**5. Applications of Hydrochar**

HC is considered a valuable material for various agricultural, environmental, and industrial applications. The high versatility of HC allows for many applications, including soil amendments in agriculture, solid fuel in power generation, electrode materials in energy storage technologies, adsorbents in contaminant removal, and materials used as sensors and fuel cell catalysts [34]. HC could show different properties based on HT technology, process conditions, and feedstock used. Due to the formation of hydrocarbons on the surface, the surface area and porosity of HC are generally low, hindering its application. High surface area and porosity are vital for contaminant adsorbents and catalysts/catalyst support applications. The physicochemical properties of HC can be altered and improved through different activations. Chemical activation is achieved by impregnating HC with one or a mixture of chemical agents, followed by an activation process under a nitrogen flow. The most used chemical activating agents for the chemical activation process are potassium hydroxide (KOH), phosphoric acid (H3PO4), Zinc chloride (ZnCl2), sodium hydroxide (NaOH), and potassium carbonate (K2CO3) [69]. Biochar has a high degree of carbonization, a highly aromatized carbon structure, lower H/C, and O/C ratios, with a strong anti-decomposition ability (environmentally more stable). But the cost of biochar production was higher. Alternatively, HC has higher H/C and O/C ratios than biochar and is mostly composed of aliphatic hydrocarbons; hence, its environmental stability is lower. The lower H/C and O/C ratios of biochar indicate a higher degree of aromaticity and maturation than HC [70].

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

Due to low bulk and energy densities, high moisture, and ash contents, untreated biomass is a poor-quality fuel. Biomass is usually pelletized to increase bulk density. But, long-term storage of biomass pellets causes moisture adsorption and biochemical and microbiological activities [71]. Granulated HC showed higher densities. HC densities in the range of 180 to 482% than raw biomass pellets made from four types of biomasses (pine sawdust, coconut fiber, coconut husk, and rice husk) have been obtained. HC pellets showed lower moisture uptake than raw biomass [72]. Consequently, HC has better fuel characteristics compared to biomass. HC needs to meet certain fuel characteristics, including energy density, combustion behavior, grindability, hydrophobicity, and thermal stability, to be effective as a replacement for coal. Its fuel-related characteristics are similar to lignite and can be used in power generation. But the chemical composition is different. HC has a significantly higher amount of volatiles than lignin. Also, the oxygen content of HC is higher than lignite. HC has a similar HHV as lignite. HC could be easily incorporated within existing coalbased processes. During the HTC process O/C ratio of the solid fraction is reduced, leading to increased HHV [28, 32, 37, 69, 73]. A high HHV (20.6–29.2 MJ/kg) of HC has been reported by many researchers as comparable to soft coal (20.93–33.5 MJ/kg) [51, 73, 74]. In addition to increased HHV, HC has lower volatile content (compared to biomass), which ensures better combustion. The low ash melting temperature of biomass makes burning more complicated. Due to the removal of many ash-forming minerals in liquid fraction, HC might have reduced mineral content. As a result, HC can achieve similar ash melting temperature as lignite. Only low-ash HC from certain biomasses (low ash) is suitable for power generation. High ash biomasses include sludge from wastewater treatment plants, agricultural residues, grasses, straws, etc. [28, 32, 37, 73].

The direct use of HC in agricultural and environmental applications might be complicated due to the presence of phenolic and organic acid compounds on the HC surface, which can cause negative plant and microbial responses. -Post-treatments, such as composting or anaerobic digestion, can reduce the toxicity of HC and make it suitable for soil application [70]. The factors, such as feedstock used, production process and process conditions, nature of the HC, morphological properties, and nature of the soil (loamy clay, fertile, sandy, or infertile), are important for HC application of crop improvement in the agricultural sector. The application of HC could result in either productive or counterproductive crop yield response. As a result of the low quantity of polar functional groups on the surface of the freshly produced HC, it shows hydrophobicity. However, due to oxidation by interacting with atmospheric oxygen, HC becomes more hydrophilic in nature by creating phenolic and carboxylic functional groups on the surface over time when mixed with soil. The water-holding capacity, cation exchange capacity, and nutrient retention capacity would increase significantly due to these functional groups on the surface [69]. The ability of HC to increase the nutrient supply for plants and decrease leaching losses makes it a good soil amendment to improve soil nutrient retention capacity. HC has a porous structure, low surface area, charged surface, and functional groups, including carbonyl, carboxyl, hydroxyl, and phenolic hydroxyl groups, to remedy the soils contaminated with heavy metals and organic compounds through adsorption [54]. Increased fertility has been achieved by the direct application of HC as a soil amendment, in addition to the formation of stable carbon sinks [75].

