*3.3. Hydrochar Properties*

To verify the results and to form a basis for comparison, a van-Krevelen-diagram was created (Figure 2) and common HC properties such as solid, carbon, and energy yield were calculated (Table 4). For reasons of clarity and comprehensibility, only a selection of HCs as well as DSS is shown in these illustrations. The HCs produced at the mildest (160 ◦C, 30 min, pH 7) and harshest conditions (200 ◦C, 90 min, pH 2) are shown to illustrate the range of reaction severity. To estimate the individual effect of the different reaction parameters, each parameter was varied from the mildest to the harshest condition while the other two remained at the mildest.

**Figure 2.** van-Krevelen-diagram of selected hydrochars (HC) and digested sewage sludge (DSS) at different process conditions.


**Table 4.** Hydrochar (HC) properties in relation to reaction conditions.

T = reaction temperature; t = holding time; C = carbon; N = nitrogen; HHV = higher heating value.

By visualizing the atomic H/C versus O/C ratios of DSS and HCs in Figure 2, insights into the governing reactions can be gained. Generally, carbonization decreases both H/C and O/C ratios and, depending on how the ratios change in relation, an assumption about the major governing reaction can be made. In this case, the atomic ratios of the HCs primarily follow dehydration, which confirms the finding of Wang and Li [33] that reactions at temperatures below 180 ◦C are governed by dehydration. When comparing the HCs that were both produced at 160 ◦C and pH 7, but at differing holding times of 30 and 90 min, it becomes apparent that varying the holding time between these time intervals does not have a noticeable effect. Contrarily, the variation of pH or temperature leads to a more severe conversion of the feedstock. The pH variation resulted in HCs with lower H/C ratios than caused by dehydration reactions only.

Considering the lower nitrogen contents of pH-adjusted HCs (Table 4), it seems like the pH adjustment favored deamination, which is besides decarboxylation the major conversion mechanism for proteins [22]. This causes the additional removal of hydrogen, resulting in a lower H/C ratio. When analyzing the influence of temperature variation, HC produced at 200 ◦C possesses almost the same atomic ratios as the HC produced at the harshest combined conditions (200 ◦C, 90 min, pH 2).

It can be seen from the data that the initial calculated HHV based on dry matter of DSS is slightly higher (13.4 MJ kgdb−1, Table 1) than the HHV of some of the produced HCs (12.7–15.0 MJ kgdb−1). This is due to the accumulation of ash in HC (increasing ash content from 40.9%db in DSS to 39.2–53.4%db in HC). Calculating HHV on a dry and ash free basis (daf) results in the expected increase when comparing the resulting chars (26.0–30.9 MJ kgdaf−1) with the initial DSS (22.6 MJ kgdaf−1). The HC characteristics solid, carbon, and energy yield were calculated according to the described approach (Equations (1), (2), and (4), respectively). They follow the expected trajectory that more severe reactions result in lower yields. Conducting an analysis of variance for the respective yields showed for reaction temperature (*p* < 0.01), initial pH (*p* < 0.001), and their interaction (*p* < 0.05) to have statistically significant effects on the yields. The regression models in terms of actual factors (T in ◦C, t in min and pH) are as follows:

$$\text{Solid yield} \,(\%\_{\text{dB}}) = 40.14 + 0.16 \text{x}\_{\text{T}} + 13.64 \text{x}\_{\text{pH}} - 0.06 \text{ex}\_{\text{T}} \times \text{x}\_{\text{pH}} \tag{14}$$

$$\text{Carbon yield } (\%\_{\text{dB}}) = 56.28 + 0.057 \text{x}\_{\text{T}} + 11.45 \text{x}\_{\text{pH}} - 0.055 \text{x}\_{\text{T}} \times \text{x}\_{\text{pH}} \tag{15}$$

$$\text{Energy yield } (\%\_{\text{dB}}) = 59.31 + 0.079 \text{x}\_{\text{T}} + 13.04 \text{x}\_{\text{pH}} - 0.065 \text{x}\_{\text{T}} \times \text{x}\_{\text{pH}} \tag{16}$$

