1. Introduction
Demographic growth and economic development cause considerable challenges for the contemporary global economy. Among them, one of the most important is the growing energy demand and the amount of municipal waste generated. According to the forecasts for 2040 by the International Energy Agency, the demand for electricity will increase by 30% [
1]. Similarly, the waste management sector will continue to grow. According to research by the Organization for Economic Co-operation and Development (OECD), an increase in national income by 1% increases the amount of municipal waste by 0.69% [
2]. Although the circular economy based on waste recycling and zero-waste trends are gaining support, the growing population and higher living standards result in more waste. The World Bank report states that by 2025, the volume of waste will increase by 2.2 billion tons a year worldwide [
3].
A synergistic solution to waste recycling and an alternative source of fuel is the product of thermal treatment of municipal solid waste (MSW), which is carbonized refuse-derived fuel (CRDF) [
4]. It is a type of biochar produced from the combustible fraction of municipal waste. It is a product that can be a renewable low-emission fuel. Its fuel properties, such as the lower heating value (LHV) ranging from 19.6 MJ·kg
−1 to 25.3 MJ·kg
−1, compete with conventional energy generation solutions [
4]. In addition, the higher heating value (HHV) of biochar, including CRDF, depending on the substrate used, can reach up to 35 MJ·kg
−1 [
5]. This HHV is comparable with energy content in different types of coal, such as hard coal (HHV > 23.9 MJ·kg
−1), non-agglomerating highly volatile coals (17.4 < HHV < 23.9 MJ·kg
−1), or lignite (HHV < 17.4 MJ·kg
−1) [
6]. Also, biochar is characterized by high energy density, hydrophobicity, improved abrasiveness and low ash content [
7].
The transformation of MSW into CRDF allows for solving the problem of waste storage and disposal, and the CRDF produced can be a fully-fledged renewable fuel [
8,
9,
10]. Initial economic evaluation of MSW torrefaction has been published by Stępień et al. [
11] where some basic calculations of heat demand for the process were determined. Authors concluded that the heat demand for drying and torrefaction of MSW is ~1.27 GJ·Mg
−1. Assuming the heat utilization rate of 90%, the chemical energy introduced with fuel into a boiler is ~1.41 GJ·Mg
−1. Assuming the use of natural gas (~
$3/GJ; U.S. pricing), the total cost of drying and torrefaction is ~
$4.21·Mg
−1 of MSW. Obviously, that cost will differ for other markets due to fuel prices and fuel type. Additional operation costs were not included [
11], but despite this, the comparison of MSW torrefaction costs with other MSW treatment methods shows that this is a competitive technique [
12,
13].
The production of CRDF can, therefore, be considered as a viable solution to the problem of management of emerging municipal waste. Thermal treatment reduces MSW volume and mass. For example, in Poland, organic waste accounts for over 80% of the total MSW [
14]. Transforming it into CRDF via torrefaction or pyrolysis would allow for energy recovery and limiting the demand for disposal and storage. Thus, CRDF can be considered as a future-proof product. Besides the many advantages of CRDF, there are also challenges, e.g., effective bulk storage and transport. CRDF suffers from low bulk density and would therefore incur high transportation and storage costs. It has been reported that biomass densification can improve feedstock uniformity and enhance the handling and conveyance efficiencies [
15,
16].
The combination of pelletization with the thermal process, i.e., pyrolysis or torrefaction, improves the fuel properties of the product as well as the conditions of storage and transport. Solutions combining these processes into one technological line with continuous reactor operation are increasingly used [
17]. The strength of the material is a parameter defining the limit value at which the body will be destroyed or irreversibly deformed. It depends on the type of material, shape, and the size of the sample, as well as the applied load and time [
18]. Two basic static tests are used to determine the basic strength properties and deformation characteristics of the materials: compression and expansion. This makes it possible to determine the maximum compressive strength (CS) followed by the destruction of the material.
