*2.4. Characterization of Feedstock and HTC-Products*

DSS and hydrochars were analyzed for DM and ash content. To avoid losses of mercury due to volatilization, all samples were previously dried at 40 ◦C [34] for further analyses and the initial DM content (% DM40C) was recorded. As a second step, subsamples were taken and dried at 105 ◦C until constant weight was reached (% DM105C). The effective DM content (% DM) was then calculated by Equation (1):

$$\text{DM}(\text{wt.\%}) = \text{DM}\_{\text{40\%}}(\text{wt.\%}) \times \text{DM}\_{\text{105\%}}(\text{wt.\%}) \div 100 \tag{1}$$

After determination of the DM105C, samples were incinerated in a muffle furnace (L 40/11 BO, Nabertherm GmbH, Lilienthal, Germany) and the ash content was measured according to DIN EN 14775. Ultimate analyses for carbon (C), hydrogen (H) and N were undertaken with a CHN-analyzer (TruSpec Macro, LECO Instrumente GmbH, Mönchengladbach Germany) and sulfur was measured at an external lab with a CHNSanalyzer (vario EL cube, Elementar Analysensysteme GmbH, Langenselbold, Germany). All samples for ultimate analyses were dried at 105 ◦C. Oxygen content was calculated by difference (Equation (2)):

$$\text{O } (\text{wt.}\%) = 100 - \text{C } (\text{wt.}\%) - \text{H } (\text{wt.}\%) - \text{N } (\text{wt.}\%) - \text{S } (\text{wt.}\%) - \text{Ash content } (\text{wt.}\%) \tag{2}$$

Total content of HM (Cd, Cu, Ni, Pb and Zn) and phosphorus (P) was measured for liquid and solid samples spectroscopically by ICP-OES (Agilent 5100, Agilent Technologies, CA, USA). 2 mL of liquid sample or 0.35 g of solid sample (dried at 40 ◦C) was added to 8 mL of aqua regia and microwave digested (Speedwave Four, Berghof Products + Instruments GmbH, Eningen, Germany) for 35 min at 175 ◦C (in dependence on SN EN 13346, protocol C). After the acid digestion samples were transferred to a 25 mL volumetric flask and diluted with ultrapure water. Final results for solid samples were corrected by the remaining water content (% DM105C). Acid digestion using aqua regia did not lead to a complete digestion for solid char samples. Therefore, measurement results were utilized for material balance and recovery efficiency was determined. The results showed good overall recovery with >83% for HM (excl. Pb, Cd and Hg) and nutrients. Dilution effects in liquid samples faced detection limitations, especially for elements with low

concentrations as Pb, Cd and Hg. Phosphorus contents for hydrochars were calculated by difference. All samples were digested in triplicates and average results are reported. For the determination of mercury (Hg) samples dried at 40 ◦C were measured according to DIN EN 1483: 08.97. Mercury was analyzed using a cold vapor atomic absorption spectrometer (CV-AAS) (novAA® 350, Analytik Jena GmbH, Jena, Germany) equipped with a hydride generator (HS 60A, Analytik Jena GmbH, Jena, Germany).

Liquid samples (process water and leaching extract) were analyzed for total organic carbon (TOC), measured as non-purgeable organic carbon (NPOC), according to ASTM D7573 and total bound nitrogen (TNb) according to DIN EN 12260 with a TOC-LCSH analyzer equipped with a TNM-L unit (Shimadzu, Kyoto, Japan). For TOC measurements the samples were automatically acidified with HCl to pH < 3 and sparged with purified air to remove inorganic carbon. Purgeable organic carbon (POC) may also be lost during this sample treatment. The pH values of fresh process water and leachate were measured with a portable multi-parameter meter (HQ40d, Hach Lange, Düsseldorf, Germany).

Hydrochars were further characterized for its fuel properties. Regarding energy content, the higher heating value (HHV) of the raw material and the hydrochar was measured using a calorimeter (IKA C 200, Breisgau, Germany). The volumetric emissions of CO2 and SO2 were calculated according to Equations (3) and (4) adapted from Kaltschmitt [35], on the assumption of a complete combustion. Weight percentage of carbon (c) and sulfur (s) are given by the elemental analysis:

$$\text{CO}\_2 \text{ emission} \left(\frac{\text{m}^3 \,\text{CO}\_2}{\text{kg DM}}\right) = 22.41 \,\frac{\text{c}}{12} \tag{3}$$

$$\text{SO}\_2 \text{ emission} \left(\frac{\text{m}^3 \,\text{SO}\_2}{\text{kg DM}}\right) = 22.41 \,\frac{\text{s}}{32} \tag{4}$$

The hydrochar yield and energy efficiency (EE) were calculated by Equations (5) and (6), respectively [36–38].

$$\text{Hydrochar yield } \left( \% \right) = \left( \frac{\text{DM}\_{\text{hydrochar }} \left( \text{kg} \right)}{\text{DM}\_{\text{raw material }} \left( \text{kg} \right)} \right) \times 100 \tag{5}$$

$$\text{Energy efficiency} \left(\% \right) = \left(\text{Hydrochar yield} \right) \times \left(\frac{\text{HHV}\_{\text{hydrochar}} \left(\frac{\text{MI}}{\text{kg}}\right)}{\text{HHV}\_{\text{raw material}} \left(\frac{\text{MI}}{\text{kg}}\right)}\right) \tag{6}$$
