Co-Firing Combustion Characteristics of Woodchips and Spent Mushroom Substrates in a 400 kWth Stoker-Type Boiler

Abstract

The simultaneous firing characteristics of woodchips and spent mushroom substrates (SMS) were studied in a stoker-type industrial boiler. The type of spent substrate intended for combustion consisted of oyster mushrooms. SMS from mushroom farms generally have a high water content. Dryers are therefore used for combustion. The moisture content of SMS was reduced to achieve low moisture to combust sufficiently at about 20%. First, the basic characteristics of the boiler were confirmed by conducting a woodchip combustion test under various operating loads of 30, 50, 70, and 100%. Thereafter, a simultaneous combustion test of woodchips and SMS was performed. During the experiment, exhaust gas concentrations in the boiler combustion chamber were measured, such as the temperature of oxygen (O₂), carbon monoxide (CO), and nitrogen oxides (NOx). In addition, industrial and basic analyses were performed on woodchips and SMS. The main differences in the fuel analysis results between woodchips and SMS were ash, nitrogen, sulfur content and net calorific value. According to the analysis, the nitrogen content of SMS was 2.6%, which was 8.7 times higher than that of woodchips, and the ash content was also 14.8%, which was 18.5 times that of woodchips. As a result of the combustion experiment, the woodchip experiment revealed that the values of O₂ and CO decreased and the combustion chamber temperature increased as the amount of fuel increased. Due to higher combustion temperature, thermal NOx also increased. When comparing this combustion test with the co-firing test, there was no significant difference in O₂, CO, and combustion chamber temperature. However, with regard to the NOx value, the results showed a sharp increase from 64 ppm to 135 ppm. Although the NOx value increased, SMS had enough heat to be burned as fuel. Therefore, the utility of various agricultural byproducts as fuel has prospects for achieving an effective approach to energy cost reduction.

1. Introduction

The boiler is a facility that is closely related to daily life settings and manufacturing processes. Fossil fuels such as coal, oil and natural gas are widely used worldwide. As a result, environmental problems related to carbon dioxide (CO₂), such as abnormal climatic phenomena, have occurred. Therefore, it is necessary to increase the proportion of renewable energy in order to reduce the use of fossil fuels. Up to now, wood pellet and wood chip have mainly been used as a renewable energy in biomass combustion. There are also many by-products in the field of agriculture, such as rice husk, empty fruit bunch (EFB), palm kernel shell (PKS) and SMS. SMS is especially abundant in agriculture, and its disposal costs are also incurred. So, if SMS is burned in a combustion boiler, fuel and disposal costs can be reduced. To combust solid biomass fuel in a boiler, there are some technologies such as: (1) a fixed bed; (2) a gasification combustor; (3) a moving grate; (4) a fluidized bed combustor; (5) a pulverized burner and so on.

