Char Preparation Research Articles

Biochar gasification: Insights from pyrolysis atmospheres and gasification heating rates

Gasification is a promising technology for the synthesis of syngas and hydrogen, with char reactivity being a pivotal factor in the design of industrial gasifiers. Despite extensive studies on the gasification characteristics of biochar, the influence of the pyrolysis atmospheres on the evolution of biochar pore structure and its reactivity in gasification remains poorly understood. This study aims to bridge this gap by comprehensively investigating the effects of char preparation atmospheres (i.e., CO2, H2O, and N2) on ensuring gasification reactivities at varied heating rates. Biochars were synthesized in a horizontal tube furnace at 700 °C with a low heating rate (10 °C min−1) under three different atmospheres: N2, CO2, and H2O. Subsequent gasification reactivities were investigated using i) a thermogravimetric analyzer (TGA) at a low heating rate (10 °C min−1) and ii) a wire mesh reactor (WMR) at a high heating rate (1000 °C s−1), both under CO2 atmosphere at 950 °C. The results showed that the presence of an active atmosphere (CO2 and H2O) during pyrolysis significantly impacts the biochar’s aromaticity, crystalline structure, and surface morphology. The functional group concentration and surface area trends indicated that CO2 and H2O atmospheres during pyrolysis enhance biochar reactivity compared to the N2 atmosphere, with the highest reactivity observed for biochars produced under H2O. Furthermore, biochar gasification reactivity was markedly higher using the WMR compared to TGA, underscoring the pronounced effects of heating rate on gasification behavior. The high heating rate in WMR preserved the biochar’s porous and amorphous structure, whereas the TGA gasification encountered diffusional limitations and thermal annealing effects. These insights underscore the need to consider pyrolysis atmosphere and heating rate in gasification studies, offering a more comprehensive understanding of biochar’s behavior under practical gasification conditions.

Optimization of nitrogen-doped sludge char preparation and mechanism study for catalytic oxidation of NO at room temperature

Catalytic oxidation of NO at room temperature was carried out over nitrogen (N)-doped sludge char (SC) prepared from pyrolysis of municipal sewage sludge, and urea was adopted as nitrogen source. The effects of different N-doping methods (one-step and two-step method), dried sludge (DS)/urea mass ratios (5:1, 4:1, 3:1, 2:1, and 1:1), SC preparation procedures (pyrolysis only, pyrolysis with acid washing, and pyrolysis with KOH activation and acid washing), and different pyrolysis temperatures (500, 600, 700, and 800°C) on the catalytic oxidation of NO were compared to optimize the procedure for SC preparation. The results indicated that N-doping could obviously promote the catalytic performance of SC. The one-step method with simultaneous sludge pyrolysis (at 700°C), KOH activation, and N-doping (DS/urea of 3:1) was the optimal procedure for preparing the N-doped SC with the NO conversion rate of 54.7%, whereas the optimal NO conversion rate of SC without N-doping was only 47.3%. Urea worked both as carbon and nitrogen source, which could increase about 2.9%-16.5% of carbon and 24.8%-42.7% of nitrogen content in SC pyrolyzed at 700°C. N-doping significantly promoted microporosity of SC. The optimal N-doped SC showed specific surface areas of 571.38 m2/g, much higher than 374.34 m2/g of the optimal SC without N-doping. In addition, N-doping also increased amorphousness and surface basicity of SC through the formation of N-containing groups. Finally, three reaction paths, i.e. microporous reactor, active sites, and basic site control path, were proposed to explain the mechanism of N-doping on promoting the catalytic performance of NO.

Combustion and flue gas evolution characteristics of rice husk in various mixtures and layers with Al2O3 particles and char prepared with different pyrolysis parameters

During pyrolysis and combustion experiments, rice husk and Al2O3 were combined in two ways, namely blending (rice husk was blended with Al2O3) and covering (rice husk was covered by Al2O3) modes, respectively. Rice husk biomass (RHB) char was prepared under different pyrolysis conditions. The resulting combustion characteristics and corresponding gaseous evolution of the biochar were compared. The maximum combustion rate decreased as particles accumulated, causing a shift in the thermo-gravimetric curve to higher temperature ranges. The combustion reaction was hindered in the covering mode. The combustion reactivity of the prepared char decreased as the char preparation time increased. During the char oxidation process, the release amounts of H2O and CO2 from char combustion increased first and then decreased, while the release amounts of CO, CH4, and organic components containing C=O gradually decreased with increasing char preparation time. Char prepared with the covering mode exhibited higher burn-out rates and combustion indices but lower activation energies required for combustion reaction. Additionally, the covering mode delayed the timely release of gases from the biomass heating in air, and the quantity of combustion gas released from the char produced at covering mode was greater than that released from the char produced at blending mode. The results obtained can improve the understanding of stacked biomass particles combustion process.

