What is Hydrothermal Carbonization?
Hydrothermal carbonization (HTC) is a thermochemical conversion process that uses heat to convert wet biomass feedstocks to hydrochar. HTC is performed in a reactor at temperatures ranging from 356 to 482°F, i.e., 180 to 250°C, under autogenous (automatically generated) pressure, with feedstock residence time ranging from 0.5 to 8 hours [1,2]. The major advantage of HTC over other high temperature thermochemical conversion techniques such as pyrolysis, is the HTC process treats wet waste, which allows feedstocks to be converted without pre-drying. A wide variety of feedstocks, including aquatic biomass, agricultural residues, and industrial and animal wastes, are suitable for HTC (Figure 1) [3]. Water acts as a good medium for heat transfer in HTC, but if variability in the feedstock particle size is too large and reaction time is too short, there might be some mass transfer limitations. Hence, the particle size should be homogeneous to ensure uniform heat and mass transfer. The feedstock fed into the reactor is heated to a set temperature and held for a specified residence time. Gases (primarily CO2) and an aqueous slurry (mainly water with a small fraction of organics and solids) are produced during HTC. The aqueous slurry is centrifuged or filtered to separate the process water and solids (wet cake). The wet cake is then dried to produce a carbon-rich hydrochar. 
Multiple reactions occur during the HTC process, namely hydrolysis (reaction with water), dehydration (removal of water), decarboxylation (removal of carboxyl groups which results in the liberation of CO2), and aromatization (formation of aromatic compounds). These reactions occur under high temperature and pressure and play a vital role in lowering the hydrogen to carbon (H/C) and oxygen to carbon (O/C) ratios to produce the carbon-rich hydrochar. Hydrochar is regarded as the primary product, because its properties enable it for use in a variety of applications, such as a solid fuel, an adsorbent for removing pollutants from water/wastewater streams, e.g., phosphorus from agriculture runoffs, and a soil amendment [4]. The distribution of solid, liquid, and gaseous products is largely influenced by the choice of feedstock and process conditions, primarily temperature and residence time.
What Affects Hydrochar Yield?
Hydrochar yield, the ratio of hydrochar to feedstock dry weight (Eq. 1), depends upon the type of feedstock, the solids loading (ratio of feedstock to water), and the process temperature and residence time (Table 1). In general, hydrochar yield decreases with increased severity of process conditions, i.e., higher temperature and longer residence time, which decomposes more of the cellulosic and hemicellulosic fractions in the feedstock. Despite low yield, at higher temperatures and longer residence times, the hydrochar has a higher carbon content with a higher heating value (HHV) [5].
$$ \text{Hydrochar yield (%)} = \frac {\text{Weight of hydrochar}} {\text{Weight of dry feedstock}} \text{ (Eq.1).}$$
| Feedstock source | Feedstock type | Temperature (°C) | Residence time (min) | Hydrochar yield (%) | Reference |
|---|---|---|---|---|---|
| Forest | Jeffrey pine and white fir (Tahoe mix) | 215-275 | 30 | 51-69 | [6] |
| Industry | Sawdust | 250 | 120 | 40 | [7] |
| Sewage sludge | 250 | 30 | 68-76 | [8] | |
| Agriculture | Palm shell | 180-260 | 30-120 | 39-71 | [9] |
| Palm residue | 150-250 | 20 | 62-76 | [10] | |
| Aquatic | Microalgae | 190-210 | 30-120 | 25-46 | [11] |
| Animal waste | Dairy manure | 180-260 | 240 | 36-57 | [12] |
What are the Mass Balance and Energy Requirements of the HTC Process?
The mass balance and energy requirements for the HTC process vary depending on the nature of feedstock, solids loading, reactor size, desired product quality, residence time, and operating temperature. As an illustration, the mass balance and energy analysis of HTC of biosolids, based on data from a study [13], are presented in Figure 2. In this study, the HTC process produced approximately 7% gases, 65% process water, and 15% solids, with an additional 13% water evaporated during drying of the post-HTC wet cake to obtain the hydrochar. The overall yield of hydrochar, on a dry basis, was 61%. The overall heat requirement for the HTC reactor operation was 22.1×105 MBTU/year (2.3×106 MJ/year), in addition to the energy required for drying operations. The energy content of the hydrochar produced was 107.5×105 MBTU/year (11.3×106 MJ/year). The process water separated from the slurry could be utilized as make-up water for the HTC process, thereby reducing the requirement of fresh water. The excess water could be used for the other purposes, such as irrigation.
 |
| Figure 2. An example of mass balance and energy analysis of HTC of biosolids (modified from Bhatt et al., 2018) |
What are the Benefits of HTC and Hydrochar?
