1. Introduction
Each EU member must reduce greenhouse gas emissions by 10 to 50% by 2030. Energy production from renewable energy sources (RES) will have to be increased in order to meet the tightened targets. Biomass is considered one of the main renewable energy sources to replace fossil fuels with the rapid development of agriculture, the use of agricultural biomass in energy has become extremely relevant [
1,
2] in low- and medium-power combustion plants.
Corn cobs, being the non-consumable post-harvest remnants of corn, contribute to a substantial volume of corn-related waste across the world each year [
3]. Global production of corn cob residues is about 18% of the global corn production of about 797 million tons [
4]. The EU’s harvested production of grain maize and corn–cob mix weighed about 60 million tons per year. After minimal preparation and grinding, this agro-waste could be used in thermochemical processes, such as gasification or direct combustion, and produce a high amount of heat and electricity energy. That kind of fuel could increase independence from fossil fuels and decrease environmental pollution worldwide [
5]. Despite this attractive characterization of corn cob, that kind of raw material fuel has some disadvantages related to chemical composition. Corn cob has completely different concentrations of potassium (K), calcium (Ca), phosphorus (P), sodium (Na), chlorine (Cl), sulfuric (S), and other chemical elements compared to traditional wood biofuel. Agro-biomass is a specific material as a fuel due to different planting environments, harvesting seasons, and different fractions of agro-biomass [
6]. The chemical composition of such fuel can differ significantly, i.e., Al, Ca, Fe, Mg, P, K, Si, Na, Ti, Zn, and Pb. Because of this, direct combustion causes difficult to control problems. The use of such fuel becomes completely unattractive for heat producers, although its quantities are quite significant. During combustion in moving-grate or fixed-grate furnaces becomes complicated due to the high concentration of Cl, S, and alkali metals (K and Na) in such biomass, deposits rapidly begin to accumulate on the combustion surfaces, as well as slag accumulation on heat exchangers and slag accumulation caused by silicate (Si) melt on water-cooled walls [
7]. According to Shao et al. [
8], potassium (K) evaporation from fuel starts from 650 °C, and the highest rates are reached at ~1000 °C. Other authors determined that at temperatures from 600 to 815 °C, evaporated potassium (K) reacts with chlorine (Cl) and forms potassium chloride (KCl), which participates in slagging and fouling and causes corrosive problems [
9]. The formation of these compounds is treated as the initial stage of further aerosol formation [
10]. Larger aerosol formations gradually form around K and Na sulphates and chlorides. If the amount of S and Cl in the fuel is lower, carbonates begin to form faster, and K and Na in the fuel are bound [
11]. Volatile heavy metals, such as Zn and Pb, are also detected in aerosols. The concentrations of these elements in aerosols directly depend on the raw fuel used for burning (poor quality).
Fluidized bed boilers are a well-known technology and provide a number of advantages, like fuel flexibility, higher efficiency of heat and mass transfer, considerably low combustion temperatures (up to 800 °C), and lower pollutant emissions [
12,
13]. For example, municipal waste combustion was investigated in a two-stage fluidized bed combustor [
14]. Low-temperature combustion of 400–800 °C was performed in the first stage, and it allowed the reduction of heavy metal concentrations from 16 to 82% in the flue gas. Another work showed promising results on phosphorus (P)- and potassium (K)-reduced emissions during the thermal conversion of the sewage sludge and wheat straw mixture in the fluidized bed at 750 °C [
15,
16]. Karel et al. [
17] investigated conifer bark and eucalyptus cutting waste combustion in the fluidized bed at 600–800 °C and, by changing the air equivalence ratio, were able to achieve stable combustion without defluidization. Experiments with oat seeds and sunflower husks in a 75 MW circulating fluidized bed combustor revealed that the combustion at 750 °C neglects ash agglomeration in the bed, but slagging on the walls leads to more intense defluidization as pieces of slag drop to the bed [
18]. Other studies [
19,
20] showed that agricultural biomass combustion in a low-temperature 600–650 °C fluidized bed could result in incomplete combustion and increased NOx, CO, and particulate matter (PM) emissions, which is related to the air equivalence ratio and volatile evaporation rate from fuel in the bed.
Using fuel additives that have varied chemical compositions is an effective strategy for lessening and offsetting the negative impacts observed in combustion processes. Such additives aid in the establishment of new chemical bonds, thereby converting problematic elements responsible for ash formation into stable materials possessing high melting points [
21,
22]. Since the use of fuel additives is related to economic factors, it is very important to determine what amount of additive would be sufficient to solve the desired problems. Calculating the required quantity of fuel additive often relies on stoichiometric calculations, indicating the necessity for more laboratory investigations to elucidate the phenomena in true scenarios. Furthermore, it is essential to find further fuel additives that boast high stability and reactivity and are economically feasible [
22]. The most appropriate option would be if the fuel additive was a waste production material, the price of which being low and the available quantities being large. Using additives and combined fuel aims to change the composition of the raw material and additionally reduce the share of volatile alkaline elements in the fuel used. Research has shown that blending low-quality fuel with additives can lead to a decrease in the formation of solid particles and other combustion by-products, such as NOx, SO
2, and HCl, during the combustion process [
23]. During gasification, dolomite undergoes calcination to yield CaO, which acts to prevent agglomeration by favoring the formation of K
2CO
3 over K
2OnSiO
2 [
24]. This effect was observed in gasification processes occurring at temperatures ranging from 650 to 850 °C, which were associated with optimal heating values, gas yields, carbon conversion rates, and cold gas efficiencies. The use of dolomite as an additive resulted in reduced agglomerate formation up to 850 °C, demonstrating its efficacy in tar cracking and anti-agglomeration. However, dolomite’s tendency to fragment at higher temperatures poses a notable challenge [
25].