The CS of materials is determined by exerting axial thrusts on the analyzed samples using universal strength machines or hydraulic presses. The durability of pelletized biochar is very important due to its transport and storage. As mentioned earlier, the strength of the material, and thus the resistance to deformation, can be expressed using various parameters, e.g., abrasion resistance, brittleness, tensile strength or compressive strength.
The pelleting process increases the energy density, affects the unification of the material and gives it a regular shape. Also, pelleting affects the increase in grindability of the material [
19]. This process is mainly used to improve fuel properties, as, in addition to increased energy density, humidity decreases, and a regular shape facilitates transport and subsequent burning in boilers [
20]. Additives are often used during pelletizing for better compaction and binding of the material. However, binders play a major role in wood pellet characteristics. Additives improve pellet durability and physical quality, reduce the dust potential, improve pelleting efficiency and reduce energy costs [
21]. The maximum content of 2% of additives is permitted in woody pellets [
22]. No limitation exists for non-woody pellets [
23], though it is a requirement to indicate the type and quantity used.
The most common additives are (1) water (used if the moisture content of the mixed material is too low) and (2) binders, which act as glue between the particles if the lignin content of the material is not enough to hold a pellet together. Lignin is a natural, optimal binder of biomass because it melts under the heat of the pellet mill [
24]. However, if the lignin content of the biomass is low, it may be necessary to add other additives. One of the simplest binders is vegetable oil, but the most widely used substance overall is starch [
25]. Obtained CRDF is similar to coal. Typical binders used for coal briquetting are starch, poly(vinyl acetate), molasses, sulfide liquors, carboxyl methylcellulose, tar, pitch, crude oil, clay, cement and sodium silicate [
26].
One feasible additive is sodium silica, also known as water glass [
27,
28], which has been used for the preparation of briquettes from coal [
26,
29,
30] with a ratio of up to 12%. Also, water glass has been proposed to be used as a coating film, making pellets waterproof [
31]. Thus, analogous to coal, for CRDF pelletizing, the use of sodium silica as a binder and coating has been proposed in this research.
To date, there is no work on the structural modification of CRDF through pelletization as preparation for effective storage and transport due to the relatively low exploration of torrefaction of MSW. In this research, the authors propose to address the challenges above by the structural modification of CRDF through the densification of the material and the creation of so-called biochar pellets, which will be similar in mechanical properties to commercially available biomass pellets. To date, there are no published reports on pelletizing CRDF. Thus, the very practical questions to advance CRDF concept are: (1) the required pressure needed for pelletizing, (2) the determination of resulting CS, and (3) the need for pellet binders or coatings. Therefore, this research aimed to determine if:
- (a)
the CS of CRDF pellets increases with the applied pressure during pelletization;
- (b)
the CS of CRDF pellets increases with the addition of water glass as a binder and as a coating, and;
- (c)
the CS of CRDF pellets is comparable to conventional biomass pellets.
4. Discussion
In this work, for the first time, the feasibility of CRDF pelletization obtained from MSW was carried out. Previous work on biochar pelletization concerned products made of lignocellulosic biomass [
15], and torrefied lignocellulosic biomass (woody tropical trees [
16]) which in many cases did not have the status of waste. The analyzed CRDF from the torrefaction of municipal waste at 260 °C and 50 min of retention time was characterized by physicochemical properties similar to those described in the literature. CRDF of a LHV of 25.95 MJ∙kg
−1 (
Table 1) was similar to CRDF obtained in earlier studies [
4] and to biochar from grass produced in a similar temperature range (250 °C to 350 °C) by [
5], which had a calorific value of 25 MJ∙kg
−1 to 30 MJ∙kg
−1. The HHV of CRDF used in this experiment (27.315 MJ∙kg
−1) could define it as hard coal (HHV > 23.9 MJ·kg
−1), according to the classification given by EUROSTAT [
6]. The moisture content of the analyzed material (1.54%) was in the 1% to 6% range presented by Jakubiak and Kordylewski [
19]. Small differences in properties could result from both different CRDF production parameters (temperatures, the residence time in the reactor), as well as from the high heterogeneity of municipal waste used for the production of CRDF.
