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.