However, agricultural fuel incineration technology is still complex. Furthermore, in the fluidized bed combustion of biomass, the agglomeration of the silica bed material stands out as the predominant operational difficulty linked to ash, especially when burning biomass with a substantial alkali concentration [
26]. For these reasons, this work focuses on the combustion of agro-biomass rich with alkali elements at low bed temperatures and the determination of optimal parameters to avoid bed agglomeration. The bed temperature’s influence on the formed compounds from agro-biomass combustion were also determined by sampling particulate matter from the FBC.
2. Materials and Methods
2.1. Fuel Selection, Preparation, and Analysis
In the process of developing an experimental biomass fuel combustion bench, different agricultural biomass types were evaluated for their chemical compositions. The bench is designed to burn a range of fuel types, specifically targeting the use of the lowest quality agricultural biomass.
For this biomass comparison, types cited in the scientific literature were used: elephant grass, wheat straw, rice husks, sugarcane fiber, corn, and, particularly, corn cobs. Their ash chemical compositions were sourced from the “Phyllis2” database [
27], which lists chemical and physical properties, ash melting temperatures, and other relevant data on various biomass types. For this,
Figure 1 presents the composition of different types of biomass originating from agriculture and indicates that corn cobs contain the highest concentration of potassium oxide (K
2O)—these crushed corn cobs were selected as the reference sample with a notably poor composition for combustion tests leading to high-risk ash-related problems.
The dried corn cob sample was smashed using a Retsch GmbH, SM 300 (Haan, Germany) mill to a fraction of 1 mm. In the following, all tests were carried out with samples of such a fraction. Proximate, ultimate analyses and other physical and chemical parameters of corn cob are given in
Table 1. The elemental composition of the corn cob and dolomite, which was used in combustion tests, is given in
Table 2. All parameters were determined by an accredited laboratory according to ISO standards. With Binder, FD 115 (Tuttlingen, Germany) furnace total content of moisture of corn cob was performed according to the LST EN ISO 18134-1:2016 standard [
28]. Nabertherm GmbH, Muffle Furnace 15/13/P330 (Lilienthal, Germany) was used to determine ash content of corn cob by the LST LT EN ISO 18122:2016 [
29] standard. Ion chromatography system Dionex, ICS 5000 (Sunnyvale, United States) was used for sulphur and chlorine determination. Analysis was performed based on LST EN ISO 16994:2016 [
30] standard. Main element analyzer ThermoFisher Scientific, Flash 2000 (Waltham, MA, USA) was used for total content of carbon, hydrogen, and nitrogen determination. For this analysis, LST EN ISO 16948:2015 [
31] standard was used. Analysis of the chemical elements was performed using inductively coupled plasma with optical emission spectrometer Perkin Elmer, ICP-OES, Optima 8000 (Waltham, MA, USA) device. Major elements were analyzed: Al, Ca, Fe, Mg, P, K, Si, Na, Ti based on LST EN ISO 16967:2015 standard [
32]. Minor elements were also analyzed: Zn, Pb, Cr, Cu, Mn based on LST EN ISO 16968:2015 standard. An automatic bomb calorimeter IKA-Werke, IKA C6000 (Staufen, Germany), according to the requirements of the LST EN ISO 18125:2017 [
33] standard, was used for calorific value determination. Ash melting behavior was analyzed according to the requirements of ISO 21404:2020(E) standard [
34] with Carbolite Gero Ltd., CAF Digital furnace (Shelfield, UK).
Raw material of selected experiment was fully analyzed, and the obtained results were compared with non-specified wood pellets from the literature and databases. The comparison of these materials is necessary to show difference between a well-known fuel as basic with a low-quality fuel.
2.2. Experimental Setup and Procedure
Experiments were carried out in a specially designed 500 kW thermal power laboratory stand. The main components are shown in
Figure 2, and parameters are presented in
Table 3.
