**3. Results**

### *3.1. Influence of pH on Bio-Coal Composition*

The yields and ultimate analysis of the bio-coals derived from the swine manure are given in Table 2. The results show that, with decreasing pH, there is increased removal of ash, with lower-pH treatments having the lowest ash content for their respective temperature. The exception is acetic acid (pH 2.88), which appears to have slightly lower ash content than formic acid (pH 2.38). Yields on a dry basis also appear to be influenced by pH, with higher yields associated with lower pH. The exception to this is sulphuric acid at lower temperatures, with lower yields observed at 120 ◦C than that for the higher-pH treatments. This lower yield is in part because of the reduced ash content; however, the reduction in ash alone does not account for the reduced yield, and the results sugges<sup>t</sup> that the lower yields appear to be due to enhanced oxygen removal. Due to the variation between ash contents with differing pH, Table 3 gives the ultimate analysis on a dry ash free (daf) basis, to enable direct comparisons between the organic chemistry of the different treatments. The results show that, for the 120 ◦C treatment in sulphuric acid, there is a higher carbon density and lower oxygen content for this treatment than the higher-pH treatments at this temperature.

**Table 2.** Ultimate analysis and yields of fuels on a dry basis; n/a—not applicable.



**Table 3.** Ultimate analysis proximate analysis and yields of fuels on a dry ash free basis.

A van Krevelen plot of the bio-coals from different pH and temperatures is presented in Figure 1. The results indicate that, with increasing temperature, the bio-coal has a more coal-like property, indicating increasing levels of dehydration with increasing temperature and decreasing pH. The pH appears to have a significant influence on dehydration, particularly at lower temperatures (120–170 ◦C), with sulphuric acid (pH 1) promoting the greatest levels of dehydration and sodium hydroxide (pH 13) showing the least dehydration. This effect of pH on dehydration, however, becomes less as the temperature is increased.

**Figure 1.** Van Krevelen plot of the bio-coals from different pH and temperature.

It should also be noted that the most dehydrated/coalified fuel for the 250 ◦C treatment is actually formic acid, followed by acetic acid (pH 2.38 and 2.88, respectively). The dry ash free ultimate analysis data presented in Table 3 also indicate that these two acid-treated bio-coals have the highest carbon density with 71% (daf). This higher carbon content could, however, be due to formic and acetic acid adding to the carbon in the bio-coal, lowering the O/C ratio. The use of a mineral acid at pH 2.38 and 2.88 could give an O/C ratio similar to that of water in Table 3 and Figure 1. This is because temperatures

above 200 ◦C bring about high dissociation of H<sup>+</sup> and OH− in the water, leading to the decomposition of monosaccharides to organic acids, rapidly dropping process water pH to approximately 3 [37,38]. The results in Table 3 sugges<sup>t</sup> that the bio-coal from sulphuric acid at 250 ◦C has the lowest carbon content at 66% (daf). It should, however, be noted that the sulphuric acid samples acquired sulphur from the acid, which makes up 4% (daf) of the fuel, while the other samples are low in sulphur; this would reduce the relative carbon content of the fuel when compared to the other treatments, even when correcting to a dry ash free basis. The analysis of the 250 ◦C bio-coals in Table 3 would sugges<sup>t</sup> that the bio-coal is similar in property to lignite A coal, as described in Smith et al. [39], for all pH treatments.

When the yields are corrected on a dry ash free basis, as shown in Table 3, it indicates that decreasing pH increases yields at 250 ◦C but decreases yields below 200 ◦C. This is likely due to the lower pH enhancing the rate of hydrolysis. The generation of hydronium ions due to the presence of acids is known to catalyse the hydrolysis of hemi-cellulose and cellulose into monosaccharides in lignocellulosic biomass [40].

Figure 2 shows the combustion profiles of the bio-coals following di fferent treatments with di fferent pH and temperature, and it shows a distinct volatile burn peak at around 300 ◦C, which is normally consistent with the presence of cellulose within a fuel [29,30]. Following HTC at 250 ◦C, this peak is absent, suggesting that the cellulose was removed. The lower yields associated with lower pH in the 120 ◦C, 170 ◦C, and 200 ◦C treatments are due to hydrolysis of the cellulose or "cellulose-like" component within the feedstock, resulting in its removal from the resulting bio-coal. For the 250 ◦C treatment, the increased yield at lower pH would sugges<sup>t</sup> that pH is catalysing repolymerisation.

**Figure 2.** Derivative thermogravimetric (DTG) burning profiles for (**a**) 120 ◦C, (**b**) 170 ◦C, (**c**) 200 ◦C, and (**d**) 250 ◦C treatments.