Effect of Fenton pre-oxidation on the physicochemical properties of sludge-based biochar and its adsorption mechanisms for ammonia nitrogen removal

Fenton pre-oxidation, as a promising technology for sludge deep dehydration, has been used intensively; however, the outputs (i.e., low-water-content sludge cakes) need to be further disposed for a full line sludge resource utilization. Carbonization presents a promising approach for disposing of the sludge cake within the process. Here, the physicochemical properties of sludge-based char obtained after Fenton pre-oxidation were studied. Both Fenton pre-oxidized sludge-based char (FSC) and raw sludge-based char (RSC) were produced within the temperature range of 400–600 ℃, and the adsorption performance of the prepared chars, exemplified by their ability to adsorb ammonia nitrogen (NH4+), was tested under various conditions. FSC exhibited superior char yield and adsorption performance compared to RSC, with chemical adsorption being the primary controlling factor. The optimum conditions for FSC were observed at 400 ℃, resulting in a char yield of 72.2 %, and an adsorption capacity of 221.7 mg/g. Notably, FSC demonstrated an increase in C-C, C-N, OC-O, C-O and CC functional groups in comparison to RSC, which suggests that pre-oxidation increases organic matter decomposition and chemical reactivity of the sludge. The introduction of Si-O-Si, Si-O-C and Fe-O-H functional groups in the FSC facilitated ion exchange with NH4+ in an aqueous solution. The key findings indicate that Fenton pre-oxidation not only enhances the adsorption performance of FSC but also reduces the required carbonization temperature for excellent-char preparation. These results demonstrated the potential to integrate sludge deep dewatering, including pre-oxidation, with char preparation.

Establishment of correlation between reaction kinetics and carbon structures in the char gasification process

For a gasification process, the char-CO2 gasification is the controlling step worthwhile to be deeply investigated. The article chosen corn stalk (CS), poplar sawdust (PS) and bagasse residue (BR) as the typical waste species derived from agricultural, forestal and industrial sources. The char-CO2 gasification behavior, reaction kinetics and carbon structure were studied to reveal the intrinsic factors determining the reaction kinetics. Generally, the carbon conversion and maximum conversion rate were influenced by the feedstocks species and char preparation temperatures, as influenced by ash proportion, potassium content in ash and carbon structure of char. The char-CO2 reaction for CS was subject more to the catalytic effect of alkali compositions, while pore structure affected more the gasification reaction for PS char. The isoconversional kinetic analysis indicated that the gasification reaction became stable at carbon conversion of 0.5. Subsequently, sectionalized kinetic parameters were calculated for the initial gasification temperature to the temperature reaching 50% conversion. The result showed that high initial gasification temperature increased the char-CO2 gasification barrier to hardly start the reaction but accelerate the reaction rate. The carbon structure analyses further clarified that the reaction activation energy was highly related to the microcrystalline structure of carbon, while the reaction rate was more determined by carbon pore structure.

Effect of Carbonization Temperature of Rice Husk Char Preparation on SiC Structure and Composition of By-Products in SiC Synthesis with Magnesiothermic Reduction Method

The structure of rice husk char is one of the factors that influence the morphology of SiC and the composition of by-products in the magnesiothermic reduction. This structure can be improved by carbonization. Therefore, this study aims to examine the effect of the carbonization temperature of rice husk char on the structure of SiC and identify the by-products formed by the magnesiothermic reduction. The rice husk char was made by carbonization at various temperatures of 700, 800, 900, and 1000 °C. They were converted to SiC by magnesiothermic reduction at 600 °C, washing with hydrochloric acid, calcination at 700 °C in an air atmosphere, and washing with a mixture of hydrofluoric acid and acetic acid. The products were characterized by FT-IR, XRD, and SEM-EDS. The results showed that creating SiC using the magnesiothermic method with carbonization at a temperature of 1000 °C produced a polytype 2H-SiC, and the by-products were MgO, Mg2SiO4, and Si. MgO was successfully separated in the washing step with a hydrochloric acid solution, while Mg2SiO4 was separated from SiC by washing using a mixture of hydrofluoric acid and acetic acid.