Figure 3 shows main fluidized bed reactor parts and locations of installed thermocouples. To keep uniform combustion temperature and initiate fuel self-ignition the lower combustion chamber part was lined with a heat-resistant refractory. Other combustion chamber sections were water-cooled. The thermocouple T
1 was installed directly into the fluidized bed, and other thermocouples (T
2, T
3, T
4, and T
5) were installed at different combustion zone heights. An additional sand was introduced with the dosing auger to maintain a constant fluidized bed height (0.3 m). The fluidized bed height was measured in accordance with a pressure drop of the bed. Moreover, an additional dosing auger was installed for agglomeration inhibition chemicals. For the experiments with inhibitors, the dolomite was chosen. Elemental analysis of dolomite is presented in
Table 2. The supply of dolomite (3% based on fuel supply) was tested at 650 and 700 °C bed temperatures to investigate the effect on agglomeration inhibition.
Before the experiments, the feedstock was prepared: corn cobs were crushed into uniform-sized pieces with a fuel crusher, collected into bags of 1 t, and loaded into a fuel bunker with manual hoisting equipment. Furthermore, the fuel was supplied into the combustion chamber using a speed-controlled auger. Before the combustion experiments, this auger was calibrated with feedstock at different speeds, and the precise amount of fuel was determined. During combustion tests, stable bed fluidization was ensured by supplying the primary air and flue gas mixture at the ~180 m3/h rate and 4000 Pa pressure. The bed temperature of 650 and 700 °C was controlled by setting the proper flue gas recirculation flow rate and oxygen concentration in the mixed gas flow. In the upper sections, the secondary air was supplied to fully combust generated gases from the fuel. The secondary air flow rates at different heights are controlled with damper to achieve effective combustion process, e.g., to obtain lowest CO and NOx and to control sufficient O2 concentration in flue gas. From combustion chamber, hot gases at 600–1000 °C go into specially designed tube section for dust and aerosol sampling. This tube is not water-cooled to maintain constant temperature and avoid possible alkali aerosol condensation. In this tube, dust sampling nozzle was mounted.
2.3. Particulate Matter Sampling and Its Chemical Analysis
During the experiments, samples of particulate matter were collected after reaching airflow, biomass feeding, and gaseous emission values at steady-state conditions. During experiment, steady-state conditions were determined and reached after 30–40 min of continuous combustion. When steady-state was achieved, readings were recorded, and particulate matter was taken. After each combustion session, the sand was replaced with new sand. Ten samples of particulate matter were collected from separate areas of boiler, upstream of the cyclone, immediately downstream of the incinerator. Five samples at 650 °C fluidized bed temperature and five at 700 °C fluidized bed temperature were collected. All the samples were ground by ball milling to obtain homogeneous powders. Major (Al, Ca, Fe, Mg, P, K, Si, Na, and Ti) elements and minor (Zn, Pb, Cr, Cu, and Mn) elements in particulate matter from boiler of biomass were determined by ICP-OES.
4. Conclusions
The experimental investigation of corn cob combustion at low temperatures was performed in a specially design fluidized bed combustor, and its performance was tested under 500 kW thermal power. During the tests, optimal parameters were established and determined that the optimal air equivalence ratio should be 1.15–1.24 depending on the bed temperature, and the bed temperature is controlled by adjusting the primary air and flue gas mixture ratio as the overall flow should be maintained constant to ensure sufficient fluidization velocity. During the combustion process, when the excess air coefficient reaches 1.3–1.4, higher oxygen concentrations (5–6%) are formed in the flue gas, and corrections of the PA and FGR ratio in the mixture is required (the amount of primary air is reduced) to maintain a 10–12% oxygen concentration in the mixture and to maintain a stable temperature of the fluidized bed. However, when supplying the PA/FGR mixture of the above composition, the combustion chamber is cooled more intensively, and it is difficult to maintain a stable combustion process. Pulsations appear, and CO concentrations increase.
It was found that the bed temperature of 650 °C leads to higher CO emissions by about 50% compared to 700 °C. However, the addition of dolomite led to improved combustion efficiency, and CO, OGC, and PM emissions decreased by 14%, 70%, and 43%, respectively. Keeping the fluidized bed temperature at 700 °C resulted in similar reductions in CO, OGC, and PM emissions by 10%, 70%, and 17%.
Adding dolomite acts to impede the melting of potassium and ensures its incorporation into the ash, facilitating the creation of compounds with elevated melting points. The decomposition of MgCO3 and CaCO3 within dolomite leads to an increased presence of high-melting-point minerals in the ash. By incorporating dolomite, ash is enriched with thermally stable minerals, including orthoclase (KAlSi3O8), CaMgSi2O6, wollastonite (CaSiO3), periclase (MgO), and magnesium silicate (MgO:XSiO2•H2O). Dolomite plays a crucial role in inhibiting silicate melting, reducing the formation of a liquid phase, and decreasing both the adhesive properties and the emission of chemical elements with particulate matter in corn cob fly ashes. It was found that the bed temperature of 650 °C leads to a decrease in high exchange elements (Mg, Cu, Si, Ca) in a range from 12% to 36%, and 700 °C leads to a decrease in a range from 15% to 29%.
During extended experiments lasting 8 h, no agglomeration of the fluidized bed was observed. Moreover, the proposed configuration of the FBC and its operational parameters prove suitable for facilitating the efficient combustion of agricultural biomass.