Research ArticleAPPLIED SCIENCES AND ENGINEERING

# Bio-coal: A renewable and massively producible fuel from lignocellulosic biomass

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Vol. 6, no. 1, eaay0748

## Abstract

Development of renewable energy is essential to mitigating the fossil fuel shortage and climate change issues. Here, we propose to produce a new type of energy, bio-coal, via a fast pyrolysis coupled with atmospheric distillation process. The high heating values of the as-prepared bio-coals from the representative biomass are within 25.4 to 28.2 MJ kg−1, which are comparable to that of the commercial coals. Life cycle assessment further shows that the bio-coal production process could achieve net positive energy, financial, and environmental benefits. By using available biomass wastes as feedstock, China is expected to have a total bio-coal production of 402 million tons of standard coal equivalent, which is equal to 13% of national coal consumption. It would grant China an opportunity to additionally cut 738 million tons of CO2 emission by substituting an equal amount of coal with bio-coal in 2030.

## INTRODUCTION

Excessive exploitation and consumption of fossil fuel will not only gradually exhaust its storage in the earth but also cause severe climate change and environment pollution problems (1, 2). In comparison, biofuels can be massively produced and are recognized as a promising alternative for future energy (3, 4). The Energy Independence and Security Act of the United States has anticipated the yield of 16 billion gallons of cellulose-derived biofuels in 2022 (5). However, most of the current biofuels are produced from grain, which inevitably impairs the global food supply. Renewable bio-oil obtained from the fast pyrolysis of lignocellulosic biomass has been found to be an alternative for grain-derived biofuels and undergoes practical applications (6, 7). Nevertheless, since bio-oil is a multicomponent mixture including water, hydrocarbons, and oxygenated compounds (8), it has some undesirable properties, such as strong corrosivity, low heating value, and chemical instability (9). Therefore, great efforts have been made to upgrade bio-oil to obtain high-quality liquid fuels or chemicals (10). For instance, hydrocarbon biofuels (gasoline, diesel, and jet fuel) were produced through several conversion routes from pyrolytic bio-oil (11). An integrated catalytic approach to convert pyrolytic bio-oil into industrial commodity chemicals (C2 to C6 monohydric alcohols and diols, C6 to C8 aromatic hydrocarbons, and C2 to C4 olefins) has been developed by Huber and co-workers (12). Although great progress in upgrading bio-oil has been achieved and a suite of technologies have been demonstrated at pilot scale, the inherent drawbacks of bio-oil such as thermal polymerization and causing poison of catalysts still present as a big challenge to impede its massive applications.

Atmospheric distillation is a simple and cost-effective technique to separate components from mixtures and has been widely used in industries for over one century. However, such a mature technique has not been successfully used to separate bio-oil because only a small quantity of distillates can be recovered owing to the thermal polymerization properties of bio-oil. Bio-oil is very thermo-unstable and would form coke when being heated, which hinders further distillation. Although molecule distillation has been used to separate bio-oil, the recovery efficiency of carbon content in light distillates is low (~17% of C in bio-oil) (13). To date, lack of effective separation methods has hindered the development of renewable fuel production from biomass pyrolysis.

In pyrolysis, lignin, cellulose, and semicellulose, the main components of biomass are quickly thermo-chemically decomposed in seconds to form small-molecule compounds, in which about 50% of the volatiles can be condensed to form bio-oil. The carbon content of bio-oil ranges from 30 to 50%, and the high heating value (HHV) is about 15 MJ kg−1. Apart from organic compounds, bio-oil contains about 30% water, which notably reduces the HHV. Thus, it is highly desirable to elevate the HHV of the residues by removing the moisture than obtain the light distillates with a low HHV. Therefore, instead of alleviating the thermal polymerization of bio-oil by racking one’s brain, we propose a new strategy to treat bio-oil. We aim to obtain a solid fuel by accelerating the thermal polymerization of bio-oil. Such a thermally polymerized residue of distillation with a high HHV is named bio-coal. Producing bio-coal has several merits: (i) Bio-coal could be quickly prepared at large scale to partially replace coal; (ii) “carbon-neutral” utilization of biomass is realized to mitigate the global warming problem; (iii) liquid chemicals are harvested without the need of catalysts; (iv) bio-coal can be long-term stored and conveniently transported; and (v) bio-coal can be a carbon warehouse when it is not used. This work would open a shortcut for partially resolving the fuel and environmental crisis faced by the world.

