Analysis of the Characteristics of Bio-Coal Briquettes from Agricultural and Coal Industry Waste (1)

1. Introduction
In the modern world, there is a steady increase in the consumption of energy resources, which is determined by the high rates of industrial development and the growth of the world population.

Climate change, which is caused by aggressive human activities, is the cause of dangerous wide-scale disturbances in nature and affects the lives of billions of people on our planet. The report of the Intergovernmental Panel on Climate Change (Sixth Assessment Report (AR6)) showed that over the next two decades the world will face the inevitable multiple climate hazards associated with global warming of 1.5 °C (2.7 °F), some of which will become irreversible. To mitigate these catastrophic changes, accelerated action is needed to reduce greenhouse gas emissions.

The solution of this issue is difficult due to the introduction of energy capacities using wind, solar, etc. This is caused by the high cost of energy produced by solar and wind stations. The most promising solution is the replacement of the energy source itself: classical organic solid or liquid fuel with biofuel.

Environment, health, and ecology issues are the main concerns associated with the operation of fossil fuels both worldwide and in Kazakhstan.

According to British Petroleum, carbon dioxide emissions in Kazakhstan have tripled over the past 70 years. During the period from 2010 to 2021, carbon dioxide emissions increased from 183.9 to 219.4 million tons per year, i.e., by 19.3% [1]. According to national and international experts, climate disasters such as drought and the shallowing of rivers will become a common occurrence in Kazakhstan. According to forecasts, pasture carrying capacity in the country will decrease by 10% by 2030, water deficit will be 50% of the need by 2040, and more than 50% of the current glacial mass will be lost by 2100.

In this context, the ambitious goal of Kazakhstan to achieve carbon neutrality by 2060 has become an important step in the fight against climate change. Currently, Kazakhstan is working on the doctrine of carbon neutral development until 2060. Among the key measures are the abandonment of new coal-fired generation projects and the gradual phase-out of coal combustion (2021–2025), the implementation of a program to plant 2 billion trees (2025), doubling the share of renewable energy sources in electricity generation (2030), one hundred percent sorting of municipal solid waste (2040), sustainable agriculture on 75% of arable land (2045), 100% electrification of personal passenger transport (2045), use of only green hydrogen and the complete elimination of coal production (2050), etc.

One of the options for reducing harmful emissions is the use of biomass, including in combination with industrial waste (for example, coal and coke dust). Biomass has great potential because it is renewable, unlike fossil fuels. Biomass in its usual form is difficult to store, transport, process, and exploit due to the following factors: high moisture content, low bulk density, and variability in feedstock sizes.

These problems can be overcome by using briquetting technology—the process of compacting waste into a homogeneous solid briquette. Co-combustion of biomass with coal will expand the use of biomass energy and improve the properties of low-grade coals [2].

Biomass, especially agricultural waste, appears to be one of the most promising energy resources for developing countries. This type of waste is available as free, local, and environmentally friendly energy sources [3]. The choice of raw materials for biomass depends on the availability of waste in each specific region of the world, and this list is quite wide: sugarcane bagasse, sisal, cassava bran, dried banana leaves, rice husks and bran, corn, sunflower husks, etc. [4,5,6,7,8].

Studies have shown that mixing coal and biomass will make it possible to obtain a bio-coal briquette with high physical and combustion characteristics [9]. Coal and biomass are different in elemental composition: coal includes a high carbon content and heat content, but a low volatile yield, whereas biomass has a high volatile content, but a low carbon content [10].

Many experiments have been carried out in order to study the characteristics of briquettes from various plant and coal wastes. For instance, the physical and combustion characteristics of briquettes from a mixture of peanut husks and coal were determined using starch as a binder and with the addition of Ca(OH)2 as a desulfurizing agent [9]. In the course of this work, the physical–mechanical and thermal properties of the obtained briquettes were studied. The results showed that the moisture content is in the range of 2.43–6.44%, the compressive strength is in the range 7.72–10.85 N/mm2, the ash content is in the range 24.18–29.15%, calorific value is in the range 21,714.17–25,027.18 kJ/kg, combined carbon is in the range 16.77–53.22%, ignition time is in the range 22.23–45.20 s, and burning rate is in the range 16.10–28.32 g/min. These are quite good values of the thermal properties of bio-briquettes. Briquettes made from coal and peanut husk in the ratio of 60:40 have a high compressive strength of 10.85 N/mm2, have a high calorific value of 23,628.83 kJ/kg, ignite much faster—35.76 s, and have a burning time of 22.56 min. These results allow us to state that these bio-briquettes have significant thermal properties compared to other mixtures of briquettes.

