**4. Conclusions**

The influence of pH most strongly influences ash chemistry, with decreasing pH increasing the removal of ash. This reduction in ash has the biggest influence on the volatile carbon and energy content of the fuel, with lower ash contents bringing about higher energy densities when calculated on a dry basis for a given temperature. The pH also influences dehydration, with fuel dehydration increased with decreasing pH, although, with increasing temperature, the influence pH has on dehydration becomes less. The pH and temperature appear to influence yield, with lower pH increasing yields above 250 ◦C but decreasing yields below 200 ◦C. The lower yields below 200 ◦C appear due to the acids catalysing hydrolysis of "cellulose-like" fibres within the swine manure, whereas the higher yields at 250 ◦C could be due to the low pH catalysing polymerisation due to its influence on the electrokinetic potential of the hydrothermal suspension.

The water experiments at 120 ◦C would sugges<sup>t</sup> that around 90% of the sodium and potassium is in free ionic form within the pig manure, along with 40% of the magnesium and 25% of the phosphorus. This free ionic sodium and potassium are more readily released into the vapour phase, and they are likely to bring about issues with fouling if combusted. Slagging and fouling indices sugges<sup>t</sup> that they cannot be safely combusted without treatment. Nonetheless, the ash fusion test suggests reasonably high deformation temperatures, suggesting low slagging and fouling. This paradox is brought about through the high calcium and phosphorus content of the fuel forming the stable calcium potassium/sodium phosphate complexes. Hydrothermally treating the fuels achieves almost complete removal of sodium and potassium and their associated issues with fouling. Increasing reaction temperature appears to immobilise calcium, magnesium, iron, zinc, and phosphorus within the bio-coal unless treated at low pH, which enables mobilisation of the phosphorus and alkaline earth metals. Treatment at 250 ◦C results in a more coal-like combustion fuel, with fuel properties similar to that of lignite coal and an HHV between 21 and 23 MJ/kg depending on pH. The removal of the alkaline earth metals and iron reduces the reactivity of the fuel treated at pH 1. Despite the mobilisation of calcium and phosphorus using strong acid, su fficient calcium and phosphorus is retained within the ash to give very favourable ash chemistry in terms of slagging and fouling. The use of sulphuric acid does result in residual sulphur within the fuel; however, this sulphur may be beneficial due to the influence that thermally derived sulphur trioxide has on the collection e fficiency of electrostatic precipitators and particulate removal, if appropriately blended with another low-sulphur fuel.

**Author Contributions:** Conceptualisation, A.M.S., U.E., and A.B.R.; methodology, A.M.S.; validation, A.M.S.; formal analysis, A.M.S.; investigation, A.M.S.; resources, A.B.R.; data curation, A.M.S. and U.E.; writing—original draft preparation, A.M.S.; writing—review and editing, A.M.S. and A.B.R.; visualisation, A.M.S.; supervision, A.B.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Engineering and Physical Sciences Research Council (EPSRC) Doctoral Training Centre in Low Carbon Technologies (EP/G036608/1), the Niger Delta Development Commission (NDDC), the European Commission ERDF Interreg IVb NEW "Biorefine" project and Grønt Udviklings- og Demonstrationsprogram (GUDP) (34009-18-1435).

**Acknowledgments:** The authors would like to thank the University of Leeds farm for the supply of swine manure and would also like to thank Simon Lloyd, Karine Alves Thorne, and Adrian Cunliffe from the University of Leeds for their technical assistance.

**Conflicts of Interest:** The authors declare no conflicts of interest.