In HTC, the hydrolysis of components such as hemi-cellulose and cellulose results in the formation of monosaccharides within the process water, which then undergo dehydration and fragmentation processes, giving rise to di fferent soluble products such as furfural and hydroxymethylfurfural (HMF), benzenetrol, carboxylic acids, and aldehydes (acetaldehyde, acetonitrilacetone) [40,41]. These decomposition products undergo polymerisation and condensation to form insoluble polymers, often known as humins, which make up a portion of the bio-coal [42,43]. The pH should play a key role in this polymerisation due to the influence pH has on the zeta potential. Zeta potential, or, as it is more correctly known, electrokinetic potential, is a key indicator of the stability of colloidal dispersions, and the magnitude of the zeta potential indicates the degree of repulsion of like-charged particles in that dispersion. A high zeta potential will result in the solution or dispersion resisting agglomeration and flocculation. Bio-coals have a negative zeta potential. This is because they have oxygenated functional groups on their surface, making them behave like weak acids [44]. When a base is added to a suspension with a negative zeta potential, the particles tend to acquire a more negative charge; this reduces the chance of the suspension flocculating and, in the case of hydrothermal suspensions, polymerising to form bio-coal [44]. This is demonstrated in the sodium hydroxide 250 ◦C bio-coal, which has the lowest yield. If an acid is added to the suspension, a point is reached where the negative charge is neutralised [45]. At this point, the zeta potential is at zero, and it is called the isoelectric point. At this moment, the suspension is most likely to flocculate. For the water- and acid-treated samples, the lower pH increases the yields for the 250 ◦C treatments, which would sugges<sup>t</sup> that lowering pH reduces the electrokinetic potential of the decomposition products within the aqueous phase, enabling them to polymerise and increase char yield.

The hypothesis that lower pH catalyses flocculation and polymerisation would also support the findings of Ghanim et al. [15], who performed HTC of poultry litter at 250 ◦C for 2 h using various initial pH with sulphuric acid and found that increasing sulphuric acid content increased bio-coal yield and decreased ash content. This, however, does not agree with the findings in Chen et al. [46], who used sulphuric acid and bagasse and found the reverse. Shorter retention times were, however, used in Chen et al. [46] (5 min, 15 min, and 30 min) and, while the samples underwent polymerisation in Ghanim et al. [15], the extent of repolymerisation was less in Chen et al. [46], as repolymerisation and aromatisation are considerably slower than hydrolysis, decarboxylation, and dehydration reactions which initially occur [3].

When looking at the bulk properties, excluding sulphur content (carbon, hydrogen, nitrogen, oxygen, fixed carbon, and volatile carbon), there is only limited di fference between the compositions of the bio-coals produced at 250 ◦C. This would sugges<sup>t</sup> that, under these conditions, temperature is a more important parameter than pH. The bio-coals produced using the addition of formic and acetic acid result in an increased carbon content in the bio-coal; however, this could be due to the addition of carbon in the form of organic acid, as opposed to an influence of pH. A major increase in sulphur content (4%) is observed in the bio-coal produced using sulphuric acid, indicating incorporation of sulphur, most likely though Maillard chemistry [47].

### *3.2. Influence of pH on Fuel Inorganic Chemistry*

The metal analysis of the bio-coals and unprocessed pig manure is given in Table 4. The results show that, despite apparent increases in ash content, the overall composition of mineral matter in the bio-coal changes depending on the temperature and the pH. For all treatments, most of the potassium, sodium, and strontium is removed into the aqueous phase. The 120 ◦C treatment with water demonstrates that, for all three metals, around 11% of the original metal is retained within the char, with the remaining 10% gradually reducing with increasing temperature and almost complete removal at 250 ◦C, regardless of pH. The exception is for the sodium hydroxide treatment, where, at 120 ◦C, additional sodium is added from the solution. This sodium addition, however, diminishes as the temperature increases and, by the 250 ◦C treatment, the bio-coal has the equivalent of only 12% of the original sodium in the feedstock along with almost complete removal of the potassium.



The results of the 120 ◦C treatment with water are significant, as autoclaving fuel samples at 120 ◦C for one hour in water is a method provided in BS EN ISO 16995-2 as a methodology for determining free ionic salts within a biofuel. Consequently, metals removed in this treatment can, under BS EN ISO 16995-2, be regarded as in free ionic form. This result would sugges<sup>t</sup> that around 90% of the sodium, potassium, and strontium is in free ionic form within the pig manure, along with 40% of the magnesium and 25% of the phosphorus. The calcium, aluminium, silicon, manganese, iron, copper, and zinc do not appear to be in a water-soluble ionic form. In higher plants, typically over 90% of potassium and sodium is in ionic form, while 60–90% and 30–85% of magnesium and calcium is ionic [48]. The results for the 120 ◦C treatment with water would sugges<sup>t</sup> that, for pig manure, there is a similar proportion of free ionic potassium and sodium in the fuel, but a greater proportion of the calcium, magnesium, and phosphorus is either organically associated or present in mineral form than in lignocellulosic biomass [16,49]. The relatively low extraction of phosphorus in water at 120 ◦C would sugges<sup>t</sup> that phosphorus is present as low-solubility salts such as calcium and magnesium phosphate.