Reduced N2O emission was observed in studying the effect of corn-based HC on the soil. HC was less effective as it is less stable than biochar. However, its production costs are less. Other studies have found that HC reduced N2O emissions and

contributed to NH3, CO2, and CH4 emissions [69, 76]. The nutrient level of HC (especially plant biomass derived) is low; still, it can be used as fertilizer. HC can reduce loss from the surface runoff of fertilizers, enhancing fertilizer use. A decrease in total nitrogen, total phosphorus, nitrate, phosphate, and organic carbon has been reported using fertilizer containing an HC additive. HC addition improved fertilizer runoff and retention. Adding HC to soil could increase nutrient capacity and water retention in soil. Nutrients accumulated in the HC pores are released as needed. Adding HC to soil could change water aggregation, pH, cation, and anion exchange capacity. The addition of HC might improve the water-holding capacity of sandy soils. The colonization of the HC surface by fungi has been observed due to the hydrophobicity of the HC, which had a negative effect on water retention [76].

Increasing levels of HC in soil have been shown to deteriorate the growth of the Taraxacum plant. Some studies reported the negative effects of HC caused by nitrogen on crop yields. The absence of nitrogen migration in the first week of HC addition and the slow release of nitrogen with time has resulted in the nonavailability of nitrogen to the plants. Mixing HC into the soil for several weeks before planting has been suggested to overcome this. More research is needed to understand the ecotoxicological properties of HC and evaluate the impact of HC on soils. This will help lower the negative effects of HC in soil improvement for agricultural applications [69].

Energy storage becomes more critical with the increasing use of renewable energy. Carbon-based materials are widely utilized in capacitors. Thus, HC is an attractive material for energy applications. Its properties include surface area, electrical conductivity, tunable pore structure and size, easy accessibility, and strong mechanical properties. These are favorable for applications as supercapacitors and as anode and cathode material for batteries and fuel cells [69, 77, 78]. Due to surface area, polarity, porosity, aromaticity, and stability, HC has gained attention for electrochemical devices such as supercapacitors and batteries. Compared to the lower cycle stability, discharge/charge rate, and higher energy density of rechargeable batteries, HC-based supercapacitors usually exhibit higher cycle stability and power density [69]. Very high capacitance has been reported of HC derived from different sources and activated with KOH at different temperatures. Successful results have been reported of HTC nanospheres as anodes in Li+ and Na+ batteries. Their reversible capacity has reached up to 370 mA h/g at a 1C rate, which was better than that of traditional graphite electrodes. In another study, corn straw-based carbon spheres have been used as an anode in Li+ batteries. The device showed excellent cycle stability with a specific capacity of 577 mA h/g after 100 cycles at 0.2C [77, 78]. Corncobs-based HC having a high surface area has been investigated as a carbon source. Sulfur loaded on HC achieved a discharge capacity of 1600 mA h/g and a reversible capacity of 554 mA h/g after 50 cycles [69]. Walnut shell-derived HC, activated HC/ZnO composites, and activated HC has been investigated for supercapacitors. The specific surface areas of 819 and 1073 m2 /g have been obtained for activated HC/ZnO composite and activated HC, respectively. The specific capacitance of activated HC/ZnO composite was 117.4 F/g at a current density of 0.5 A/g in KOH aqueous solution, which was found to be stable for 1000 cycles [79].

HC can also be used as an adsorbent to remove impurities in aqueous solutions. Depending on the raw material and manufacturing conditions, HC has a wide range of sorption properties [76]. The factors that determine the adsorption efficiency of HC include the specific surface area, pore structure, and surface functional groups. The functional groups on the surface give the HC a high chemical affinity and hydrophobicity, which has good potential adsorption applications [75, 78]. Relative

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

to biochar, HC has a lower surface area. However, the adsorption capability of the HC is higher than biochar due to the abundance of oxygen-rich functionality and the presence of functional groups such as carbonyl, carboxyl, and hydroxyl groups on the surface [69]. Some of the weaknesses of HC include poor sorption properties compared to other adsorbers, high amounts of volatiles, low pore volume, low surface area, and negative surface charges, which repel negatively charged compounds such as phosphate. Despite the weaknesses, sorption properties for polar and nonpolar contaminants have been confirmed [76, 80]. HC subjected to chemical activation has better properties, which can be done during or after the HTC process. After chemical activation, enhanced sorption of heavy metals has been reported. Lanthanum activation has shown to be effective in phosphate removal as it can the negative surface charges of the HC. The results showed a maximum absorption of 61.5 mg/pg. [76, 81].

Heavy metal removal using HC has attracted many research interests. Different mechanisms, such as complexation, physical adsorption, precipitation, and electrostatic interactions, can be used to extract heavy metals. The research focused on heavy metal remediation using high-temperature HC has been reported. HC produced at high temperatures has improved surface area, less volatiles, increased ash content, and reduced functional groups. Higher adsorption of Cu by HC (compared to biochar) has been reported due to large quantities of functional groups found in HC. Removal of lead using HC derived from pine wood and rice husk has also been reported. The presence of oxygen functional groups on the HC surface was the primary factor influencing the strong removal capacity of heavy metals [51]. The comparison of the performance of switchgrass-based HC, KOH-activated HC, and activated carbon to remove Cu and Cd from the aqueous solution has shown close to 100% adsorption for Cu and Cd in 24 h by activated HC relative to the HC and activated carbon. The adsorption of lead from an aqueous solution using pinewood and rice husk-derived HC produced by HTL was favored at high temperatures as it is a physical endothermic process [69]. HC has effectively removed Cr (VI) from an aqueous solution. Low pH has been shown to give maximum adsorption efficiency. Pinewood sawdust-derived HC activated with H2O2 has been reported to have enhanced adsorption efficiency. The uptake of Pb2+ ions was 42 times higher [78, 82].