Previously published research has shown that torrefaction/pyrolysis influences the mechanical strength of the biochar. Emmerich and Luengot [
40] have shown that carbonized material can achieve similar or even better strength parameters than other materials, but it should be produced from a very durable material, e.g., the Brazilian native palm tree. Additionally, the pyrolysis temperature influence on the strength of the material has been reported [
41]. It has been observed that up to a certain temperature (600 °C), the strength decreases due to the decrease in density caused by the ingress of volatiles and moisture, but after evaporation, there is an increase. Noumi et al. [
42] conducted experiments on the strength of biochar from eucalyptus trees formed under different temperature conditions (350 °C and 600 °C), rates of temperature increase (1 °C and 5 °C min
−1) and pressure (2 bar and 6 bar) and concluded that the best strength parameters were associated with biochar produced at a higher temperature, shorter retention time and lower pressure. Additionally, the correlation of mechanical stability with the structure of biochar (porosity) and density was demonstrated.
In this research, it has been proven that densification via pelletization increases the mechanical strength of the biochar. Most previous work on solid fuel densification was done on different types of biomass. Biomass pellets are usually produced at pressures between 1.5 MPa and 300 MPa [
43], and generally, higher pressures give more durable pellets [
44]. Two studies using very low pressures (1.5 MPa) produced poorer pellets compared to standard pellets [
45,
46]. Higher pressures increase durability in cereal residues [
47] and reduce pellet relaxation after formation [
48]. A study on olive pruning residues found no difference in durability in pellets produced between 70 MPa and 175 MPa, although interactions between pressure and other factors suggested 170–180 MPa was optimal [
49]. Another study suggested that only marginal improvements in durability could be achieved in beech and Scots pine above 250 MPa [
50].
In this study, CRDF pellets were produced with pressure ranging from 8.5 MPa to 72.6 MPa, which generally falls in the range of pressures used for pelletizing biomass and confirms the durability increase with the increase of used pressure for pelletization, which is consistent with the results of densification of biochar from woody residues [
44].
The pressure applied during pelletization at an industrial scale can be affected by a number of factors including the motor power, the rolling speed, the density of the feed, and the dimensions and material of the pellet channel [
51]. Two studies suggest an interaction between pressure, temperature and moisture content [
44,
50], i.e., heat and moisture can ease the flow of material through the die, and therefore would need to be optimized for CRDF on a technical scale to ensure that the desired durability is achieved. Another factor may be the type of feedstock and pre-treatment procedure (e.g., application of torrefaction or pyrolysis).
The increase in material density is directly proportional to its strength [
5]. It may be concluded that higher applied pressure result in higher sample density, which is related to the reduction of space in the biochar/biomass structure (lower porosity of the material). This relationship was described by Weber and Quicker [
5]. However, the densification degree, or CS improvement degree, have limitations—threshold values above which the increase of applied pressure during pelletization does not increase the CS of pellets. In this research, this concept has been confirmed. The first (to date) CS analyses of CRDF pellets showed that the CS of compressed material produced with pressures over 50.8 MPa does not result in a significant improvement of CS: i.e., from 3.43 MPa for 50.8 MPa to 3.94 MPa for 76.2 MPa. Taking into account technological and economic considerations, it was decided that the pressure of 50.8 MPa was sufficient to obtain robust CRDF pellets. The strength of CRDF pellets obtained at 50.8 MPa statistically (
p < 0.05) did not differ from those produced at higher pressure, and the compaction process itself was easier and therefore less expensive to implement. The same rationale was used to test the effects of water glass addition.
In the present study, it has been shown that the addition of water glass reduces the mechanical stability of the material. Only a 10% addition of the binder caused a reduction in strength by 0.59 MPa (from 3.43 MPa to 2.84 MPa). In the experiments carried out by Chinmayananda et al. [
27], a slight improvement in CS was found with the use of water glass (ratio 30–35%) as a binding agent. Nevertheless, Chinmayananda et al. [
27] did not specify the origin and properties of the biochar used in research. Thus, further investigation on the influence of the type of feedstock and process temperature on the durability of pellets with water glass should be carried out. In contrast, the strength of the investigated combination coated with water glass was similar to pellets with a 10% addition of water glass. However, authors do not recommend the use of water glass as a binder or as a coating material for CRDF pellets. Additionally, it was found that the water glass coating was troublesome due to the hydrophobic CRDF structure.
