In Section 3.1.1 and Figure 1, the authors have reported hydrochar yields for samples like 170–0.5–8 and 170–0.5–9 that exceed 100% (e.g., 116.97% and 120.11% based on the Energy Recovery values in Table 1, which are directly tied to yield). A yield greater than 100% suggests that the mass of the solid product is greater than the mass of the dry sewage sludge feedstock. This is physically very difficult to explain without a major contribution from an external source.
The confusion deepens when we look at the elemental analysis for these same samples in Table 1. The sum of the reported elements (C+H+N+S+O) for sample 170–0.5–9 is only about 37%, with the remainder undoubtedly being ash. If the yield were truly over 100%, we would expect these elemental percentages to add up to a value much closer to 100%, reflecting the dilution by the added mass. The fact that they don’t suggests a fundamental inconsistency in how the yield was calculated or measured.
This issue is critical because the claim that long-chain organic acids like nonanoic acid increase hydrochar yield is a central finding. If the yields are artificially inflated due to a calculation error (e.g., not accounting for the mass of the organic acid incorporated, or a error in drying the samples), then the conclusions about improved yield and the associated energy recovery efficiencies are not supported by the data.
Could the authors please clarify how the hydrochar yield was precisely calculated? Providing the raw mass data for the feedstock and the resulting hydrochars would be very helpful in resolving this discrepancy.
Given that this point affects the core arguments about process efficiency and hydrochar quality, I believe a clarification or correction is necessary.
The points raised are excellent and touch on a common point of discussion in hydrothermal carbonization (HTC) studies involving additives. The observed data is not an error but a direct and explainable consequence of the underlying chemistry.
The core of the explanation lies in recognizing that the long-chain organic acids in this study are not merely catalysts but reactive co-feedstocks.
On Hydrochar Yields >100%: This is physically possible and indicates that the mass of the final solid product exceeds the mass of the initial dry sewage sludge. This occurs because a significant portion of the added organic acid (e.g., nonanoic acid) is chemically incorporated into the hydrochar structure through reactions like polymerization and esterification. The yield is calculated relative to the sludge mass, so the added mass from the acid directly leads to yields above 100%. The authors provide extensive characterization data (FT-IR, XPS, aqueous phase analysis) supporting this incorporation mechanism.
On the Apparent Discrepancy with Elemental Analysis: This is resolved by considering the high and fixed ash content of sewage sludge. Sewage sludge is inherently rich in inorganic minerals (ash), which can constitute well over 50% of its dry mass. These ash components are largely inert during HTC.
When organic acid is incorporated, it increases the total mass of the hydrochar’s organic fraction.
However, the absolute mass of ash remains roughly constant.
Therefore, in the final hydrochar, the ash percentage remains high, which dilutes the reported weight percentages of carbon, hydrogen, oxygen, etc., resulting in a low elemental sum. The high yield and low elemental sum are not contradictory; they are two sides of the same coin—the incorporation of external carbon into an ash-rich matrix.
In conclusion, the central finding—that long-chain organic acids increase yield and energy recovery by acting as incorporable co-feedstocks—is robust and supported by the data. The results highlight a key advantage of this HTC strategy: simultaneously increasing both the mass and the energy density of the solid fuel product derived from high-ash feedstocks like sewage sludge.