In the paper [11], various mixtures of biomass with coal were used. Namely, the mixtures used were coconut fiber mixed with brown coal, rice straw mixed with coal, and corn cobs mixed with brown coal. In all cases starch and molasses were used as a binder. The results of the study show that the studied compositions of briquettes have a good calorific value. However, briquettes from 60% coal and 40% corn cobs showed better combustible qualities compared to other compositions. This type of briquettes has the following characteristics: ash content 20.17%, moisture content 2.5%, density 0.414 g/cm3, volatile matter 32.50%, calorific value 124.45 kJ/kg, ignition time 29.56 s, and burning time 19.76 min.

In another work [12], briquettes were made from coal waste and bananas at various composition ratios (100:0, 90:10, 80:20, 70:30, and 60:40) using calcium hydroxide as a desulfurizing agent, and starch as a binder. The results showed that the content of moisture, volatile matter, and ash content of composite briquettes ranged from 6.74 to 9.36%, from 25.25 to 39.78%, and from 6.25 to 8.75%, respectively. The carbon content, porosity index, calorific value, ignition time, burning rate, and thermal efficiency of composite briquettes ranged from 54.16 to 76.32%, from 23.42 to 44.48%, from 31.62 to 31.43 MJ/kg, from 57.24 to 180.96 s, from 0.035 to 0.083 g/min, and from 12.73 to 15.63%, respectively. The higher calorific value and lower volatile content of combined briquettes compared to biomass briquettes make them more suitable as solid fuels.

In the paper [13], the authors mixed low-grade South Sumatra coal and palm shell charcoal to produce bio-coal briquettes in order to improve fuel properties. The results of the experiments showed that the calorific value of a bio-coal briquette is greatly influenced by the composition of the raw material and the type of binder. The highest calorific value was 6438 kcal/kg for a sample with the following composition: low-grade coal—65%, palm shell charcoal—20%, and a binder (combination of starch powder, water, and liquid caustic soda in a mass ratio of 1:1:1)—15%.

The study [14] shows an analysis of the properties of bio-coal briquettes obtained by mixing cassava stalks and coal in various ratios using starch as a binder and Ca(OH)2 as a desulfurizing agent. Among the briquettes obtained in this way, a sample with a content of 40% biomass showed the best combustible qualities.

All these studies have shown that bio-coal briquettes show promising results as an alternative solid fuel with properties within the limits set by international standards. These results encourage the search for other options for combining biomass and coal as a feedstock.

However, all the listed bio-coal briquettes contained a binder in their composition. The presence of a binder in the fuel briquette improves its strength, but at the same time, increases its cost.

Across the territory of Kazakhstan, a huge amount of industrial (around 700 million tons) and agricultural (about 13 million tons) waste is generated annually, which is not properly disposed of and is placed in landfills [15]. As of 1 October 2022, the availability of oilseeds, including sunflower, was 511, 794.1 tons [16]. Sunflower is the leading oil crop in Kazakhstan. It accounts for up to 70% of the sown area occupied by oil crops and 85% of the gross harvest. Due to the great economic benefits, sunflower areas are constantly growing. Over the past 10 years, they have increased fivefold and have reached almost 1 million hectares [17]. Sunflower husk is the main waste in the processing of sunflower seeds. Sunflower husk as a fuel has good characteristics, has a low ash content, is highly flammable, and has a high calorific value. Regrettably, no information is currently available on the possibility of obtaining bio-coal briquettes and their properties obtained through mixing sunflower husks with coal or coke dust. In the current studies, the possibilities of bio-coal based on sunflower cake have been presented [18], or the burning of sunflower husks in a vortex layer was investigated [19]. However, burning husks in a vortex layer has a number of considerable drawbacks. Firstly, currently there are no industrially produced boilers and furnaces designed for burning husks. Secondly, the husk can be satisfactorily burned in flare-layered and shaft-type furnaces only, with a low forcing of the combustion process. This combustion option is suitable only for low power boilers or with a 1.5–3-fold decrease in evaporative capacity of medium power boilers. There is a need to search for the possibility of utilizing biomass in another way, e.g., its briquetting.

Apart from the problem of disposing these biowastes, there is a problem of disposing coal and coke dust produced in mining and processing. According to experts, their volumes can be from 30 to 70% of the main volume of production and processing [20]. For example, according to the data of the petroleum coke calcination enterprise (UPNK-PV LLP (Pavlodar, Kazakhstan)), the output of coke dust as a waste is 200 tons annually. Thus, a thorough analysis of the possibility of using bio-coal briquettes from local raw materials as an alternative fuel is necessary.

2. Materials and Methods
The aim of this research is to obtain bio-coal briquettes from biomass (sunflower husks, leaves) and industrial waste (coal and coke dust) and to analyze their characteristics. The starting material for the test briquettes was waste of organic origin (leaves, sunflower husks) and industrial waste (coke and coal dust).
The preparation of the feedstock was as follows:
-  cleaning from foreign inclusions (glass, plastic, metal, etc.);
-  drying in the open air until air-dry state;
-  grinding to a size of no more than 2 mm for a fine fraction and no more than 6 mm for a coarse fraction.