## RESULTS AND DISCUSSION

### Properties of bio-coal

The bio-coal preparation process is illustrated in Fig. 1A. Briefly, the renewable biomass (e.g., rice husk, saw dust, corn stalk, etc.) was first fast pyrolyzed at 500°C under anaerobic atmosphere to produce bio-oil and biochar. Then, the bio-oil was distilled under air atmosphere from room temperature to approximately 240°C to obtain the liquid chemicals and bio-coal. The preparation of bio-coal involved conventional chemical engineering processes and did not need complicated and costly refinery equipment or operation, which could be readily scaled up.

The rice husk–derived bio-oil was first distilled to prepare bio-coal, and its properties were characterized. The element analysis shows that C and H in the bio-coal were notably higher, and O was lower than that in the bio-oil after the distillation (Table 1), indicating that the oxygen-containing groups were taken off. The as-prepared bio-coal was a blocky solid with glossy black surface, which is similar to commercialized coal in color. The scanning electron microscopy (SEM) images show that bio-coal was an amorphous and imporous bulk (Fig. 1, B and C). More images of the bio-coal are provided in fig. S1.

Table 1 Element analysis (weight %) and physical properties of bio-coal.

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Thermogravimetric analysis (TGA) was then applied to evaluate the thermostability of the obtained bio-coal. The bio-coal had a negligible mass loss below 250°C (Fig. 1D), which was generally attributed to the evaporation of physically adsorbed water and the nondistilled organic compounds. A considerable weight loss was observed in the temperature region of 300 to 750°C, where almost ~62% weight loss occurred. It has been reported that the weight loss of coals usually mainly occurs in a temperature range of 100° to 800°C in differential thermal gravity (DTG) curves (14), implying that the bio-coal has similar thermogravity properties with commercial coals.

Mass energy density is the most important criterion to evaluate the quality of solid fuel. The HHV of bio-coal was ~25 MJ kg−1 (Fig. 1E). The mass energy densities of coals A (Alberta sub-bituminous Coal), B (Indonesian Tinto Coal), C (Australia Collie Coal), D (China Pindingshan Bituminous Coal), and E (China Yangquan Anthracite coal), which were acquired from previously reported literature, are ~21, ~28, ~26, ~26, and ~19 MJ kg−1, respectively (1518). Thus, the mass energy density of the bio-coal was much higher than that of coals A and E and slightly lower than that of coals B, C, and D, indicating that the bio-coal is a potential alternative to the commercial coals.

The content of heavy metals (e.g., Cd, Pb, and Zn) is a very important factor to evaluate solid biofuels as they can be retained in particulate matter (PM2.5 or PM10) after combustion and then float in the air and cause health issues through breathing (19). The species and contents of heavy metals in the bio-coal are presented in Table 2. The contents of Cd, Pb, Cr, Zn, and Mn were very low, while Cu and Ni were even not detected, suggesting that the combustion of the bio-coal would not cause heavy metal–related pollution.

Table 2 Contents of heavy metals in the bio-coal (weight %).

ND, not detected.