Boilers can burn a variety of fuels. Awan et al. [1] performed a techno-economic sustainability analysis of a biomass-fired industrial boiler, looking at biomass evolution as heat and a power generation source. This study investigated the energy conservation opportunities in a biomass-fueled boiler. They also proved that rice husk has a burning capacity similar to other expensive and hazardous resources and biomass should be opted for as a source of power generation. An extensive thermodynamics analysis (using energy and exergy mathematical models) of a rice husk-based fixed bed boiler was conducted. Since not all fuels can be combusted in boilers, studies on biomass used in boilers are needed. Although studies on NOx emissions have proved rice husk to be an effective fuel for co-generation in fixed bed boilers, it is strongly recommended to use biomass only in fluidized bed boilers. All biomass fuels have higher moisture content, and fluidized bed boilers can burn high moisture content fuel more effectively. Thus, burning biomass in fluidized boilers will increase the biomass’ effectiveness as fuel. Saidur et al. [2] reviewed biomasses as a fuel for boilers, and several aspects that are associated with burning biomasses in boilers were investigated, such as the composition, estimation of the higher heating value, comparisons to other fuels, combustion, co-firing with coal, economic and social analyses, transportation, densification, problems and the future of biomasses. In this paper, it was found that using biomass in boilers offers many advantages such as the mitigation of hazardous emissions such as CO₂, NOx, CH₄, SOx and CO, the diversification of fuel supply and energy security, the potential use of oceans and low-quality soils, and the restoration of degraded lands, and economic, social and environmental benefits such as financial net saving, the conservation of fossil fuel resources and job creation. Qiu et al. [3] measured the flue gas emissions, particularly carbon monoxide (CO), nitrogen oxides (NOx) and particle emissions, of a domestic biomass boiler under various operating conditions. This paper presents an experimental method to investigate the flue gas emissions of a domestic biomass boiler. Biomass flue gas cleaning is of great concern—especially particle emission cleaning—since PM0.1–10 are major pollutants to human health and the atmosphere as well. The state-of-the-art ceramic filter cleaning technology is validated to be very effective to meet environmental targets. Rabacal et al. [4] evaluated the combustion and emission characteristics of a domestic boiler fired with pellets of pine, industrial wood wastes and peach stones. Initially, the boiler’s performance firing pine pellets was evaluated as a function of the thermal input. Subsequently, the influence of the pellet type on boiler performance was also examined. They found that boiler operating conditions have a pronounced effect on the CO and HC emissions. The fuel–NO mechanism is the main source of NOx emissions and does not seem to be affected by the specific boiler operating conditions. The type of pellets significantly affects the boiler performance, particularly the emission characteristics. The use of pellets made from industrial wood wastes and peach stones only marginally affects the boiler thermal efficiency. Industrial wood residues and peach stones have an attractive potential to be used as sustainable alternative fuels in domestic pellets-fired boilers. The emissions originated by incomplete combustion, especially the CO emissions, can be minimized through the optimization of the boiler operating conditions, in particular the excess air level. Karen, N.F. [5] studied thermal treatments technologies for the reuse of SMC and coal tailings. They found that SMC (spent mushroom compost)-coal tailing pellet combustion produced minimal acid gas emissions (NOx, SOx and HCl), although the alkali metal oxide content of the fly ash was sufficient to cause possible slagging/fouling in the system. Using these wastes for energy recovery provides a sustainable management solution to divert SMC from landfill and aid the reclamation of contaminated land, and is therefore both practical and environmentally-sound. Bhupendra, K. et al. [6] studied agricultural waste management strategies for environmental sustainability. The major portion of the agricultural wastes comes from crop residues, livestock, agro-industrial, and aquaculture routes; therefore, managing the agricultural wastes is the need of the hour and requires robust strategic planning and implementation. They say that proper agricultural waste management shall ensure the sustainability of the agricultural sector for the food and health security of the future generation and strengthen the circular bioeconomy. In studies on NOx emissions for various solid biomass fuels, Kang et al. [7] investigated the characteristics of spent coffee grounds as a fuel, the combustion characteristics in a small boiler system, such as CO, NOx, and O₂, and the heating characteristics of a heating boiler. In this study, they found the possibility of using spent coffee grounds as a heating fuel in a small-scale residential boiler. However, optimization for combustor design would be needed to reduce O₂, CO and NOx concentrations in flue gas. Lucas et al. [8] studied the recycling of spent mushroom substrate into fuel pellets for low-emission bioenergy producing systems. They clearly demonstrated the possibility of systematically converting diverse types of SMS into fuel pellets suitable for low-emission bioenergy producing systems. Alves et al. [9] studied energetic characteristics and combustion behavior of agro-pellets, produced from pure spent coffee grounds (SCGs) or blended with pine sawdust in a commercial residential pellet boiler. They found the use of pure SCG in boiler lead to a good combustion efficiency. However, energy recovery was lower than with the other fuels. Dias et al. [10] presents results from an experimental study performed on a 13 kW(th) commercial domestic boiler using pellets as fuel. Measurements were performed for boiler heat load, pellets consumption rate, flue gas temperature and composition. NOx emissions are independent of excess air for low values of nitrogen in the fuel whereas, for larger values, NOx emissions increase with the O₂ present in the combustion products. Verma et al. [11] studied the efficiency characteristics and emissions of a multi-fuel domestic boiler (40 kW) were investigated as function of the operational load for six different types of commercially available biomass pellets. The operational load dependent emissions were reported for each type of pellets. They found NOx emissions were minimum and maximum with wood and CPW pellets, respectively, at both operational loads. Emissions of NOx with CPW were 3.4 and 4.6 times higher than wood pellets at nominal load and reduced loads, respectively. Kortelainen et al. [12] studied combustion of wood chips was studied in a moving step-grate burner in different combustion conditions. This work presents new information from organic aerosol behavior in wood combustion emissions under controlled laboratory conditions in real time. Organic factors of HOA, LV-OOA, and BBOA, which are commonly found in the atmospheric data, were found to be present in the wood chip burner emissions. Caposciutti et al. [13] studied the effect of fuel size on the combustion of woodchips in a small boiler was investigated by sampling the concentration of volatile species at different locations on the fuel bed surface and by monitoring temperatures inside, above and on the surface of the fuel bed. The fuel size was found to have a significant impact on the release profiles of the early stage combustion gases. Varol et al. [14] studied co-combustion of Bursa-Orhaneli lignite and woodchips mixtures containing 10%, 30% and 50% by wt. of woodchips was studied in a circulating fluidized bed combustor in order to investigate the effect of excess air ratio (k) on the flue gas emissions. During the combustion tests, CO, O₂, NO, and SO₂ emissions in the flue gas was continuously measured and recorded by ABB-AO 2000 flue gas analyzer. Deng et al. [15] studied an incubation experiment to examine effects of SMS derived-biochar created at different pyrolysis temperatures on soil CO₂ and N₂O emissions and N transformations in moso bamboo forest soils. Results of the incubation study suggested that application of biochar derived from SMS to moso bamboo forest soils decreased soil N₂O emissions especially with N fertilization at 25 and 30 °C. Xu et al. [16] studied CH₄ and N₂O emissions from C. oleifera plantation soils as affected by biochar. The results showed that higher N₂O emissions occurred in soils with 120%WHC, due to increased abundance of AOA, AOB and nirS. MS or FS biochar differed in their effects on soil GHG emissions with different WHC. Tillman et al. [17] studied ground biomass and pulverized coal were used for co-firing test. The tests of co-firing of coal and biomass were carried out in a bench-scale bubbling fluidized bed combustor. Coal co-firing was successful with up to a 20% biomass mix boilers. Ash deposits reduce heat transfer. Shen et al. [18] studied to characterize the carbonyl emission from biomass-fired boilers, air samples collected from six typical biomass-fired boilers were analyzed by a PFPH-GC/MS method to determine 21 carbonyl species. The results showed that distinctive emission characteristics of carbonyl compounds for each boiler.

