Sr. No. |
Biomass Type |
Blending Ratio |
Process and Type of Reactor |
Process Condition |
Outcome |
Application |
Ref. |
1 |
Waste epoxy resin and fir |
Mixing ratio of fir:waste epoxy resin is 1:3 |
Co-torrefaction Conventional heating batch-type reactor |
Temperature: 120 °C–180 °C, time: 10 min–40 min |
Solid yield 76.86%. Enhancement in HHV 1.12 Energy yield 85.79% Improved evaporation of volatile compounds. Solid yield adversely affected |
Improvement of biochar |
[23] |
2 |
Sewage sludge and Leucaena |
Mixing ratio of sewage sludge:Leucaena is (75:25%) |
Co-torrefaction Microwave heating |
Microwave power level 100 W, time: 30 min, temperature: 170 °C–390 °C |
Bio-char made from pure Leucaena wood has a CO2 adsorption capacity of 53 mg/g |
Solves waste-water problem. Production of biofuels |
[44] |
3 |
Biomass and coal |
Blending ratio of biomass:coal is (30:70%) |
Vertical tubular furnace |
Temperature: 300 °C, time: 60 min |
Produced mass yield: (57.0–63.8%), energy yield: (77.0–89.0%), (18.1–22.2%) reduction in CO2 emissions |
Enhances the quality of coal |
[45] |
4 |
Microalgae and Lignocellulosic biomass |
- |
Co-torrefaction A gas chromatographic furnace with a glass reactor |
Temperature: 250 °C, time: 60 min |
Better temperatures (92.6%) result in higher energy efficiency, but the moisture content of the feed mixture quickly decreases this efficiency (16.9 to 57.3% for 70% moisture) |
High production of bio-char with high calorific value |
[35] |
5 |
Mango seed and passion shell with optoelectronic sludge |
Blending optoelectronic sludge with mango seed in a 25/75 ratio |
Wet co-torrefaction Microwave reactor |
Temperature from 120 °C to 180 °C), reaction duration from 10–40 min |
Higher heating value of 19.0 MJ/kg, 92.1% of energy yield, fuel ratios of 1.60–1.82, and an energy return on investment of 14.7% |
The production of fuel of the highest grade |
[46] |
6 |
Food sludge and lignocellulosic biowaste |
Mixing macadamia husk and sludge in a (25/75%) ratio (db%) |
Wet co-torrefaction Microwave reactor |
Temperature: 150 °C, duration: 20 min |
HHV:19.6 MJ/kg; decreased ash content; first-order kinetics; increased thermal stability and combustion efficiency of biochar; 7.4 energy return on investment; 45.2% reduction in carbon gas emissions |
Production of bio-solid and nutrient recovery |
[41] |
7 |
Empty fruit bunch pellet, used cooking oil, and waste engine oil |
- |
Co-torrefaction Microwave reactor |
Temperature: 200, 250 °C and 300 °C, heating rate: 50–65 °C/min, time: 5–8 min |
There is an 85.5 wt% mass yield Fuel ratio: 1.8. Carbon content: 68.3%. Fixed carbon: 62.3%. HHV: 28.0 MJ/kg. |
Production of solid fuel with greater improvement |
[30] |
10 |
Hemicellulose, cellulose, lignin, xylan, dextran, xylose, and glucose |
Weight ratio (1:1:1) |
Co-torrefaction Conventional heating thermogravimetry |
Temperature: 230 °C, 260 °C and 290 °C |
There is no synergistic effect of co-torrefaction on weight loss of the blend |
- |
[47] |
11 |
Textile sludge and lignocellulose biowaste (macadamia husk) |
- |
Wet co-torrefaction |
Temperature: 120 °C–180 °C, time: 10–30 min |
Amount of fixed carbon: 29.8%, HHV: 19.7 MJ/kg |
Production of biofuel |
[41] |
12 |
Mango branches (MBr), waste newspaper (Np), and low-density polyethylene (LDPE) |
Three binary mixtures prepared, with a mass ratio of 1:1 |
Bench-scale tubular reactor |
Temperature: 300 °C |
(MBr-LDPE) carbon content: 71.94% HHV: 35.84 MJ/kg |
Improved fuel characteristics that allow co-firing |
[48] |
13 |
Food sludge and six widely produced lignocellulose bi-wastes |
Blending ratios of 0/100, 25/75, 50/50, and 100/0 |
Microwave heating system |
Torrefaction temperature (120, 150, and 180 °C), reaction time (10, 20, and 30 min) |
Food sludge blended with macadamia husk (25/75 db%) highest fixed carbon content (25%) HHV: (19.6 MJ/kg) |
Renewable energy resource. |
[41] |
2. Co-Torrefaction Mechanism and Operation Parameters
2.1. Co-Torrefaction Process
Co-torrefaction occurs when two biomass blends undergo a process and are converted into a bio-solid.