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In the atmospheric distillation process, the C content in the residue increased continuously from 34.75 to 64.82% and was greatly enriched in the final bio-coal (Fig. 2A). On the contrary, the O content decreased continuously from 55.93 to 28.19%. The H content also decreased continuously from 8.04 to 5.88%, while the N content remained at around 1 ± 0.3%. The C/O and H/O ratios increased in the atmospheric distillation process, which led to the improvement of the HHV (13.2 MJ kg−1 for the bio-oil and 25.4 MJ kg−1 for the bio-coal). The bio-oil and residues retained in the atmospheric distillation process were also characterized by Fourier transform infrared spectroscopy (FTIR) (Fig. 2B). The FTIR spectrum of the bio-oil shows its high oxygen functional groups with strong absorption peaks of O─H (3408 cm−1) and C═O (1709 cm−1) moieties, as well as phenolic hydroxyl groups (1211 cm−1) and C─O─C groups (1082 cm−1). Carbon existed in the form of aromatic rings (1609, 1512, and 1450 cm−1) and aliphatic moieties (2964, 2928, and 1377 cm−1). No obvious differences were observed from the FTIR spectra of the atmospheric distillation residues and bio-coal, indicating that no new functional groups were formed in the atmospheric distillation.

### Effect of biomass type on the quality of bio-coal

Considering the universality of the proposed bio-coal production route, five common biomass wastes, including rice husk, saw dust, wheat straw, bagasse, and soybean straw, were selected as representatives to produce bio-coals (fig. S2). The bio-coal yields derived from the bio-oil were 45.2, 37.2, 33.9, 41.8, and 34.3%, respectively. Moreover, the bio-coals had similar element compositions (67 to 70% carbon, ~6% hydrogen, 22 to 27% oxygen), as well as low nitrogen (<1.6%) and sulfur (<0.8%) content (Table 3). The five types of bio-coals obtained from the different biomass wastes exhibited similar functional groups, as revealed from the FTIR results (fig. S3). In addition, the estimated mass energy densities of the rice husk–, saw dust–, wheat straw–, bagasse-, and soybean straw–derived bio-coals were 25.4, 28.0, 28.2, 26.3, and 27.6 MJ kg−1, respectively, which were comparative to those of commercial coals. These results indicate that the proposed route could be used as a universal method to produce bio-coal from worldwide biomass wastes.

Table 3 Production and element composition of the bio-coals.

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### Energetic, financial, and environmental footprints of bio-coal production

Two scenarios composed of fast pyrolysis for bio-oil production (scenario A) and fast pyrolysis plus atmospheric distillation for bio-coal production (scenario B) were assessed in life cycle assessment (LCA) (Fig. 3A). In scenario B, some chemical products could be derived from bio-oil after the two-stage distillation, leaving bio-coal as fuel simultaneously (fig. S4). Before the distillation, there were 51% oxygen, 40% carbon, 8% hydrogen, and 1% nitrogen in the bio-oil. After the distillation, the yields of the bio-coal and other chemical products were 50 and 15%, respectively.

For the different end uses of biochar, another subscenario was additionally analyzed, in which biochar was used as fuel in scenarios A1 and B1, while as soil amendment in A2 and B2. Results show that the different uses of biochar would result in a varied output while keeping the input unchanged (Fig. 3, B and C). In scenario A, the energy consumption, greenhouse gas (GHG) emission, and economy cost stemmed mainly from the pyrolysis stage. The net energy production yielded roughly 10,752 MJ/ton of dry rice husk in scenario A1, where biochar was used as a coal substitution, and 4532 MJ/ton of dry rice husk in scenario A2, where biochar was applied for soil amendment. For net GHG emission, there was a negative output around −450 kg in scenario A1, while there was a positive output around 906 kg in scenario A2. Although using biochar as fuel would result in a raised energy production, 42 g/MJ energy GHG would be discharged rather than sequestration of 200 g/MJ energy with soil application of biochar. The net economic performances remained similar between the two subscenarios, as the economic revenue of using biochar as coal substitution was cost-effective.