Effect of Char Temperature on CO2 Gasification of High Ash Coal and Biomass

AbstractInformation on comparative gasification reactivity of high temperature chars in CO2 with high ash coal (C), sawdust (SD) and rice husk (RH) under identical condition is important for clean energy development drive particularly in countries like India, where energy matrix is dominated by coal and introduction of larger quantity of biomass in energy‐mix is attracting global interests. Therefore, TGA‐studies on char‐gasification reactivity using CO2 as a gasifying agent were performed at temperatures 850–1000 °C. In case of coal and RH‐chars, CO2 reactivity found to increase with increasing the BET surface area and reverse trend was observed with SD‐chars. Both the biomass‐chars showed increased specific micro pore surface area with increasing char preparation temperature through t‐plot analysis, still keeping unusual behavior of CO2 reactivity of SD‐char unexplained. The estimations indicating the presence of catalytically active alkali and alkaline earth metals in substantial quantity is only important criteria found to explain the decreasing trend of gasification reactivity of SD‐char prepared at various temperatures despite the increase in surface area. Catalytic activity of the components decreases with the rise in char preparation temperature. Kinetic analysis reveal that activation energy values of SD‐chars varied in the range123–182 kJ mole−1, whereas those for coal‐ and RH‐chars remained in the range 204–214 kJ mole−1.

Real-time monitoring of alkali release during CO2 gasification of different types of biochar

Potassium and sodium compounds play both positive and negative roles during biomass gasification, but the detailed behavior of alkali metal compounds remain incompletely understood. In this study, alkali release during CO2 gasification of biochar is characterized online with a surface ionization method in combination with thermogravimetric analysis of the char samples undergoing gasification. For wood chars, the alkali release rate follows a slowly decreasing trend as the char conversion proceeds, but increases by up to two orders of magnitude when the conversion approaches completion. In contrast, the alkali release from straw char is 40–50 times higher than observed for wood char and decreases continuously during the whole gasification process. A high temperature and a high CO2 concentration enhance both alkali release and char reactivity. The char preparation method also influences the alkali release from pine char, while the char reactivity is less affected. Alkali release and char reactivity are linked, but other factors including mineral content, surface area and char structure may play important roles for the observed reactivity. The results provide a basis for understanding of alkali behavior during gasification and may help optimize catalytic effects and reduce detrimental issues in biomass gasification.

Adsorption of dental clinic pollutants using bone char: Adsorbent preparation, assessment and mechanism analysis

The assessment of bone char for the adsorption of toxic dental clinic pollutants is reported. Different conditions for bone char preparation via pyrolysis and its adsorption properties were evaluated using fluoride, mercury and arsenic as the target pollutants. The best pyrolysis conditions to improve the adsorption properties of this adsorbent for the removal of these dental clinical pollutants were identified. The best bone char was employed to quantify the experimental isotherms of the adsorption of mercury, fluoride and arsenic at pH 6 and 20 – 40 °C. Results showed that the dehydroxylation of hydroxyapatite contained in the bone char affected its adsorption properties for the removal of these pollutants and not only its defluoridation properties as has been already reported in the literature. It was concluded that the degree of hydroxyapatite dehydroxylation affected the ligand exchange involved in the adsorption mechanism of bone char for the removal of these adsorbates. Theoretical calculations with statistical physics and artificial neural networks indicated that fluoride and mercury adsorption on bone char was an endothermic multi-molecular process, while the arsenic adsorption was also endothermic but implying mono-adsorbate interactions. Bone char showed the best maximum adsorption capacities for fluoride (0.4 – 1.58 mmol/g) followed by mercury (0.14–0.23 mmol/g) and arsenic (0.06 – 0.14 mmol/g), respectively. Calculated adsorption enthalpies for these pollutants ranged from 25.4 to 39.4 kJ/mol. A detailed description of the adsorption mechanism and possible interactions involved in the removal of these dental clinic pollutants with bone char were also provided. This study contributes with theoretical and experimental results to support the development of an in situ solution for facing the water pollution problem caused by the dental clinics, which is an unregulated health care sector in terms of local and international legislations for water quality protection.