Enhanced hydrophobicity: The hydrochar has less moisture and is more hydrophobic than raw feedstock. These attributes decrease transportation costs and improve shelf life by impeding wettability and rot during storage [14]. However, achieving these features requires use of reactors and mechanical equipment, such as a filter press, that makes the process energy intensive.
High nutrient recovery: HTC promotes enhanced nutrient recovery as both solid (hydrochar) and process water possess essential nutrients, including phosphorus, potassium and nitrogen, which are vital for plant growth [15]. Depending upon the feedstock used, the products may also contain undesirable metals, such as Ni, Pb, Cd, Cr, which are distributed among solid and liquid fractions post HTC.
Avoids feedstock pre-drying: HTC does not require pre-drying of biomass and can utilize feedstocks of varying moisture contents, which saves energy and costs for drying before processing [16]. This is one of the major benefits of HTC compared to other thermochemical processing methods that require dry feedstocks to produce char [17].
Improved dewatering efficiency: HTC enhances the dewatering efficiency of raw feedstocks as it helps release the bound water and thus is highly beneficial for biosolids management [18].
Lower environmental impact: HTC has the potential to minimize environmental impacts of waste biomass as it recovers more energy, and emits much less pollutants and odor, than incineration, landfilling, and composting [19].
How are HTC Products Used?
Laboratory-scale studies show that hydrochar produced via HTC has the potential for a variety of applications; however, to date, there are only a few commercial-scale HTC systems and information on real-world applications is limited.
Activated carbon adsorbent: Waste biomass derived hydrochar, when activated, has the potential as an adsorbent for remediation of nutrients, e.g. phosphates from run-off water, dyes, heavy metals, and pharmaceutical waste [16]. However, the hydrochar produced from HTC needs to be activated through chemical and thermal treatments before it can be used as adsorbent.
Soil amendment: The use of hydrochar can improve soil quality by enhancing its water and nutrient retention properties [4]. However, the char may contain toxic compounds which could limit its use as soil amendment.
Bioenergy feedstock: Depending upon the feedstock, hydrochar has a calorific value in the range of 6,450–12,900 BTU/lb. (15–30 MJ/kg), which is slightly higher than typical raw HTC feedstocks, which range from 5,550 to 8,200 BTU/lb. (13–19 MJ/kg), but is lower than bituminous coal ranges from 12,900 to 15,000 BTU/lb. (30–35 MJ/kg) [20–24]. Nonetheless, some hydrochar have sufficient energy content to be used as solid fuel. In addition, hydrochar can be utilized as feedstock for synthesis of liquid fuels (bio-oil, blend-stock fuel) and gaseous fuels (syngas) [4,25].
Carbon sequestration: Carbon sequestration essentially involves capturing and storing carbon. Hydrochar is rich in carbon and has the ability to adsorb CO2, making it an effective material for carbon sequestration [26].
What is the Cost of Hydrochar Production?
The capital investment of an HTC system varies widely, depending on the size of reactor and scale of operation. Operating costs include factors such as the cost of feedstock, HTC conditions (temperature and residence time), and nature of the desired product. There are limited studies on the cost of hydrochar production as the technology is not yet widely commercialized. However, based on available literature, the costs of hydrochar produced from different feedstocks, such as a coal-miscanthus blend, compost, and grape marc vary between $106 and $170/ton [27,28]. The key factors affecting the cost of hydrochar production are size of the production plant, nature of feedstock, and hydrochar yield.
What are the Bottlenecks in Commercialization of HTC?
HTC has the potential to valorize waste biomass by converting it to hydrochar, which can be used in different applications. It also has benefits compared to other thermochemical conversion methods as it takes place at lower temperatures and can utilize feedstocks with high moisture content, thereby lowering pre-processing costs. However, there are uncertainties associated with heat transfer dynamics, product yields, and costs of commercial scale hydrochar production. More extensive studies on combinations of different feedstocks, large scale reactors, and efficient hydrochar production methods could help optimize the process and reduce resource requirement and costs. In addition to hydrochar (solid product), HTC also generates process water (liquid phase) and gases, which are highly underutilized. The process water generated could be recycled for subsequent HTC cycles and could also be utilized in anaerobic digestion to produce biogas. Effective utilization of all HTC products and by-products would minimize waste and production costs. Robust pilot-scale HTC systems are needed, which can handle a wide variety of feedstocks under different processing conditions coupled to analysis of product quality, and provide the data needed to project the cost and economics of commercial systems. Utilization of HTC to produce biofuel and bioproducts, coupled with effective management of waste, could make it an attractive process for commercial scale implementation.
Acknowledgments
Authors thank Dr. Katrina Cornish, Professor, and Mary Wicks, Program Coordinator, Department of Food, Agricultural and Biological Engineering, The Ohio State University, and Dr. Toufiq Reza, Assistant Professor, Department of Biomedical and Chemical Engineering and Sciences, Florida Institute of Technology for technical and editorial review of this factsheet.
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