The behaviour of silicon is particularly notable in the results as, for all pH treatments, it appears to undergo increasing removal with increasing temperature, with between 45% and 60% retained, depending on pH, at 250 ◦C. In previous work, it was generally found that silicon is reasonably recalcitrant, being largely retained in the bio-coal, as, to become water-soluble silicon, it has to be hydrated and become silicic acid (H4O4Si), which, unless kept bu ffered within certain boundaries, readily degrades back to insoluble silicon dioxide (SiO2) [29]. Magnesium retention appears to be strongly influenced by pH, with the highest retentions seen with the sodium hydroxide (pH 13) and reduced retention with decreasing pH. Temperature is also critical, with the highest removal of magnesium typically being observed at 170 ◦C and increasing retention at 200 ◦C and 250 ◦C at all pH.

The metal retention behaviour for magnesium, whereby there is an initial reduction in metal retention up to 170 ◦C followed by increased retention of metals with increasing temperature between 200 ◦C and 250 ◦C, is also observed for calcium, magnesium, iron, and zinc, along with phosphorus. For these elements, there appears to be almost complete retention within the bio-coal at 250 ◦C for the sodium hydroxide, water, acetic acid, and formic acid treatments. The exception is sulphuric acid, which extracts 50% of the calcium between 120 ◦C and 200 ◦C, with 60% retained at 250 ◦C. Most of the phosphorus is extracted by the sulphuric acid with 87% extracted at 120 ◦C and 200 ◦C and over 90% extracted at 170 ◦C. Additionally, 80% of the original phosphorus is extracted at 250 ◦C, leaving 20% of the original phosphorus within the bio-coal. Similar ratios of magnesium are extracted as seen for the phosphorus using sulphuric acid. Aluminium and copper appear to remain within the bio-coal irrespective of temperature and pH.

The additional removal of the metals with the addition of sulphuric acid is due to the high dissociation constant of sulphuric acid, which is a strong acid and forms hydronium ions in two stages, with the initial loss of a hydrogen (Equation (4)) and the subsequent decomposition of the bi-sulphate (Equation (5)). Here, 0.1 molar sulphuric acid gives a pH of 1, but the same pH is possible using the same concentration of other strong mineral acids such as hydrochloric acid, or higher concentrations of weaker acids such as acetic acid which only partially dissociate (hence, pH 2.88 at the same molar concentration). The justification for using acid catalysts in HTC, according to Ekpo et al. [8] and Dai et al. [2,7], is to principally mobilise the phosphorus into the aqueous phase for subsequent recovery. The relatively low extraction of phosphorus in water at 120 ◦C would sugges<sup>t</sup> that phosphorus is present as low-solubility salts such as calcium and magnesium phosphate. The high concentrations of hydronium ions generated by the sulphuric acid are required for the acid leaching of the phosphorus, with the mechanism for acid leaching of calcium given in Equation (6). With sulphuric acid, the calcium is converted to calcium sulphate (see Equation (7)) liberating the phosphorus as phosphoric acid [50]. Similar phosphorus extraction is possible using hydrochloric acid, although, when leaching iron ores, slightly higher e fficiency is observed for sulphuric acid [50]. Greater calcium extraction may be possible using hydrochloric acid, as calcium chloride is more water-soluble than calcium sulphate (745 g/<sup>L</sup> as opposed to 2.6 g/<sup>L</sup> at standard temperature and pressure (STP)). This could explain the higher retention of calcium in the bio-coal when compared with magnesium, calcium, phosphorus, manganese, iron, and zinc, as the process water may be saturated once cooled to room temperature.

$$\text{H}\_2\text{SO}\_4 + \text{H}\_2\text{O} \rightarrow \text{H}\_3\text{O}^+ + \text{HSO}\_4^- \text{ K} \\ 1 = 2.4 \times 10^6 \text{ (strong acid)} \tag{4}$$

$$\text{HSO}\_4^- + \text{H}\_2\text{O} \rightarrow \text{H}\_3\text{O}^+ + \text{HSO}\_4^- \text{ K} \\ 2 = 1.0 \times 10^{-2} \tag{5}$$

$$\text{Ca}\_{10}(\text{PO}\_4)\_6\text{X}(\text{s}) + 20\text{H}^+ = 6\text{H}\_3\text{PO}\_4 + 10\text{Ca}^{2+} + \text{H}\_2\text{X} \tag{6}$$