Effect of biochar and hydrochar from cow manure and reed straw on lettuce growth in an acidified soil

Soil acidification has become a major environmental and economic concern worldwide. Char materials (e.g biochar, hydrochar) have attracted considerable attention as soil amendments to restore degraded soil. However, the comparative study of biochar and hydrochar on plant growth in acidified soil is limited. In this study, a microcosmic experiment was used to compare the effect of biochar and hydrochar from cow manure (CBC, CHC) and reed straw (RBC, RHC) on the growth of lettuce in the acidified soil. CBC and RHC significantly increased and decreased the biomass of lettuce by 18.7% and 32.5% in the acidified soil, respectively. The increase of the lettuce growth by CBC primarily attributed to the enhancement of soil properties (SOM and pH) and soil nutrient content (Olsen-P, K, Ca, Mg, Zn, Fe and Mn). Moreover, CBC enhanced the microbial activity and complexity and increased the abundance of beneficial genera like Gemmatimonas, Ramlibacter and Haliangium, which improved soil health and might indirectly benefited the lettuce growth. Our findings highlighted the priority of char materials feedstock and preparation technology for remediating the acidified soil.

Structure evolution of lignite char in step pyrolysis and its combustion reactivity

Step pyrolysis was conducted in a laboratory-scale fixed bed reactor for char preparation. Characterization techniques such as volume solvent swelling, thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were applied to reveal char structure evolution during step pyrolysis. Combustion reactivities of the prepared chars were examined at a thermal gravimetric analyzer. Newly formed bond structures in char such as Cal-Cal and Car-Cal/O have been identified at the 1st stage of step pyrolysis, which evidences the occurrence of cross-linking reactions. As a result, step pyrolysis has higher char yield than direct pyrolysis. The char yield from step pyrolysis increases as the 1st stage pyrolysis temperature increases and reaches a maximum at 773 K. An index was put forward to assess the extent of cross-linking reactions during pyrolysis, which is well correlated with char production. The char from step pyrolysis has relatively high graphitization degree than from direct pyrolysis, and thus has lower combustion reactivity. Step pyrolysis can suppress sulfur and nitrogen release in comparison with direct pyrolysis owing to the formation of more stable sulfur and nitrogen species in char. The difference in nitrogen release varies within 8 percentage points and for sulfur release, it is within 5 percentage points under the examined experimental conditions.

Catalytic coal gasification for methane production: A review

Catalytic coal gasification for methane production is a promising technology in the clean coal utilization field. In this review, the technologies for coal-derived natural gas production, the catalytic coal gasification processes and the used reactors were compared. The compared catalysts mainly included single-component, composite, and disposable catalysts. The effects of catalyst properties included composition, preparation method, supporter and loading amount were further illustrated. The influences of coal properties included char preparation method, particle size, and ash content on catalytic performance were investigated. The effects of temperatures and pressures on gasification performance were discussed in details. The evaporation, melting, decomposition, and inactivation of catalyst under various temperatures and pressures were also analyzed. It is expected to provide comprehensive information on the researches of catalytic coal gasification for methane production.

Influence of cooling rate on structure and combustion reactivity of char during char preparation process

Different chars with various cooling rates were obtained in the novel concentrating photothermal reactor to investigate the influence of cooling rate on the structure and combustion reactivity of char. The chars were prepared at 900°C with holding time of 10s, 30s and 120s, respectively. Subsequently, the cold (ex-situ) chars were prepared from the hot (in-situ) chars by controlling different cooling rates of 180°C/s (fast cooling rate), 30°C/s (sub-fast cooling rate), 6.8°C/s (moderate cooling rate) and 1.3°C/s (slow cooling rate). The ex-situ chars were then employed for the combustion in 30%O2/70%CO2 atmosphere at 900°C for 180s. For comparison, the in-situ char was conducted by instantaneously switching the reaction atmosphere. For the chars with holding time of 10s, the mass fraction of in-situ char was higher than the char yield of ex-situ char, and the char yield was decreased with the decrease of cooling rate. The char yield with slow cooling rate was decreased by 11.16% than that with fast cooling rate. The maximum and average mass loss rate of char in O2/CO2 was decreased as the cooling rate decreased. Reduced influence of cooling rate on the aromatic ring systems of char with holding time of 30s and its reactivity was displayed, while almost no difference of those with holding time of 120s was illustrated. The reactivity was closely related to the chemical structure of the char, but no positive correlation was found between the combustion reactivity and the specific surface area of char. The residues proportion of volatile components was the decisive factor of char reactivity.

