*3.2.1 HTC mass and energy balance*

With reference to the section of the plant reported in **Figure 3** and destined to the hydrochar production and thickening of leachate, the mass balance is reported in **Table 4**. The flow ID are referred to in **Figure 3**.

The yield of hydrochar referred to the initial biowaste results to be 7%. The amount is quite similar to the compost yield, obtained by using the aerobic stabilization instead of the HTC process. The main differences are the following:


The energy balance of the HTC section is proposed in **Table 5**.

The heat necessary for the heating of the slurry from the input temperature (exit of filter) up to 220°C is 866 MJ/Mg (at 42.6 bar); once the reaction temperature is reached, the carbonization begins by absorbing heat from the environment until exothermic reactions begin. The heating time has been fixed in 0.4 h (1440 sec), so an installed heating power of about 10 MW is necessary to provide the heat in the specified time interval.

The thermal energy necessary to provide for the evaporation of water has been obtained by subtracting that requested to bring the water into vapor phase at 100°C and 1 bar to the thermal energy of the water medium present in the reactor. The amount of heat to carry out this process is 1366 MJ/Mg that corresponds to a thermal power of 9.7 MW, by assuming an evaporation time equal to that necessary for slurry's heating. By using the energy content of water after the reactions are completed, it is possible to obtain the evaporation for about 78% of water


**Table 4.** *HTC mass balance.*


### **Table 5.**

*Evaluation of enthalpy flows for the unit processes E and F.*

(10.19/13.01); the remaining fraction remains in the liquid form by forming a thick leachate with solute.

The hydrochar can be separated by water, by centrifugation, or by other standard dewatering systems.

The most important feature is that the leachate produced by the facility is reduced at 44% of that produced by anaerobic digestion sector, without increase of thermal heat, but that used for HTC reaction.

Further evaporation is technically possible and can be also economically feasible if heat demand is fulfilled by the third section of the plant: the gasification with energy recovery.

The overall feedstock energy balance is reported in **Table 6**. The energy balance shows in brief that:


The energy content of waste corresponds to about 3800 MW of chemical energy that can be used to produce energy by means of a gasification process, described in the following paragraph.

## *3.2.2 Gasification mass and energy balance*

With reference to the section of the plant reported in **Figure 3** and destined to the conversion of waste into energy and heat, the data in **Table 7** are the basis for calculation. Data refer to the the typical waste resulting from the sorting of biowaste treated in the reference facility, just before being fed to digestors. These data are the starting point for calculation of calorific value, stoichiometric oxygen demand, bottom ash production rate, and other process parameters.


*Exploitation of Digestate in a Fully Integrated Biowaste Treatment Facility: A Case Study DOI: http://dx.doi.org/10.5772/intechopen.92223*