Figure 2 illustrates the process of co-torrefaction. In
Figure 2, two residual/waste biomass were blended together in various blending ratios (0:100, 25:75, and 50:50%). The feedstock was placed into the furnace for pre-treatment (co-torrefaction) and run under vacuum with a supply of nitrogen, so that an inert atmosphere was created in the furnace in order to avoid the possible ignition of the sample. The duration of this step depended on the flow rate and size of the furnace. After purging the supply of nitrogen was interrupted, the sample was kept in a crucible placed in the central furnace with a heating rate of 10 °C/min in the temperature range of 200–300 °C for a residence time of 30 min to 2 h
[49]. Three products (bio-oil, bio-solid, and bio-gas) were obtained from the furnace following the co-torrefaction of blending the residual biomass. The main outcome of the co-torrefaction process is a high-quality bio-solid product
[50]. The co-torrefaction process is endothermic at low temperatures, but progresses toward an exothermic process when char is formed during the thermal degradation of lignocellulosic biomass at high temperatures
[51]. During the degradation of the components, various reactions are involved. The first stage is to remove the moisture content at 110 °C. The following stage is to remove inbound moisture or a fully moisture-free environment when the temperature increases to 200 °C. At 200 °C, the torrefaction process begins to decompose volatile matter to produce solid, liquid, and gaseous products. At 200–250 °C, the stage of decomposition of hemicellulose occurs that is characterized by limited devolatilization, and a solid structure is formed. During this stage, C-C, C-O, and inter- and intramolecular hydrogen breakdowns occur, which form condensable liquids and non-condensable gases. The stage of 250–300 °C is the extensive part of the torrefaction process in which hemicellulose decomposes into volatiles and solid products are formed
[42].
Figure 2. Co-torrefaction process.
2.2. Synergistic Effect
When two or more biomass wastes combine to generate a more significant impact than either of them could produce alone, this is called a synergistic effect. When materials are combined, synergistic effects may be used to increase co-torrefaction yields. Furthermore, the combination of OS with MIse and Passiflora edulis shell (PEsh) for wet torrefaction (WT) has a synergistic impact on the increase in HHV content in co-torrefied bio-solid, especially in a 75/25% ratio
[46]. As a consequence of these results, combining OS with fruit bio-waste is an additional effective way to help the process, involving the betterment of the bio-solid as a product. Therefore, it is likely to be used instead of traditional fuels in the future (e.g., coal)
[52].
2.3. Operating Parameters
The co-torrefaction process utilized a variety of biomasses that were thermochemically processed and acquired desirable qualities. During the co-torrefaction of biomass, numerous operating parameters affected the co-torrefaction process, such as the role of temperature, residence duration on mass and energy yields, and the HHV of biomass, and the Van Krevelen diagram.
2.3.1. Studying the Role of Temperature and Residence Time on Mass and Energy Yields
The mass and energy yields of the co-torrefied biomass varied with temperature and the reaction time. The increase in temperature and residence time decreased the mass and energy yields, while the energy density increased. The mass yield of OS decreased when the co-torrefaction temperature increased from 120 °C to 180 °C, from 98.4% after 10 min at 150 °C to 79.9% after 30 min at 180 °C. The main constituents of raw sewage (such as low-molecular-weight hydrocarbons) were degraded with the increasing co-torrefaction intensity. This reaction had an energy density of 1.14 and a 100% energy yield at 150 °C for a reaction time of 30 min
[46]. During co-torrefaction at a temperature of 150 °C and a reaction time of 10 min, a further 99.4% energy yield was obtained with an associated energy density of 1.01
[46]. As a result, unnecessary energy consumption is reduced, and a high HHV of bio-solid is obtained
[46]. During 20 min of torrefaction at 150 °C, 95.2% of the energy was extracted, with a maximum energy density of 1.20.
