass and reduced chemical losses in CO and OGC. Additionally, less fuel needs to be loaded into the burner inside the combustion chamber to reach the nominal heat output, resulting in higher efficiencies, which was also highlighted in Ref. [4] during experiments conducted on large-scale boilers.

5. Conclusions

The experiments conducted in this study showed that, in the case of a small-capacity retort boiler, the combustion of rape straw pellets and their blends—fuels with high K and Cl contents—can be problematic in terms of emissions and boiler performance. The addition of halloysite offers a potential way to mitigate these issues. Our experiments demonstrated that adding halloysite to fuel may improve ash quality, which, in turn, may influence other important combustion process parameters.
The improved ash properties were achieved through a higher sintering temperature and a reduced concentration of KCl. The higher sintering temperature was attributed to the reduction in the formation of K2CO3 and Na2CO3, which have relatively low melting points. According to the literature, this was possible due to the formation of high-melting-point compounds, such as alkali aluminosilicates, through the reaction of Al and Si from halloysite with K and Na from the biomass. The reduction in KCl concentrations in ash and TSP was made possible by the formation of stable compounds such as KAlSi3O8 and K2SiO3 facilitated by the presence of halloysite. A higher sintering temperature and a reduction in KCl concentrations in ash and TSP influenced the following:
  • Slagging: Slagging was reduced in the case of rape straw pellets and their blends, as the sintering temperature of the ash was increased due to the addition of halloysite.
  • CO and OGC emissions: CO and OGC concentrations in the flue gas were reduced. This occurred due to the reduction during slagging, which improved air/fuel contact during combustion.
  • TSP and SO2 emissions: Both TSP and SO2 concentrations in the flue gas were slightly reduced. The reduction in TSP may be linked to the lower KCl concentrations, as KCl can condense into fine particulate matter when combustion gases cool. For SO2, Al and Si in halloysite can capture SO2 under certain conditions, forming aluminum sulfates or calcium/aluminum/silicate/sulfates, which reduces SO2 concentrations.
  • Efficiency: Rape straw pellets with halloysite addition showed higher efficiency compared to pellets without it. This improvement was due to better air/fuel contact and enhanced heat exchange, achieved through a reduction in slagging and TSP emissions.
  • Boiler corrosion: A lower KCl concentration in bottom ash and TSP can reduce the risk of corrosion. The KCl concentration serves as an indicator of corrosion potential, especially when direct observations of corrosion are not feasible due to the long timescales required for such studies.