Early-stage kinetics and char structural evolution during CO2 gasification of Morupule coal in a wire-mesh reactor

Understanding the intrinsic gasification kinetics and structural evolution of char is crucial in the effective design of industrial gasifiers. This work studies the kinetics of Morupule coal gasification in atmospheric pressure CO2 at isothermal temperatures of 900–1050 °C using a rapid heating wire-mesh reactor (WMR). The gasification reaction order was determined by varying the concentration of CO2 in the reactor atmosphere. The bulk chemical structure and surface chemistry of the early-stage residual chars were characterised using Raman and X-Ray photoelectron spectroscopy, respectively. Gasification reaction rates were significantly higher than those presented in published literature with an activation energy of 320 kJ mol−1 and a pre-exponential factor in the order of 1010 s−1. In a chemical reaction controlled kinetic regime, the reaction order was found to be zero and increased to 0.6 in a mixed chemical reaction plus pore diffusion controlled kinetic regime. The bulk chemistry of the chars was largely unaffected by CO2 in the conversion range studied. However, surface chemistry analyses revealed a decrease in the carbon content, through the depletion of CC/CC bonds, resulting in the development of char surface porosity. Residual chars from gasification experiments showed a lower combustion reactivity to those produced under inert pyrolysis conditions. This study provides insights on direct gasification (i.e. no char preparation in a separate pyrolysis stage) for kinetic determinations and a concomitant extensive characterisation of char structural properties.

Production of Silicone Tetrachloride from Rice Husk by Chlorination and Performance of Mercury Adsorption from Aqueous Solution of the Chlorinated Residue.

The production of silicone tetrachloride (SiCl4) from rice husk char by chlorination was investigated, and the effect of the char preparation temperature on SiCl4 volatilization and the coexisting element species in the char was examined. The behavior of chlorine (Cl) and the change in pore properties during char chlorination were analyzed, and the reaction mechanism was discussed. The performance of Hg ion removal of the chlorination residue was also investigated. At 1000 °C chlorination, the optimum rice husk pyrolysis temperature for attaining high ash-release extent was 800 °C. Ash volatilization during char chlorination with heat treatment mainly occurred at >300 °C and reached a release extent of ∼75% by 1000 °C. Si and P volatilization started at >300 °C and reached 70–75% by 1000 °C. In contrast, Na and K the volatilization occurred at >700 °C, with a 50% volatilization extent by 1000 °C. Mg and Ca had a volatilization rate of <20% by 1000 °C. When the char was held at 1000 °C, the release extent of Si and P reached 75–80% by 10 min. Na and K volatilized almost completely by 10 min, and the release extent of Mg and Ca increased with increasing holding time and became 10–50% by 60 min. The Cl content in the residue obtained at each chlorination temperature increased from 300 to 700 °C and then decreased with increasing temperature. The majority of Cl taken up in the residue was an H2O insoluble form. The surface area and pore volume of the chlorinated residue tended to increase with increasing chlorination temperature, with the former increasing to 335 m2/g at 1000 °C and 10 min holding. The maximum mercury adsorption amount of the chlorinated residue obtained at 1000 °C, 10 min holding was 620 mg/g, indicating the mercury ion adsorption performance of the chlorinated residue.

Volatile-char interactions during biomass pyrolysis: Effect of char preparation temperature

In this study, the effect of char preparation temperature on the interactions between cellulose volatiles and acid-washed sawdust char was investigated experimentally on a fixed-bed pyrolysis system. The results indicated that significant volatile-char interactions did exist at the pyrolysis temperature of 500 °C as evidenced by the great changes in the composition and distribution of pyrolysis products. The oxygen-containing functional groups as well as the aromatic ring systems in the char both acted as active sites during the volatile-char interactions. The changes in chemical structure of biochar caused by the different preparation temperatures would notably affect the final products of cellulose pyrolysis. Meanwhile, the acid-washed sawdust char was still found to participate in the reaction process, lower temperature chars would have higher reactivities, and an obvious weight loss of char was also observed after interactions. In addition, volatile-char interactions significantly increased the yields of non-condensable gases, especially those of CO and CO2, while decreased the yield of condensable vapors. The introduction of biochar into cellulose pyrolysis could promote the ring scission of pyranoses as well as the decarbonylation/decarboxylation and dehydration reactions, thus caused the yields of anhydrosugars and monoaromatic compounds decreased and the yields of light ketones and acids increased.