The mass and energy yields were affected by various blend ratios and types of biowaste used
[29]. The OS and bio-waste co-torrefied together produced more than 80% of the total mass and energy yield. These were the same yields reported for the microwave-based torrefaction of OS, which may have been due to the heating of the samples from the inside at lower temperatures and for shorter periods, leading to their higher energy efficiency
[53]. This might be because microwave-irradiation heating modes are more energy efficient, as they can heat the interiors of materials at lower co-torrefaction temperatures for shorter periods of time
[54]. The bulk bio-solid yields decreased when the ratio of the OS/biowaste blend was reduced from 75/25 to 25/75%, especially in the case of the PEsh OS blend
[46]. This phenomenon occurs because biomass has a higher microwave-absorption capacity than sludge, resulting in the considerable devolatilization of biomass as the percentage of bio-waste in the mix increases
[55]. Because MIse and PEsh have lower energy densities than those of pure OS, they increase the energy density of pure OS. When OS is combined with MIse and PEsh bio-wastes, the energy and mass yields are the same. When the OS/bio-waste blending ratio was altered from 75/25% to 25/75%, the bio-solid mass yields decreased from 95.1% to 92.1% for OS combined with MIse and 93.4–65.2% for OS mixed with PEsh. These results are consistent with the other research investigating the co-torrefaction of sewage sludge and Leucaena using microwave heating
[54]. Note that when the OS–PEsh and OS–MIse co-torrefied bio-char was mixed at 50/50 and 25/75%, the mass recovery and energy yields of the OS–PEsh co-torrefied bio-solid were substantially lower than those of the OS–MIse co-torrefied bio-solid
[46]. Furthermore, when bio-waste and sludge are mixed for co-torrefaction, heat can degrade a significant amount of hemicellulose and cellulose, reducing the mass and energy yields of bio-char while maintaining its higher energy content
[55]. Furthermore, bio-solids produced from co-torrefied food waste offer an improved substitute for peat in terms of their thermal qualities when combined with sugarcane bagasse, rice straw, Pisdium guajava, Annona squamosal, macadamia husk, and pistachio husk, respectively
[41].
2.3.2. Studying the Role of Temperature and Residence Time on HHV
The quality of a bio-solid can be significantly influenced by the proportions of biomass used in the mixing process. The combination of OS with MIse and PEsh bio-waste generates a bio-solid with different HHVs. The Mlse and PEsh biowastes were observed to have experimental HHVs of 19.4 and 18.6 MJ/kg, respectively, which was significantly higher than OS (15.5 MJ/kg) after 30 min of torrefaction at 150 °C; microwave-assisted WT was used to mix textile sludge and lignocellulose bio-waste, and bio-char HHV increased in the same proportion as the blending ratios of the two types of bio-waste increased. Following 30 min of torrefaction at 150 °C, it was revealed that the maximum high-heating values of OS mixed with MIse and PEsh were better than those obtained with the other blending ratios (75/25 and 50/50%). These were 19.0 and 18.3 MJ/kg, respectively
[46]. The resulting bio-solid had a maximum of 19.2–21.1 MJ/kg HHV, which was an increase over lignite coal (19.2–21.1 MJ/kg)
[56]. A total of 55% of the carbon in bio-char was fixed carbon compared to raw food sludge (FS). The fixed carbon and ash contents of biomass increased when the FS and bio-waste were mixed. As a result, agricultural bio-waste can be appropriately disposed of by reusing it as renewable energy
[57]. As the ratio of blending for bio-waste increased, the HHVs increased more than the FS; the energy density of the subsequent bio-solid also increased. Sewage sludge and Leucaena co-torrefaction produced a similar outcome. When bio-solid was created from torrefied food scraps, it had a significantly higher HHV than bio-solid created from torrefied food scraps alone (19.2–20 MJ/kg). The HHV content of FS with MH (25/75% dry basis) presented the highest amount of investigational HHV
[41].
