**1. Introduction**

Historically, animal manures were returned to land and used in agriculture to increase soil organic matter and provide plant nutrients. The expansion of concentrated animal husbandry over the latter half of the 20th century, however, resulted in thousands of animals often being concentrated into small geographical areas, overwhelming the nutrient needs and soil-absorbing capacity of the nearby land. Excessive nutrients can then leach into groundwater, potentially leading to surface and groundwater pollution. As such, disposal of animal manures is a problem [1]. Hydrothermal treatment, including hydrothermal carbonisation (HTC), is an emerging technology which is well suited to processing wet wastes such as manure, and it has potential for recovery of nutrients from biomass and wastes such as phosphorus and nitrogen while simultaneously producing a solid fuel for energetic purposes [2].

Hydrothermal treatment involves the processing of biomass in water at temperatures above 100 ◦C at elevated pressure to ensure the water is in the liquid phase. HTC typically uses a temperature range of 180 to 250 ◦C, while temperatures below 180 ◦C are typically regarded as thermal hydrolysis. Under hydrothermal conditions, water acts as both reagen<sup>t</sup> and medium for a series of aqueous and solid-phase reactions to take place, leading to the carbonisation of biomass, resulting in a hydrochar or bio-coal, which has similar properties to a low-rank coal. [3]. Animal manures are typically composed of faeces, urine, discarded bedding, and waste feed. They have high moisture content, and they are, therefore, well suited for conversion by HTC [1]. During the hydrothermal processing of plant biomass, a number of key plant nutrients, including potassium and phosphorus from soluble phosphates, can be extracted into the aqueous phase and subsequently precipitated and recovered [4,5]. The extent to which the phosphorus is extracted is feedstock-dependent, with the inorganic content of the feedstock, particularly calcium content, often a key variable [6].

Previous studies investigating the HTC of manures found that phosphorus within manures is not easily extracted, leading to the immobilisation of the phosphorus in the bio-coal. This prompted the application of acids in HTC to aid phosphorus extraction [2,7,8]. The addition of acids during HTC was widely investigated, and it is thought to improve the overall rate of reaction in HTC [9–13]. In a study reported by Reza et al. [14], the influence of feedwater pH on the HTC of wheat straw was investigated using acetic acid and potassium hydroxide. The results indicated that the feedwater pH influences carbon density and higher heating value (HHV) in wheat straw, with higher carbon densities associated with lower pH.

At present, the application of acid catalysis for the processing of manures is primarily focused on increasing the extraction of phosphorus. Ekpo et al. [8] and Dai et al. [7] investigated the influence of acids on the recovery of phosphorus and nitrogen in swine and cattle manures, respectively. Ekpo et al. [8] investigated the addition of sodium hydroxide, sulphuric acid, acetic acid, and formic acid at 0.1 molar concentration and demonstrated that the presence of acidic additives improves the extraction of phosphorus and nitrogen. This study showed that phosphorus extraction is pH- and temperature-dependent and enhanced under acidic conditions. Phosphorus was most readily extracted using sulphuric acid, reaching 94% at 170 ◦C, while largely retained in the residue for all other conditions [8]. Dai et al. [7] performed HTC at 190 ◦C for 12 h using hydrochloric acid at varying concentrations. The results indicated that HTC in 2% hydrochloric acid extracted almost 100% phosphorus and 63% nitrogen. Decreasing the pH results in a small increase in carbon content and a large decrease in oxygen content, which will increase energy content (not stated). Decreasing the pH, however, also impacts the yields of bio-coal, which reduces from 70% (db) to 53% (db), and this reduction appears to be predominantly associated with the removal of oxygen. Fuel volatile matter is also seen to decrease, corresponding to an increase in fixed carbon at low pH. Ghanim et al. [15] investigated the HTC of poultry litter at 250 ◦C for 2 h at di fferent pH using acetic and sulphuric acid. Once again, the results indicate that operation at low pH increases the carbon content and HHV of the bio-coal. Increasing sulphuric acid content appears to both increase yield of bio-coal and reduce ash content. These results sugges<sup>t</sup> that performing HTC in dilute acid can simultaneously facilitate nutrient recovery from manure while upgrading the manure to a higher-quality bio-coal.

The studies performed to date did not consider the implications of operating at low pH on the inorganic chemistry, and how this a ffects the combustion behaviour of the bio-coal. The presence of inorganics and heteroatoms is a particular issue during thermochemical conversion of biomass and feedstocks that contain large amounts of potassium, sodium, sulphur, and chlorine; it can result in corrosion and slagging, or fouling in furnace and retorts [16]. Slagging is a process that occurs when ash deposits melt due to exposure to radiant heat, such as flames in a furnace. As this ash begins to melt, it starts to fuse, becomes sticky, and eventually forms a hard glassy slag known as a clinker, making ash removal di fficult. A high ash melting temperature is desirable as most furnaces are designed to

remove ash as a powdery residue [16]. Fouling occurs when potassium and sodium chlorides within the fuel partially evaporate on exposure to radiant heat and then condense on cooler surfaces such as heat exchangers forming alkali chloride deposits, which reduces their e fficiency on heat exchangers. These alkali chlorides can also play a role in the corrosion as they can react with sulphur in the flue gas, forming alkali sulphates and liberating chlorine within the deposit. This chlorine then catalyses the active oxidation and corrosion of the steel on which the deposit is formed [16,17].

To reduce the chance of a fuel slagging or fouling, it is important to minimise the alkali metal content in the ash along with chlorine. Leaching of alkaline metals and chlorine during HTC was demonstrated in a number of studies that concluded that slagging, fouling, and corrosion can be reduced by reducing alkali metals [18–24]. This reduction in slagging and fouling propensity following HTC was first demonstrated by Reza et al. [25] and later developed by Smith et al. [26], who demonstrated the e ffect of alkali metal removal on ash melting temperatures for HTC bio-coal using ash fusion analysis. This was later validated by subsequent studies [27–30].

The work presented in Smith et al. [26], however, demonstrated that the reduction in slagging and fouling propensity of HTC bio-coal is only partially due to a reduction of alkali metals, and it is also influenced by the retention of calcium and phosphorus within the bio-coal. Calcium and phosphorus are important as, while alkali metals, such as potassium and sodium, act as a flux for alumina–silicate ash, alkaline earth metals, such as calcium and magnesium, tend to increase melting temperatures [16]. In addition to the alkali and alkaline earth metals, the presence of phosphorus can prevent alkali metals forming low-melting-temperature alkali silicates, instead forming thermally stable phosphate compounds [31]. Phosphorus is also important from a fouling perspective, as potassium and sodium chlorides present within the ash can bind with calcium-rich phosphates to produce potassium or sodium phosphates, which then further react with calcium oxides. The resulting calcium potassium phosphate/calcium sodium phosphate complexes are stable and remove the potassium/sodium available to form low-melting-temperature potassium silicates [31,32]. Calcium oxide, calcium carbonate, and calcium hydroxide would otherwise dissolve into potassium/sodium silicate melts, bringing about the release of the potassium or sodium into the gas phase [33,34]. The removal of inorganics by the addition of acids during HTC may have a profound e ffect on the ash chemistry and a ffect the properties of the bio-coal during subsequent thermochemical processing.

In this work, swine manure was hydrothermally treated between 120 and 250 ◦C in water or 0.1 M NaOH, 0.1 M H2SO4, or 0.1 M organic acid (CH3COOH and HCOOH). The influence of pH on the on the HTC process was assessed, and the combustion properties of the resulting fuels were assessed.