Adsorption of organic contaminants, including common inflammatory drugs (diclofenac sodium (DCF) and ketoprofen) and fungicides (diphenylamine), have been performed successfully using cellulose-derived carbon spheres. Successful removal of DCF from aqueous samples using KOH-activated HC produced from municipal woody and herbaceous pruning has been demonstrated [78, 82]. Absorption of pharmaceuticals (salicylic acid, flurbiprofen, and DCF) using orange peel-derived HC activated with H3PO4 has shown to be effective [83].

HC could serve as an adsorbent for capturing CO2 to mitigate CO2 quantities in the atmosphere. HC is economical, but for effective adsorption of CO2, it needs posttreatments. The CO2 adsorption potential of HC derived from woody and herbaceous pruning activated with KOH at higher temperatures was tested. The HC showed uptake of 84.5 mg CO2/g. HC has excellent prospects as a cost-efficient and environmentally friendly material for CO2 capturing, treating, and monitoring wastewater [78]. Sugarcane bagasse-derived HC and KOH-activated HC have been used to study CO2 adsorption, which resulted in higher affinity for N2 and CO2 at 50°C by activated HC [69]. In another CO2 adsorption study, silver fir sawdust-derived HC and KOHactivated HC were used in a pressure swing adsorption that resulted in 6.57 mmol/g of CO2 adsorption at 5 bars with HC. The performance of HC was higher than activated HC and some traditional sorbents [84].

Dye removal by chemical and biological methods is effective, but they produce a lot of by-products. Natural physical adsorbents are environmentally safe, cheap, and abundant and can provide a better solution to this issue [48]. Activated carbon is the widely employed adsorbent for dye removal. The use of activated carbon is limited due to its high cost. HC has proven to be an efficient and economically viable adsorbent for the treatment of dye-contaminated water. Studies have shown that the adsorption performance of HC is most in favor of cationic dyes (methylene blue, rhodamine b, and methyl green) rather than anionic dyes (methyl orange and acid red 1). LCB-derived HC and sewage sludge have shown slightly lower adsorption capacity than commercial activated carbon. The same study reported a higher dye adsorption capacity of HC (128.6 to 160.5 mg/g) compared to biochar (12 to 130 mg/g). HC has been shown more effective in removing cationic dyes such as malachite green (10– 40 mg/g) and methylene blue (15–45 mg/g) from the aqueous solution than anionic dye methylene orange (< 6.5 mg/g). HC could be used as an industrial adsorbent and a potential low-cost replacement for activated carbon after undergoing activation [51, 75]. Chemically activated HC could be used as a low-cost adsorbent to remove malachite green [85]. A maximum adsorption capacity of 34.9 mg/g of HC has been achieved in removing methylene blue dye from an aqueous solution using coffee husk-derived HC. In another study, the maximum adsorption capacity was 97 mg/g of HC in removing congo red dye using bamboo-derived HC [69]. A new chitosan-based adsorbent has been synthesized and used to remove methylene blue from wastewater. The adsorbent had outstanding reusability and a maximum adsorption capacity of 215.73 mg/g at 318.15 K based on Langmuir isotherm. Another novel chitosan-based adsorbent has been prepared and tested for methylene blue, methyl orange, and rhodamine B removal. The adsorbent had excellent reusability and ease of separating them from the solution using a magnetic field, and the maximum adsorption capacity for rhodamine B was 191.57 mg/g at 25°C. Carbon-coated polyacrylonitrile nanofibers have been used to adsorb methylene blue, and the adsorption capacity was 153.37 mg/g at room temperature. Even after 5 cycles, adsorption efficiency remained high [48].

Applications of HC in wastewater treatment have been extensively studied. HC has been shown to remove about 55% of gross pollutants from blended (50% of raw +50% of post HTL) wastewater. In the same study, ammonia, COD, nitrate, and phosphate removal efficiencies were 48, 53, 59, and 60%, respectively [21, 51, 86]. Sewage sludge-derived HC and KOH-activated HC has been used to remove orthophosphate (anions) and copper (cations) from wastewater, revealing that 97% of orthophosphates were removed through activated HC at 6 g/L, a higher adsorption capacity than HC [69]. HC is porous, has functional groups, and has a high content of microand macronutrients; therefore, it is a highly favorable and suitable environment for microorganisms to grow and can be used as an additive for anaerobic digestion. The addition of HC has been reported to increase methane yield in anaerobic digestion. HC has been shown to increase COD removal capacity in anaerobic digestion [51].