In general, hexanaldehyde and propionaldehyde were the most abundant carbonyl compounds with a percentage of 29–47% and 19–31% in the total carbonyls, respectively, followed by formaldehyde and acetone. Lv et al. [19] studied the problems of the high exhaust gas temperature in the biomass boiler which moves normally are analyzed, and the anti-balance method to get the boiler efficiency is used to get the relationship between the exhaust gas temperature and the boiler efficiency. Chandrasekaran et al. [20] studied detailed particulate and gaseous emission characterizations were conducted on six commercially available residential-scale wood pellet boilers. CO emissions, an indication of combustion efficiency, were found to be higher for the grass pellets, indicating less complete combustion. NOx and SO₂ emissions were also higher for grass and grass/corn blends, attributable to the higher fuel N and S. PM samples collected on. Dioxin and dibenzofuran emissions were found to be substantially higher for grass pellet emissions compared to wood pellet emissions at both high and low loads for all of the appliances. Oberweis et al. [21] studied characteristics of different biomass samples, such as the moisture, ash and fixed carbon content and the volatile matter. Furthermore, the paper presents a performance and emissions evaluation for 40 kW biomass boiler. The results show that biomass is a valuable option for energy generation and the reduction of GHG gases.

To test whether the SMS can be used as a fuel, we burned SMS in a 400 kWth boiler with capacity. As a result of the mixed combustion of wood chips and SMS, we found that SMS has a high amount of NOx emission but sufficient calorific value to be used as a fuel.

2. Materials and Methods

SMS was secured from a mushroom factory. The substrate type was derived from oyster mushrooms. The moisture content of the obtained SMS was checked to confirm whether it was in a combustible state. Thereafter, we dried the SMS in a dryer. The dryer used to dry the SMS is shown in Figure 1. And Figure 2 shows spent mushroom substrate before and after being added to the dryer. When drying through a dryer, we measured the weight and moisture of the fuel before and after drying.

After the fuel to be used for combustion was prepared, the experiment was conducted in two ways. As the first method, a single combustion experiment on woodchips was conducted. Based on the consumption of 200 kg of woodchips burning, a combustion experiment was performed with woodchips fuel for each of the four load stages: 30%, 50%, 70%, and 100% (first experiment). The woodchips used for combustion are shown in Figure 3. In the second experimental method, we tested co-firing the fuel amount of woodchips at a 70% load, with the remaining mushroom medium at a 30% load. In the second experiment, a total of two combustion experiments were performed. Experiment 1 had a relatively higher moisture content of SMS. Images of the SMS used in the co-firing experiment are shown in Figure 4. And Figure 5 shows mixture of woodchips and SMS.

