Digestate is primarily used as fertilizer but can also be transformed into fuel pellets. The composition of digestate pellets depends on the organic waste used as feedstock in biogas plants. It is possible to use digestate as a combustion material, but it has a lower calorific value than pellets made of wood. The energy efficiency of digestate pellets is around 44 kW, and the competence of the process is 85%. For comparison, the combustion of wood pellets is higher than 90%
[23][24][56,57].
As mentioned above, waste from agriculture and the food industry is mainly used in biogas production. Recently, research has been conducted on using algae as a substrate in the co-digestion process. The process occurs simultaneously, and the fermentation of a homogeneous mixture of different wastes occurs. As a result, more relevant parameters for the anaerobic digestion process are obtained
[25][58].
Regardless of the selected parameters of the HTC process, the most important factor is the safety of the reactor operation. Due to changes in pressure and temperature, the reactor’s parameters should always be selected very carefully. In hydrothermal carbonization, different reactors can be used: batch, semi-batch and continuous, equipped with a stirrer, direct heating by steam injection or through the reactor’s walls. The cost of a high-pressure reactor can be relatively high. For that reason, most research is limited to studies concerning reasonable temperature and residence time ranges while considering the potential interest of investors. As a result of the formation of reaction gases and water vapour, the pressure is usually in the range of 0.6–6.4 MPa. In the HTC process, the raw material is immersed in liquid water, whereas when it has direct contact only with water vapour, it is called steam carbonization (VTC)
[26][61].
Another type of HTC process is a “Two-stage HTC”, which consists of dividing the process into two phases. The first is hydrolysis, where products with a low molecular weight are obtained. It occurs at relatively low temperatures, up to 175 °C. The second phase occurs at higher temperatures, up to 280 °C, called carbonization. The hydrolysis products are dehydrated and polymerized. Consequently, “Two-stage HTC” reduces power consumption by up to 25% compared to the conventional HTC process. Besides, studies indicate that extending the hydrolysis time to 200 min increases the energy content of the produced hydrochar
[27][63].
There is also a process called co-hydrothermal carbonization (co-HTC). This kind of process affects the properties of hydrochar. Studies were conducted for different materials, for example, sewage sludge with the addition of various types of organic waste, e.g., lignocellulosic, wood, food, agriculture, municipal solid waste, and sewage sludge. The latter kind of waste is characterized by its heterogeneity of structure and its fairly low carbon content. In addition, the high nitrogen and ash content reduces the efficiency of hydrochar as a solid fuel
[28][64]. According to the data, wastewater treatment plants in Europe produce approximately 22.5 kg of dry matter of sludge per capita per year. It is estimated that annually this number reaches 17 million tonnes of dry sludge
[29][65].
Furthermore, the HTC process has improved the drainage of the sewage sludge. Hydrochar, adding 20% fir enhanced the best properties of the obtained fuel
[30][67]. Wilk et al.
[31][68] also demonstrated that when the HTC process is applied to sewage sludge, it reduces the activation energy of hydrochars by 50%
[31][68]. Liu et al.
[32][69] used co-HTC for cellulose from wheat straw, xylan and soy protein and proved co-HTC has an impact on the efficiency of hydrochar. The highest hydrochar yield was observed for cellulose. In addition, co-HTC reduces the ratio of O:C and raises N:C in the resulting hydrochar. In addition, the effect of the co-HTC process with aqueous phase recirculation has been investigated
[32][69]. Sample studies were conducted for aquatic plants of water hyacinth and cattail, as they have cellulose, hemicellulose, and lignin in their composition. According to previous studies, hydrochar obtained in the HTC process from those plants was characterized by better combustion efficiency and lower ash content
[33][70].
Furthermore, the research confirmed the effectiveness of reducing the tested contaminants using these technologies
[34][76]. Studies conducted for maize silage digestate as a substrate indicate that during the HTC process, increasing the temperature from 180 °C to 220 °C causes weight loss. The best parameters for the obtained hydrochar and process water were recorded at a temperature of 180 °C with a residence time of 30 min. Lower temperatures and shorter residence times cause a lack in the production of non-biodegradable or toxic substances in the process water
[11][40].
In addition to the HTC process, two other processes stand out: hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG). The first occurs within a temperature range of 200–400 °C and at a pressure of 10–25 MPa in an aqueous environment. As a result, the production of bio-petroleum occurs. In the HTG process, the conversion of biomass into gaseous components such as hydrogen, methane, and carbon dioxide. These gases can be used in heat production
[15][44]. According to previous research, hydrochars contain greater values of higher calorific value and fixed carbon content than the initial feedstock. However, volatile matter and ash content are higher for agricultural waste
[35][62].