In comparison, two-stage distillation was applied in scenario B for the production of bio-coal and chemicals, which would further increase the overall input compared to scenario A. In the distillation process, the energy consumption, carbon dioxide emission, and capital costs for every ton of dry rice husk were 2531 MJ, 478 kg, and US$64, respectively. For the distillation products, the bio-coal was used to substitute the equivalent coal, which could additionally provide 6505 MJ energy and US$24 profit per ton of dry rice husk compared to scenario A. Moreover, chemical products would bring US$546 under scenario B (details are listed in tables S1 to S3). Since the bio-oil is not used as fuel, about 456 kg of carbon dioxide emission could be reduced, bringing a cost compensation of US$12.3 (scenario B1) and US$39 (scenario B2). The LCA results show that the net energy and economic revenues were positive under both scenarios A and B. However, for GHG emission, the revenue would be negative when biochar was used as fuel. Use of biochar as soil amendment could enhance the fixation of both carbon and nitrogen from fertilizer (20, 21), and thus, there should be an extra benefit for GHG net for scenarios A2 and B2, which is not considered in the present analysis. In addition, since the GHG emission in China is large (22), the government has to make a plan to improve environmental quality (23). Here, in terms of mitigation of climate change, it will be more advisable to apply biochar as soil amendment (scenarios A2 and B2). In system B2, only half of bio-oil was converted into bio-coal as fuel. In this case, the output of net energy production was reduced from 6600 to 3974 MJ in system A2, and the net GHG sequestration was reduced by 22 kg. However, the net economic revenue of scenario B2 was expected to reach US$525, as the chemical products doubled the benefit of bio-oil.

### Benefits for developing bio-coal–based renewable energy

The above results show that it is feasible to use the widely produced and easily accessed biomass such as forestal and agricultural wastes to prepare bio-coal by using our proposed route. It is estimated that the global biomass production is around 146 billion tons/year (24), of which about 3% is agricultural wastes. Thus, it would be a great chance for developing countries to substitute the conventional fossil fuel–based energy with bio-coal–based renewable bioenergy. Taking China as an example, about 402 million tons of bio-coal could be produced with agricultural and forestal wastes, which is equal to 384 million tons of standard coal equivalent (tce) (table S4). Heilongjiang, Henan, Shandong, and Jilin provinces are found to be the four highest bio-coal production areas, which account for 37% of total agricultural and forestal wastes production (Fig. 3D). At present, coal is still the dominant energy source in China. In 2017, a nationwide energy of about 4.49 billion tce was consumed, 60% of which stemmed from coal (25). By using bio-coal as an alternative, 14% of coal equivalent could be reduced, which would reduce the pressure for coal demand.

## SUPPLEMENTARY MATERIALS

Table S1. Consumptions in thermal process.

Table S2. Prices of the bio-chemicals (found in the quotes online).

Table S3. Prices of products of the system-B (US\$/dry rice husk metric ton).

Table S4. Estimated available biomass residues (Qar) and bio-coal (Qbc) amount (million tons).

Table S5. Ultimate and proximate analyses of rice husk (weight %, on dry basis).

Table S6. Ultimate analyses of products of fast pyrolysis.

Fig. S1. SEM images of the bio-coal derived from rice husk.

Fig. S2. Photographs of the five typical biomass wastes used in this work.

Fig. S3. FTIR spectra of the five bio-coals derived from the typical biomass wastes.

Fig. S4. Compositional variations of products after atmospheric distillation.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

## REFERENCES AND NOTES

Acknowledgments: Funding: We acknowledge the financial support from the National Natural Science Foundation of China (21677138 and 51821006). Author contributions: H.J. and H.-Q.Y. designed this work. B.-H.C., Y.-L.C., X.S.Z., and S.F.J. performed the experimental investigations. B.-C.H., R.Z., and Y.L. developed the model and performed the data analyses. All authors contributed to the interpretation of the findings. B.-C.H., H.J., and H.-Q.Y. led the writing of this paper. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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