Volatile–char interactions during biomass pyrolysis: Understanding the potential origin of char activity

To understand the potential origin of char activity responsible for volatile evolution during biomass pyrolysis, the interactions between benzyl phenyl ether (BPE, a typical lignin dimer) and pinewood chars prepared under a series of thermal, acidy, and steamy conditions were investigated. The results showed the activity of low-temperature char on BPE conversion was mainly attributed to the surface O-containing functional groups. The BPE conversion decreased as the temperature for char preparation raised, resulting from the elimination of char surface functional groups to a large degree at high temperature. The low activity of high-temperature char on BPE conversion could be recovered by acid-washing to release metal-occupied carbon based active sites (e.g., small aromatic rings), and further promoted by steam activation to modify the surface property and porous structure, finally achieving a full conversion of BPE and high selectivity to the products of phenol and toluene.

Effects of total pressure and CO2 partial pressure on the physicochemical properties and reactivity of pressurized coal char produced at rapid heating rate

Abstract The effects of the total pressure (0.1–1.3 MPa) and the CO2 partial pressure (0–0.45 MPa) on the physical structural properties, chemical structural properties and reactivity of Shenhua coal pressurized char at 1273 K were investigated in a pressurized drop tube furnace (PDTF). There were reverse-direction effects on char yield between total pressure and CO2 partial pressure for char preparation. Porosity characteristics were performed using an ASAP2020 instrument. It was found that same-direction increment effects on BET surface and porosity between total pressure and CO2 partial pressure for char preparation. Carbon structure and surface structure were performed by Raman and XPS methods. The results showed that both total pressure and CO2 partial pressure can promote the transformation of small aromatic structures to large aromatic structures and increase the graphitization degree of the char. With increasing total pressure and CO2 partial pressure, the unstable N-5 content decreased, while the N-6 and N-Q content increased.

Catalytic effect of rice straw‐derived chars on the decomposition of naphthalene: The influence of steam activation and solvent treatment during char preparation

AbstractRice straw char (RSC) was prepared as the catalyst for naphthalene decomposition in this work. The effects of steam activation and solvent treatment on the catalytic naphthalene decomposition during the char preparation were investigated. Four chars were prepared: RSC, steam‐activated RSC at 500°C (SRS500), steam‐activated RSC at 800°C (SRS800), and solvent‐treated RSC (ResC). The catalytic activity of each catalyst was discussed, and the catalytic mechanisms were also proposed. The results revealed that the SRS catalysts gave a higher catalytic activity than the RSC, due to the enhancement of the surface area after steam activation. The SRS800 gave the highest naphthalene conversion and net gas yield: approximately 76.9% and 45%, respectively. However, its catalytic activity was more rapidly decreased by coke deposition than the other chars. Compared with the RSC, the ResC showed a slightly lower naphthalene conversion, but the net gas yield was slightly higher, especially that of H2 and CH4, due to the existence of K in silicate form.

The influence of manure feedstock, slow pyrolysis, and hydrothermal temperature on manure thermochemical and combustion properties

Slow pyrolysis and hydrothermal carbonization (HTC) of organic wastes for char preparation has been proved as an effective way for livestock manure management. Livestock manure chars were prepared by slow pyrolysis (400, 500, 600 °C) and hydrothermal carbonization (180, 210, 240 °C) at different reaction temperatures. The influences of manure type and reaction condition to element content, calorific value, char yield, energy yield, and combustion characteristic were investigated. The results illustrate that thermochemical process can strongly affect the properties of pyrolytic char and hydrochar. Compared to pyrolytic char, the hydrochar had higher heating value, higher energy yield, and lower ash content with respect to the same feedstock. The livestock manure type could also influence the properties of biochars/hydrochars. Hydrochars from swine manure, broiler litter, and layer chicken litter achieved the highest energy yield of 65.5%, 56.9%, and 64.4% at 210 °C. Dairy cattle manure and beef cattle manure displayed higher energy yield and higher comprehensive combustibility index than other manures. Furthermore, HTC can narrow the weight loss temperature range in differential thermogravimetric curve of manures. Therefore, HTC is considered as a more effective approach in carbonizing animal manure for solid biofuel compared to slow pyrolysis.