The obtained results of CS of CRDF pellets have been compared to biochar pellets obtained by other authors (
Table 6). The CS is reported in units of pressure and force to enable comparisons between different studies. The necessary pressure-to-force conversion involved using the cross-sectional area of a pellet. The CS of most durable CRDF pellets without water glass, expressed in kN, were in the range of 0.405 kN to 0.465 kN, but the addition of or coating with water glass reduced the CS to 0.340–0.346 kN. In comparison, pellets obtained from biochar produced from five different tropical trees had CS values in the wide range of 0.165 kN to 1.469 kN (
Table 6) [
16], which shows that the CS of CRDF pellets is located in the lower half of the values of biochar pellets from tree biomass. This indicates the strong influence of the type of feedstock on the final durability of the carbonized product.
As a part of ‘
Waste to Carbon’ technology development, pellets with comparable strength properties to conventional biomass pellets should be achieved. This is needed to implement easily manageable, durable, highly calorific fuel to the market. The potential end-users have access to mature technology for pellet manufacturing, storage, handling, and utilization technology. On the other hand, carbonized (not densified) CRDF material has properties resembling powdery dust, raising concerns about safety (e.g., self-ignition), storage, transportation, handling, and utilization. Therefore, the CS of CRDF pellets was compared with the CS of biomass pellets available on the market. The tested lignocellulosic pellets and corn husk pellets showed better strength properties than CRDF pellets. Similar conclusions about higher CS associated with fibrous biomass have been reached in [
5], i.e., that biochar produced from biomass has generally poorer strength compared with unprocessed biomass and coal. The CS of lignocellulosic and corn husk pellets was in a comparable (i.e., the same order of magnitude) range of values with the CS of CRDF pellets.
The tested CS of pine pellets was similar (
p < 0.05) to the CS of CRDF pellets produced with the pressure of 50.8 MPa. Obtained CS values for CRDF pellets are comparable to this research and other studies on CS of biomass pellets (
Table 6), which indicates that they could be competitive to biomass pellets on the market.
Based on Chinmayananda et al. [
27], in experiments where pellet binders were used in the present study, we decided to verify the usefulness of water glass addition for the increase of CRDF pellet durability. It has been shown that as a result of compressing CRDF samples, the air was displaced from the inter-granular space, the particles were closer together and the grains connected together. Therefore, CRDF compressed at 76.2 MPa resulted in a higher sample density than the ones produced at lower pressures. There are also visible differences in the volume of obtained pellets, which is also related to the porosity of the material. The higher compressive pressure caused a reduction of volume in the obtained sample. In the case of pellets with an added binder, the density of the material increased with the water glass addition. A similar observation was made in the case of structural modification involving the water glass coating. However, a bulk density analysis of (50.8 MPa) CRDF with and without the addition of water glass showed that water glass filling the spaces between the grains reduced the density of the material.
Finally, further investigation on smaller doses of water glass, and with other binding agents, could be pursued. It is recommended to conduct experiments on the influence of the addition of binding agents to CRDF on firing process conditions, properties of ash, and emitted gasses. The aforementioned test should be completed on both lab and technical scales for determination of binding mechanisms and interaction between molecules in the hydrophobic material, as well as for further evaluation of the technological, and economic feasibility of CRDF pelletization. Pelletization of the CRDF in this research was carried out on a small, lab scale testing machine. It is necessary to introduce a structure modification solution using production-scale pelletizing equipment to optimize implementation, verify performance, and to obtain data for economic analyses. This research has proven that CRDF pelletization and the production of durable CRDF is possible on a lab scale. The technology of pelletization is well known and may be easily used for CRDF pelletization. As the next step of RDF torrefaction technology development, a full-scale test is warranted to assess the energy demand for initial grinding and pelletization.