Author Contributions

Methodology, M.C.; Investigation, M.C.; Writing—original draft, M.C.; Writing—review & editing, A.S.; Supervision, A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Pellets received from coniferous wood (a), rape straw (b), and wood/rape straw blend (c).
Figure 1. Pellets received from coniferous wood (a), rape straw (b), and wood/rape straw blend (c).
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Figure 2. Small-capacity retort boiler (a) and set of analyzers (b).
Figure 2. Small-capacity retort boiler (a) and set of analyzers (b).
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Figure 3. Experimental setup. Combustion chamber of the boiler with retort burner and screw loader (1), air fan (2), biomass tank (3), electric engine (4), electronic controller (5), heat meter (6), measurement of outlet water temperature (7), measurement of inlet water temperature (8), TSP probe (9), OGC probe (10), CO, SO2, NOx, O2 probe (11), ZAM-KETY ZS10 TSP analyzer (12), Sick-Maihak FID700 OGC analyzer (13), flue gas conditioning system (14), Rosemount NGA2000—CO, SO2, NOx analyzer (15), Sick-Maihak N700—O2 analyzer (16), data logger (17), and PC (18).
Figure 3. Experimental setup. Combustion chamber of the boiler with retort burner and screw loader (1), air fan (2), biomass tank (3), electric engine (4), electronic controller (5), heat meter (6), measurement of outlet water temperature (7), measurement of inlet water temperature (8), TSP probe (9), OGC probe (10), CO, SO2, NOx, O2 probe (11), ZAM-KETY ZS10 TSP analyzer (12), Sick-Maihak FID700 OGC analyzer (13), flue gas conditioning system (14), Rosemount NGA2000—CO, SO2, NOx analyzer (15), Sick-Maihak N700—O2 analyzer (16), data logger (17), and PC (18).
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Figure 4. Flue gas components for different biomass pellets.
Figure 4. Flue gas components for different biomass pellets.
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Figure 5. OGC and CO concentrations in flue gas and maximum slag diameter for combustion of different biomass pellets.
Figure 5. OGC and CO concentrations in flue gas and maximum slag diameter for combustion of different biomass pellets.
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Figure 6. Sintering temperature and maximum slag diameter for the combustion of different biomass pellets.
Figure 6. Sintering temperature and maximum slag diameter for the combustion of different biomass pellets.
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Figure 7. Concentration of KCl in TSP and bottom ash in comparison with K and Cl concentrations in fuel.
Figure 7. Concentration of KCl in TSP and bottom ash in comparison with K and Cl concentrations in fuel.
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Figure 8. Maximum slag diameter and boiler efficiency in different biomass pellets.
Figure 8. Maximum slag diameter and boiler efficiency in different biomass pellets.
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Table 1. Proximate analysis, ultimate analysis, and heating value of the fuel.
Table 1. Proximate analysis, ultimate analysis, and heating value of the fuel.
Parameter and UnitWPWPhRPRPhMIXMIXh
Moisture, Mwb * [%]8.95 ± 0.258.99 ± 0.289.11 ± 0.269.20 ± 0.229.08 ± 0.219.20 ± 0.22
Volatile matter, VMdb [%]76.52 ± 0.1875.83 ± 0.1681.02 ± 0.1879.68 ± 0.2078.44 ± 0.1677.51 ± 0.17
Fixed carbon, FCdb * [%]22.05 ± 0.2421.45 ± 0.2312.84 ± 0.2312.19 ± 0.2318.34 ± 0.2518.13 ± 0.24
Ash, Adb [%]1.43 ± 0.142.72 ± 0.156.14 ± 0.218.13 ± 0.213.22 ± 0.204.36 ± 0.21
Cdaf * [%]50.87 ± 0.4548.88 ± 0.4943.22 ± 0.4241.20 ± 0.4148.01 ± 0.4047.98 ± 0.42
Hdaf [%]6.01 ± 0.125.71 ± 0.125.46 ± 0.185.45 ± 0.195.88 ± 0.205.85 ± 0.21
Ndaf [%]0.75 ± 0.100.76 ± 0.101.25 ± 0.161.25 ± 0.150.99 ± 0.120.93 ± 0.11
Sdaf [%]0.11 ± 0.050.10 ± 0.060.57 ± 0.060.58 ± 0.070.29 ± 0.060.30 ± 0.05
Cldaf [%]0.03 ± 0.0020.03 ± 0.0020.85 ± 0.040.83 ± 0.040.51 ± 0.030.54 ± 0.03
Odaf [%] **42.13 ± 0.3342.06 ± 0.3248.65 ± 0.3548.69 ± 0.3244.33 ± 0.3344.40 ± 0.32
HHVdb [MJ/kg]17.39 ± 0.1117.38 ± 0.1114.52 ± 0.0914.51 ± 0.0816.13 ± 0.1016.15 ± 0.09
K [ppm]1980 ± 251985 ± 2518522 ± 5618487 ± 7011218 ± 4211287 ± 50
Al [ppm]190 ± 10184 ± 11323 ± 15330 ± 16258 ± 14264 ± 14
Fe [ppm]381 ± 12377 ± 141811 ± 311831 ± 281167 ± 201122 ± 26
Na [ppm]52 ± 556 ± 5399 ± 10405 ± 11240 ± 10231 ± 8
Mg [ppm]594 ± 14602 ± 151002 ± 20998 ± 23794 ± 18805 ± 18
Ca [ppm]3207 ± 313199 ± 317007 ± 306999 ± 365320 ± 305279 ± 27
* daf—dry ash free; db—dry basis; wb—water basis. ** calculated by difference.
Table 2. Flue gas composition and boiler parameters for different biomass pellets.
Table 2. Flue gas composition and boiler parameters for different biomass pellets.
Parameter and UnitWPWPhRPRPhMIXMIXh
O2 [%]7.92 ± 0.338.01 ± 0.318.00 ± 0.367.97 ± 0.327.99 ± 0.298.03 ± 0. 39
CO [mg/Nm3(10% O2)]160 ± 3.54161 ± 4.24612 ± 5.65463 ± 4.18521 ± 6.18354 ± 7.13
NOx [mg/Nm3(10% O2)]136 ± 3.42140 ± 3.53245 ± 4.02244 ± 3.77192 ± 2.98188 ± 3,44
SO2 [mg/Nm3(10% O2)]7 ± 1.126 ± 1.4074 ± 2.1264 ± 2.2538 ± 1.8732 ± 1.95
OGC [mg/Nm3(10% O2)]22 ± 2.1221 ± 1.9895 ± 2.3578 ± 2.1475 ± 2.1263 ± 1.92
TSP [mg/Nm3(10% O2) *]40 ± 1.4138 ± 1.41312 ± 1.62285 ± 1.39150 ± 1.44131 ± 1.56
Power output [kW]25.03 ± 0.2225.01 ± 0.1424.98 ± 0.2324.99 ± 0.2724.99 ± 0.3125.02 ± 0.26
Boiler efficiency [% **]88.18 ± 0.5389.01 ± 0.2264.74 ± 0.3575.28 ± 0.2575.93 ± 0.4182.83 ± 0.41
* total solid particles. ** calculated.
Table 3. Ash parameters for different biomass pellets.
Table 3. Ash parameters for different biomass pellets.
Parameter and UnitWPWPhRPRPhMIXMIXh
KCl in TSP [ppm]<10<1041 ± 515 ± 327 ± 411 ± 3
KCl in bottom ash [ppm]<10<1048 ± 521 ± 433 ± 410 ± 3
St of bottom ash [°C]840 ± 101018 ± 10611 ± 5880 ± 7705 ± 7922 ± 5
Slag diameter [mm]<1<110–273–84–111–5
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Chabiński, M.; Szlęk, A. Effect of Adding Halloysite to Different Solid Biomass Fuels on Combustion Process in a Small-Scale Domestic Retort Boiler. Energies 2024, 17, 6287. https://doi.org/10.3390/en17246287

AMA Style

Chabiński M, Szlęk A. Effect of Adding Halloysite to Different Solid Biomass Fuels on Combustion Process in a Small-Scale Domestic Retort Boiler. Energies. 2024; 17(24):6287. https://doi.org/10.3390/en17246287

Chicago/Turabian Style

Chabiński, Michał, and Andrzej Szlęk. 2024. "Effect of Adding Halloysite to Different Solid Biomass Fuels on Combustion Process in a Small-Scale Domestic Retort Boiler" Energies 17, no. 24: 6287. https://doi.org/10.3390/en17246287

APA Style

Chabiński, M., & Szlęk, A. (2024). Effect of Adding Halloysite to Different Solid Biomass Fuels on Combustion Process in a Small-Scale Domestic Retort Boiler. Energies, 17(24), 6287. https://doi.org/10.3390/en17246287

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