All combustion experiments were carried out in the boiler shown in Figure 6 This was a 400 kWth moving grate boiler. In the woodchips combustion, the combustion test was performed immediately using the fuel, but in the co-firing experiment, the fuel was mixed using a stirrer before proceeding. Fuel was fed from each hopper prior to mixing. By using a load cell under the hopper, it is possible to measure the consumption rate of both fuels. The initial ignition was carried out only with woodchips. After the combustion chamber temperature exceeded 850 °C, SMS was fed into the combustion chamber to start the co-firing condition. As shown in Figure 7 after combustion in the grate, the exhaust gas passes through the heat exchange part inside the boiler and is discharged through the exhaust gas passage.

We measured the temperature of the combustion chamber before entering the heat exchanger unit. The measured point is shown in Figure 6 with a blue dot. For each experiment conducted, we confirmed the characteristics of O₂, CO, NOx, and the combustion chamber temperature of the exhaust gas. Exhaust gas components were acquired by using a flue gas analyzer Testo 350 (Testo SE & Co., Titisee-Neustadt, Germany). The temperature data were acquired by using the GL800 (Graphtec Corp., Yokohama, Japan) with a K-type thermocouple. The experiment was conducted for more than two hours with steady state conditions. With the flue gas analyzer, the power of the blower fan was controlled so that the oxygen concentration in the exhaust gas was about 10–14%.

In addition, we requested a fuel analysis from an authorized agency and obtained the official analysis report data of the woodchips and SMS (

Table 1. Official analysis report data of the experimental fuels.

Test Item	Unit	Woodchips	Spent Mushroom Substrate
Moisture	Mass %	26.8	17.0
Volatile matter	Mass %	58.9	59.7
Ash	Mass %	0.8	14.8
Carbon	Mass %	49.8	38.2
Hydrogen	Mass %	6.1	5.2
Nitrogen	Mass %	0.3	2.6
Sulfur	Mass %	0.02	0.21
Ash (Dry basis)	Mass %	1.1	17.8
Gross calorific value	kcal/kg	3480	2989
Net calorific value	kcal/kg	3080	2664
Fixed carbon	Mass %	13.5	8.5
Oxygen	Mass %	42.67	-

).

3. Results

3.1. Characteristics of Woodchips and Spent Mushroom Substrate

Elemental analysis and industrial analysis were performed on the woodchips and SMS through an authorized institution. As a result, the moisture content of woodchips was 26.8%, which was higher than that of the SMS (17.0%). In contrast, it was confirmed that the moisture content of ash was 0.8% for woodchips but 14.8% for SMS. Nitrogen was also confirmed to be as high as 2.6%.

3.2. Combustion Test—Woodchips (First Experiment)

In

Table 2. A single woodchip experiment and a mixed-firing experiment with spent mushroom substrate. Fuel quantities, O₂, CO, NOx, combustion chamber temperatures, and flame images are shown. CO and NOx are converted values based on 13% O₂ (first experiment).

Fuel Type (Boiler Load)	Fuel Consumption Rate (kg/h)	O2 (%)	CO (ppm)	NOx (ppm)	Temperature of Combustor (°C)	Combustion Image
Woodchips (30%)	66	14.0	172	45	765
Woodchips (50%)	102	14.0	124	50	856
Woodchips (70%)	138	12.1	87	59	909
Woodchips (100%)	200	10.5	66	64	1103

, woodchips combustion test was conducted. The findings of the experiment confirmed that the O₂ and CO values decreased as the amount of fuel was increased from a 30% load to a 100% load.

In addition, it was confirmed that the NOx value and the temperature of the combustion chamber increased. In the case of the flame shape, the fire spread more and more.

3.3. Combustion Test—Co-Firing of Woodchips and Spent Mushroom Substrate (Second Experiment)

A co-firing experiment was conducted with woodchips and the spent mushroom substrate. The SMS in the initial state was dried because the moisture content was too high to be burned without treatment.

Co-firing experiments were conducted in two experiments: Experiment 1 and Experiment 2. In Experiment 1, the moisture content of the woodchip fuel was 30.4% and that of the SMS was 45.2%. In Experiment 2, the moisture content of the woodchips fuel was 30.4%, the same as in the first experiment. In contrast, the moisture content of the SMS decreased to 20.7% (

Table 3. Weight and moisture content values of spent mushroom substrate at stages in Experiments 1 and 2.