In contrast to the majority of studies, holding time was found not to be statistically relevant in these experiments. This is because others investigated a much longer time span [18,19]. Therefore, it can be concluded that for solid, carbon, and energy yield, a variation of holding time between 30 and 90 min does not cause statistically different results, and the time could be minimized. Danso-Boateng et al. [34] developed a regression model for the solid yield of primary sewage sludge (SY = 118.49 − 0.26xT − 0.04xt, actual factors, range of 140–200 ◦C and 15–240 min) and it compares quite well with Equation (14) when set at a neutral pH of 7 (SY = 135.62 − 0.30xT, actual factors). Though holding time is included, the main effect comes from temperature, as found in this study.

The regression model of the solid yield is displayed as a contour plot in Figure 3 as a representative for the other yields, as they follow the same trend. It shows that mild conditions (low reaction temperature and neutral pH) lead to higher solid, carbon, and energy yields. While Mäkelä et al. [18] found no effect of the addition of acid on the yields when investigating industrial mixed sludge from a pulp and paper mill, these experiments show a clear effect. This is likely to occur because a wider range of pH with a more concentrated acid was tested. Unlike Reza et al. [35], who found that mass yield was more affected by reaction temperature than initial pH when converting wheat straw, the opposite is indicated here with pH having 1.5 times the effect compared to that of reaction temperature.

**Figure 3.** Regression model contour plot showing the influence of initial pH and reaction temperature on the solid yield (%db).

The dry matter content after mechanical dewatering was used as a measure of the increase in sludge dewaterability. Analysis of variance has shown that only the influence of reaction temperature (*p* < 0.001) and initial pH (*p*<0.001) was statistically relevant (see Table 2). Furthermore, their interaction (*p* < 0.001) as well as the quadratic term of temperature (*p* < 0.01) are included in the model due to their high significance level, resulting in the following equation with actual factors:

$$\text{Dry matter content} \left( \% \right) = -91.48 + 1.64 \text{x}\_{\text{T}} - 7.74 \text{x}\_{\text{pH}} + 0.036 \text{x}\_{\text{T}} \times \text{x}\_{\text{pH}} - 0.0047 \text{x}\_{\text{T}}^2 \tag{17}$$

The resulting response contour, plotted in Figure 4, shows the general trend that low pH and higher temperatures favor dewaterability. Initial pH has a clear and strong influence, as a more acidic initial pH yields increasing dry matter contents. A deviation from the expected trend exists at low pH levels, where an increase in reaction temperature does not result in ever increasing dry matter contents. This trend is caused by the quadratic term xT <sup>2</sup> in the regression model, which causes the slight curvature. However, when determining to what extent this term contributes to the model by comparing the SS of the term xT <sup>2</sup> to the total SS, it becomes clear that it contributes only with 5% to the model and should not be emphasized.

The regression model of the dry matter content is in agreement with Danso-Boateng et al. [8], Gao et al. [36], and Wang et al. [37], who also found a strong influence of temperature on dewaterability, but not of holding time. When the threshold time of 30 min is exceeded, there is no longer any holding time dependency according to Wang et al. [37]. At the selected temperatures, the typical HTC reactions already take place sufficiently and another increase in temperature will yield even faster reactions. Because dehydration and decarboxylation cause a decrease in hydrophilic functional groups, such as hydroxyl and carboxyl groups [38], the product is more easily dewatered already by a simple mechanical press. The effect of acidic pH on improved dewaterability can be explained by the enhancement of dehydration reactions, caused by sulfuric acid [39], which adds to the temperature effect. Dewaterability is additionally improved by the floc structure disintegration, which releases the bound water as free water [22]. Escala et al. [9], who conducted a comparable experiment, achieved similar dry matter contents. They were able to reach a dry matter content of 52 ± 5.5% after applying a

pressure of 40 bar to HC produced at 205 ◦C and 24 min. This is higher than the 45.6 ± 2.24% achieved in this study at comparable reaction conditions (200 ◦C, 30 min, pH 2 and 7) and is very likely due to a much lower dewatering pressure.

**Figure 4.** Regression model contour plot showing the influence of initial pH and reaction temperature on the dry matter content (%) after mechanical dewatering.
