2.1. Experimental Data and Their Interpolation
The process model, developed to simulate a continuous HTC plant with all the auxiliary equipment (see
Section 2.2), uses HTC experimental data previously obtained by our research group [
17,
18]. Experiments were performed in a stainless steel batch reactor, with an internal volume of 50 mL. Scale-up experiments performed by Hoekman et al. [
19] demonstrate that HTC experimental results obtained in batch reactors are representative of those obtained when operating in continuous mode at a different reactor scale.
The types of feedstock used in the experiments were “off-specification compost” (OSC), coded by the European Waste Catalogue as EWC 19.05.03 [
17], and grape marc (GM) [
18]. OSC is a bio-stabilized material that is discarded and landfilled following the composting process because of its large particle size (between 10 and 40 mm). GM is a by-product of the wine industry. GM could be seen as a secondary raw material for innovative food applications [
20]. Although it is often used for the production of spirits through distillation, it is still found as lignocellulosic residue at the end of this process (exhausted GM). Our experimental measures revealed moisture content of about 30% for OSC [
17] and 65% for GM [
21].
For both feedstocks, model simulations were performed at three residence (reaction) times (θ = 1, 3 and 8 h) and three reaction temperatures (T = 180, 220, 250 °C), maintaining a dry biomass to water ratio (DB/W) equal to 0.07 for OSC and 0.19 for GM: exactly the same conditions as in the experimental tests.
For modeling, the feedstock and hydrochar compositions were identified by their ultimate analysis. As a procedural hypothesis, both biomasses were considered to be only composed of carbon, hydrogen, oxygen, and ashes. Nitrogen and sulphur, present in small to negligible amounts, were considered to be part of the ashes.
In order to perform the mass balances of the HTC process, yields in solid, gas, and liquid phases were required—with yield being defined as mass of the phase produced per mass of dry feedstock. Experimental data shows that temperature affects hydrochar and gas yields more strongly than residence time. Moreover, experimental data reveals a linear dependence of yields with temperature. Thus, the yield values obtained experimentally for each combination of residence time and temperature were correlated using linear regression, as expressed by Equation (1):
where “
Y” is solid (or gas) yield, and “
x” is temperature (°C). Coefficients “
a” and “
b”, depending on residence time, are given in the
Supplementary Materials (Table S1) for both solid and gaseous products and both biomasses. Correlations for OSC are depicted in
Figure 1a,b. The liquid yield, due to liquid species derived from feedstock decomposition during HTC, was calculated by difference (liquid yield = 1 − solid yield − gas yield).
The gaseous products were modeled as CO
2, CO, H
2, and CH
4, in line with gas-chromatographic analytical data. To quantify the amount of each species present in the gas, the regression curves between molar fractions and temperature were estimated. The procedure used was the same as that for gas and solid yields, where “
Y” is, in this case, the molar fraction of each gas species. The regression curve between molar fraction of CO
2 and temperature is shown in
Figure 1c for OSC, while the values of “
a” and “
b” for both feedstocks and all the gaseous species are given in the
Supplementary Materials (Table S1).
The liquid produced during HTC consists of several organic and inorganic compounds initially present within the feedstock. No detailed compositional data regarding the liquid phase from HTC of OSC and GM is available in the literature. That said, phenol was chosen as the species representative of the whole organic liquid mixture, and it was utilized to compute the mass balances of the various elements. This assumption is based on the work by Xiao et al. [
22], who measured the composition of the liquid deriving from HTC of maize stalk and
Tamarix ramosissima, ligno-cellulosic feedstocks such as OSC and GM. Results show that the liquid was mainly composed of phenols (lignin-derived species) with different substitution patterns. Surely, considering the liquid phase as consisting of only water and phenol is a very basic simplification; nevertheless, it is a procedural hypothesis which enables one to write the C, H, and O elemental mass balances and has no practical implications on the calculation of the process energy duties, one of the main goals of this work.
The total organic content (TOC) of the liquid phase from HTC was used to compute the amount of phenol as the representative species of the organic liquids. HTC lab tests performed on OSC [
17] show that TOC values increase at increasing temperature:
Figure 1d. Time plays a marginal role, such that only temperature dependence was taken into account. The trendline can be expressed as Equation (1), where “a”, “b”, and R
2 are equal to 0.0721, −5.6959, and 0.9157, respectively. As far as GM is concerned, TOC values did not exhibit an evident trend with process conditions: thus, a mean value equal to 14.85 g/L was considered for each combination of residence time and temperature. The amount of phenol (C
6H
6O) was calculated by considering the carbon balance: each mole of phenol corresponds to 1/6 moles of carbon determined by multiplying the value of TOC and the volumetric flow rate of liquid inside the reactor.
The developed model does not take into account the inorganic elements present in the liquid phase; considering the mass balances of such inorganic species was beyond the scope of the present work. The amount of water present in the liquid phase results from the water added to the feedstock before feeding it to the HTC reactor, and the water produced during HTC. The water produced during HTC was calculated as the difference between liquid yield and the amount of phenol produced per mass of dry feedstock. The production of water during biomass hydrothermal processing was previously documented in the literature [
23].
2.2. Conceptual Process Design
Figure 2 shows the conceptual process design developed for an industrial-scale HTC plant. The biomass (stream 2) is firstly processed in a grinder (G), which reduces and homogenizes its particle size. The feedstock is then mixed with water (stream 1) to reach the expected DB/W. Downstream the mixer (M), a pump (P1) and two heat exchangers in series (H1 and H2) are present.
The pump raises pressure to feed the slurry to the HTC continuous reactor. The heat exchangers preheat the biomass slurry. Next, the mixture (stream 6) enters the top of the reactor (R), while the HTC slurry, containing hydrochar and aqueous products, is removed from the bottom (stream 8). Gases formed during the reactions escape from the upper part of the reactor, passing through a valve (VALVE2) that, together with VALVE3, regulates pressure inside the reactor. The methane burner (B1) raises the temperature of the preheated biomass slurry up to HTC temperature (180–250 °C). The HTC slurry is then depressurized in two stages in flash tanks T1 and T2. The slurry exiting T2 is conveyed to a decanter (DEC), where liquid-solid separation is performed. The solid, i.e., hydrochar (stream HC-WET), is then transferred to an air-dryer (D), fed by an air blower (B), and heated by methane burner B2. After drying, the hydrochar produced is transferred to a pelletizer. Vapors produced by the expansion of the liquid slurry (streams VAP1 and VAP2) are used to preheat the biomass slurry in H1 and H2 and the air (stream AIR-IN) to the drier in heat exchanger H3. The condensed vapor (stream V3), together with the liquid exiting the decanter (stream 11), is pumped (P2) in order to be recycled to the process. Filter F separates organic liquids (phenol in the current simplification) from process water, which is recirculated to mixer M. The presence of filter F is not mandatory: the liquid from the decanter (DEC) could be recycled to the process—totally or partially—which could be beneficial in terms of increasing hydrochar yields [
15,
16]. Vice versa, if the HTC process aqueous phase is recovered for other applications, for instance as a liquid fertilizer, the use of the filter becomes an obvious choice. In this case, the separation of phenol-like compounds from the liquid is required because phenols are toxic species for plants. Considering the above, to be conservative, we considered the presence of the filter F (activated carbon filter) in the HTC process scheme, and its pressure drop was estimated to be 1.5 bar.