Moisture Content (%)	Experiment 1	Experiment 2
Woodchips	30.4	30.4
Spent mushroom substrate	45.2	20.7

).

In Experiment 1, SMS had a high water content of 45.2, so combustion did not occur normally such as with a high CO and O₂ concentration and a low temperature in the combustion chamber. On the contrary, in Experiment 2, SMS had a relatively low water content of 20.7, so combustion occurred normally such as with a low CO and O₂ concentration and a high temperature in the combustion chamber (

Table 4. A single woodchip experiment and a mixed-firing experiment with spent mushroom substrate. Fuel quantities, O₂, CO, NOx, combustion chamber temperatures, and flame images are shown. CO and NOx are converted values based on 13% O₂ (second experiment).

	Fuel Load of Woodchips/SMS(%)	O2 (%)	CO (ppm)	NOx (ppm)	Temperature of Combustor (°C)	Combustion Image
Experiment 1	70/30	17.3	2391	66	638
Experiment 2	70/30	9.85	71	135	1049

).

4. Discussion

With regard to mushroom substrates, considerable moisture exists because a humid growing environment is necessary. If the moisture content in the fuel is high, combustion does not occur properly, so it is necessary to decrease the moisture through drying, followed by a combustion experiment. However, the combustion experiment necessitates considerations of the additional costs of the heat and time when drying. If spending less time and money is prioritized, a moisture content of 45.2% can be achieved, as shown in the primary co-firing experiment, Experiment 1. However, this finding occurs because drying is not properly performed. If the fuel is burned in a very dry state, a moisture content of 20.7% can be obtained, as shown in the secondary experiment (Experiment 2), and the combustion experiment can be performed properly. This shows a similar pattern to the woodchips single firing experiment, indicating that SMS has the potential to be sufficiently used as a fuel.

Furthermore, achieving good combustion can be confirmed from the composition of the exhaust gas or the combustion chamber temperature. Even if the same amount of fuel is burned, if combustion occurred properly, the combustion chamber temperature will be relatively high. Higher combustion chamber temperatures result in higher NOx and lower CO concentrations. In the experimental case of exclusively using woodchips, the higher the woodchips fuel load is, the better the combustion. Accordingly, the combustion chamber temperature and NOx values increased, and the O₂ and CO values decreased. Compared with the 100% load of the entire combustion, there was no significant difference in the combustion chamber temperature, O₂, and CO values in the case of the mixed combustion. However, it was confirmed that the NOx value significantly increased. This finding may have occurred because the amount of nitrogen contained in the SMS is high, as confirmed through the requested authorized analysis. However, NOx can be high despite good combustion. When comparing the combustion chamber temperature or O₂ value in the woodchip only and co-firing experiments, there was no significant difference, lending confirmation to attributing the difference to the nitrogen component.

Figure 8 depicts a graph showing the NOx and N values as various by-products are burned. The findings confirm that this experiment was conducted according to the trend.

5. Conclusions

Wood pellets and woodchips are the main materials in field of biomass renewable energy. However, agricultural by-products are possible to use for heating. So, we focused on SMS as a fuel in industrial boilers to minimize agricultural heating energy and cost.

In this study, we tested woodchips and co-firing of woodchips and SMS in a 400 kWth industrial boiler. The test results are summarized as follows:

• The water content of raw SMS is approximately 66%, so a drying process is necessary to decrease the water content to less than 20%;
• The SMS have poor fuel characteristics, especially ash (14.8%), nitrogen (2.6%), sulfur (0.21%) and net calorific value (2664 kcal/kg), compared to woodchips;
• Due to the higher value of the nitrogen content in SMS, the NOx emissions in the flue gas are much higher (143 ppm) than those encountered in the woodchip combustion condition (64 ppm);
• For water content up to approximately 20% in the SMS (30%) mixed with woodchips (70%), co-firing combustion has possibilities for use in industrial woodchip boilers. For scenarios in which the water content of SMS is greater than 20%, co-firing could be very unstable;
• In the future, it will be necessary to study the combustion of pure SMS;
• If the moisture content of SMS is less than 20%, the possibility of using SMS as a fuel a boiler co-fired with woodchips or wood pellets was confirmed;
• If SMS is dried to a moisture content of about 20%, the possibility of using it in a boiler together with woodchips or wood pellets was confirmed.