For the research, bio-coal briquettes were made from organic waste in combination with coal and coke dust in various ratios of 30:70%, 40:60%, 50:50%, 60:40%, 70:30%, and 80:20%. The following types of briquettes were made:

-  sunflower husk of fine fraction (0–2 mm): coal dust from the Karazhyra deposit;
-  sunflower husk of coarse fraction (3–6 mm): coal dust of the Karazhyra deposit;
-  sunflower husk of fine fraction (0–2 mm): coal dust of the Shubarkul deposit;
-  sunflower husk of coarse fraction (3–6 mm): coal dust of the Shubarkul deposit;
-  sunflower husk of fine fraction (0–2 mm): coke dust;
-  sunflower husk of coarse fraction (3–6 mm): coke dust;
-  leaves of fine fraction (0–2 mm): coal dust from the Karazhyra deposit;
-  leaves of fine fraction (0–2 mm): coal dust of the Shubarkul deposit.

The fuel characteristics of the industrial waste used are presented in Table 1.

Table 1. Characteristics of feedstock for bio-coal briquettes.

To manufacture briquettes, mixed raw materials were pressed by a hydraulic press with a constant pressure of 25 MPa. The resulting briquettes were dried in the room to an air-dry state. Briquettes have a cylindrical shape with a diameter of 30 mm and the weight of 4 g.

In the course of the study, physical–mechanical characteristics (density, strength, humidity) and thermophysical characteristics (humidity, ash content, volatile matter yield, lower calorific value, burning time and speed, ignition time) of fuel briquettes were determined.

The density of briquettes was determined in accordance with [21]. For this procedure, a standard container is filled with a sample and weighed. The bulk density was calculated based on the net weight and the internal volume of the container.

The mechanical strength of the obtained samples of briquettes was determined in accordance with [22]. The test samples were subjected to controlled impacts by colliding the briquettes with each other and with the walls of a special rotating chamber. The strength was calculated by sieve analysis of changes in the particle size distribution of the drum sample [23].

The determination of the moisture content of the prepared samples was carried out according to [24]. The procedure consisted of drying the sample of briquettes in an oven at a temperature of 105 ± 2 °C for 60 min and calculating the weight loss of the taken sample.

The ash content of briquettes from organic waste and briquettes containing up to 50% industrial waste was determined in accordance with the method described in [25]. The ash content was determined by calculation, based on the mass of the residue formed after burning a sample of biofuel in a muffle furnace with free air access and a temperature of (550 ± 10) °C. For bio-coal briquettes with industrial waste content of more than 50%, the combustion of a sample of fuel and the calcination of the ash residue to constant weight was carried out at a temperature of (815 ± 10) °C. After cooling and weighing, the change in weight was determined.

The yield of volatile matter was determined according to [26]. A weighed portion of the analytical sample was heated without air in a muffle furnace at a temperature of (900 ± 10) °C for 7 min. The yield of volatile matter was determined as the weight loss of the fuel sample minus the weight loss due to the moisture content in the sample.

The determination of the lower calorific value of the briquettes was carried out according to [27]. The essence of the method for determining the calorific value was the complete combustion of a sample of the fuel mass in a calorimetric bomb. The process took place in an isothermal mode at a constant volume in a compressed oxygen medium at a pressure of 29.4∙105 Pa. The researchers measured the temperature of the water in the calorimeter vessel and made corrections for the heat released during the combustion of the fire wire, and the heat of formation and dissolution of sulfuric and nitric acids in water.

The following combustion characteristics of bio-coal briquettes were measured:
-  burning time;
-  ignition time;
-  weight of the briquette.

Based on the obtained data, the burning rate of bio-coal briquettes was calculated.

Burning time measurements were made similarly to the measurements made in [28,29] by burning bio-coal briquettes placed on a perforated sheet in the open air in a gas burner flame.
The change in the mass of the briquette was monitored every 10 s throughout the entire combustion process. The end of the burning time was considered the moment when the mass of the briquette stopped changing.

The ignition time is characterized by the time from the start of the test to the onset of stable flame combustion, which is visually recorded. To perform this, a burner flame is brought to the fuel briquette and the ignition time is recorded using a stopwatch.

The mass of the briquette was measured on a scale and was subsequently used to determine the burning rate of bio-coal briquettes according to the following formula:

𝐵=𝑚1−𝑚2𝑇

where B—burning rate, g/s;
m1—initial mass of fuel before combustion, g;
m2—final mass of ash after combustion, g;
T—total burning time, s.