This is consistent with those previously described for torrefied wood and agricultural biomass after hydrothermal carbonization. The higher degree of carbonization of torrefaction significantly accelerated cellulose and hemicellulose degradation, resulting in a reduction in smoke (from fly ash, CO
x, NO
x, and SO
x) produced during biofuel combustion
[58]. The increase in the temperature and reaction time of torrefaction steadily increases the HHV. The result is affected more by the reactor time of co-torrefaction than by the temperature of the OS bio-solid. The heating value of the bio-solid was 24.1 MJ/kg, at a temperature of 300 °C at a residence time of 45 min. The blended FS bio-solid had a maximum heating value of 18.9 MJ/kg at 150 °C at 30 min. The HHV of bio-solid from FS was 21.7% higher than that of the raw sludge
[41]. The increase in torrefaction temperature decreased the mass yield from 84.2% (120 °C for 30 min) to 67.7% (200 °C for 30 min). It could be associated with protein breakdown and polysaccharides in solids of sludge
[59].
2.3.3. Van Krevelen Diagram
The Van Krevelen diagram was first used to categorize the coal and estimate the compositional change throughout maturity by plotting O/C against H/C. In order to better understand fuel quality, one must consider the atomic ratios of the constituting elements. The HHV of biomass, for example, ranged from approximately 20.5 to 15 MJ/kg as the oxygen–carbon ratio increased from 0.86 to 1.03
[60].
The Van Krevelen diagram also compares torrefied and untorrefied biomass. Torrefied biomass has a higher carbon content and decreases oxygen and hydrogen contents compared to untorrefied biomass. The other aspect is that co-torrefied biomass has lower oxygen-to-carbon and hydrogen-to-carbon ratios compared to untorrefied biomass, as presented in Figure 3. Untorrefied biomass, such as OS 100%, MIse 100%, EFB 100%, Cv 100%, and Lc 100%, have low HHVs due to the higher O/C and H/C ratios, and co-torrefied biomass, such as OS:MIse (25:75%) torrefied = 150 °C, EFB with used UCO torrefied = 300 °C, Lc 50% torrefied = 300 °C, and Lc 100% torrefied = 300 °C, have high HHVs due to the low O/C ratio, as depicted in Figure 3. This discussion shows that torrefied biomass has better fuel than untorrefied biomass.
Figure 3. Van Krevelen diagram redrawn using the data from
[61].
Figure 3 also presents the O/C and H/C values of the coal. Anthracite has elaborated low values of the O/C and H/C ratios and presents high-solid-fuel properties. After comparing the torrefied with untorrefied biomass in different literature surveys, it can be observed that the un-torrefied biomass outlies the coal value of the O/C and H/C ratios. However, the ratios of H/C and O/C of the torrefied biomass are close to those for coal. For example, EFB pellets with UCO, T = 300 °C show that the O/C- and H/C-ratio values are very similar to anthracite coal, showing that this biomass has a good fuel quality.
Fuel quality decreases as the O/C and H/C ratios increase. A decrease in the O/C ratio was compared to the raw materials Cv and Lc in bio-solids formed at 200 °C and 225 °C, indicating some deoxygenation. However, no structural alterations were observed because the H/C ratio was equivalent to the feedstock. Thermal processes, such as bio-solid torrefaction, results in O/C and H/C ratios compared to peat, lignite, and anthracite coal, further demonstrating the impact of temperature on fuel quality
[29]. A temperature-dependent decrease in the H/C ratio was also observed at co-torrefaction temperatures higher than 250 °C. This indicates that the carbonaceous structure is reorganized as more aromatic compounds are produced
[62]. The lignocellulosic structure of the bio-solid undergoes an enhanced rearrangement under high-torrefaction conditions, altering the porosity of the material by eliminating OH-binding groups
[63]. Compared to raw biomasses, bio-solids also have a higher energy content due to their lower moisture content
[61]. The calorific value of co-torrefied biomasses depends on the oxygen, hydrogen, and carbon content present in the feed. The feed contains a significant amount of oxygen and a low carbon content, so its calorific value is low and vice versa
[64]. The primary objective of pre-treatment is to improve the carbon content and reduce the oxygen level. Co-torrefaction is used to reduce the oxygen concentration of biomass, which directly affects the heating value of any fuel and presents a higher calorific value. As a result, it is challenging to convert biomass into liquid fuels with an improved heating value. Products can be produced from the biomass of high-oxygen or -hydrogen contents
[65].