$$\text{Ca}^{2+} + \text{SO}\_4^{2-} = \text{CaSO}\_4.\tag{7}$$

### *3.3. Influence of pH on Fuel Combustion Chemistry*

Table 5 gives the energy content, the volatile content, and the results of slagging and fouling indices derived from the inorganic chemistry are given in Table 4. The results show that the reaction temperature has the biggest impact on the energy content of the fuel, irrespective of the pH. The results indicate that HTC with sodium hydroxide gives the lowest energy density at all temperatures, and that undertaking HTC in the presence of acids at decreasing pH increases the HHV of the bio-coal, as demonstrated in Ghanim et al. [15]. The highest HHV is observed for acetic and formic acid due to their higher carbon contents, due to the increased carbon within the hydrothermal reaction, as previously discussed; however, there still appears a trend of increasing HHV with decreasing pH.

**Table 5.** Energy content, volatile content, and slagging and fouling indices for the bio-coals. HHV—higher heating value.


The volatile matter content of the bio-coal is also given in Table 5. The volatile matter content is important for predicting combustion behaviour, as, during combustion, the volatiles prevent oxygen from oxidising the carbon, hydrogen, and sulphur present within the fuel particle, bringing about two-stage combustion within a furnace [51]. Moreover, the escaping volatiles burn much more quickly than the char (the fraction remaining after devolatilisation); therefore, understanding the devolatilisation behaviour of a fuel is important in terms of flame ignition, flame stability, flammability limits, and the formation of pollutants such as nitrogen oxides [52]. The volatile content is also useful when determining the equivalent coal rank, with coals with higher ranks having lower volatile contents.

The volatile content appears to be most strongly influenced by reaction temperature, with lower volatile content with increasing reaction temperature. Figure 2 shows the combustion profiles of the bio-coals following di fferent treatments, with the 120 ◦C, 170 ◦C, and 200 ◦C treatments giving a distinct volatile burn peak at around 300 ◦C. By the 250 ◦C treatments, this peak is all but removed from the profiles of all fuel. For lignocellulosic biomass combustion, the volatile burn is often closely associated with the thermal decomposition of the hemi-cellulose and cellulose [53,54]. Hemi-cellulose and cellulose are readily degraded at hydrothermal temperatures of 180 ◦C and 200 ◦C, respectively [55], and the distinct volatile burn peak at around 300 ◦C is normally consistent with the presence of cellulose within a fuel [29,30]. This would then sugges<sup>t</sup> the presence, and removal, of fibrous material within the pig manure in a similar manor to that seen for lignocellulosic biomass.

The results presented in Ghanim et al. [15] stated that low pH with sulphuric acid brought higher volatile matter contents in the bio-coal; however, this contradicts the findings in Chen et al. [46], who used sulphuric acid and bagasse and found the inverse. The volatile contents presented in Table 5 would initially support the findings of Ghanim et al. [15]; however, the reason that the volatile matter appears to increase is due to the decrease in ash content of the fuel, increasing the relative amount of volatile matter present. Due to the variation between ash contents with di ffering pH, the volatile contents are calculated on a dry ash free basis in Table 3 to enable direct comparisons between the volatile chemistry of the di fferent treatments. These results show there is little change between pH, perhaps even suggesting a small decrease in the volatile matter content of the 250 ◦C treatments with decreasing pH. In the work presented in Chen et al. [46], the fuel was considerably lower in ash than the samples used in Ghanim et al. [15]; thus, there would not be the apparent increase due to the decrease in ash content of the fuel. Consequently, the results presented here would support the findings of both studies.

Combustion of these HTC bio-coals is considered as a coal substitute for coal-powered power plants, enabling utilisation of pre-existing infrastructure. Coal-powered power plants are usually designed to burn a specific type of coal, normally a coal obtained in and around the locality of the plant. Consequently, power stations have a design fuel specification that sets out, amongs<sup>t</sup> others, the ash content, energy content, particle size, and slagging and fouling properties of the fuel. When changing from a design fuel specification, care is required to ensure that the new fuel achieves a stable flame, required to ensure safe boiler operation [56]. Biomass often has di fferent combustion characteristics to that of coal, principally due to a higher proportion of volatile carbon and a much smaller char fraction than those seen in coals [57]. When burning high volatile fuels, combustion starts with the ignition of volatile gases surrounding the fuel particle and prevents oxygen from reaching and igniting the char, resulting in a two-phase combustion called homogeneous ignition, whereas, for typical coals used in pulverised fuel applications, devolatilisation, ignition, and combustion of the volatiles and char combustion occur almost simultaneously [58]. Issues with two-phase combustion occur when you start getting two areas of burning within the furnace that can then draw the flame from the burner and higher in the furnace, bringing about flame instability [52]. This can be a particular issue when co-firing biomass fuels with coal or two fuels with di fferent burning characteristics, as mismatched burning characteristics can result in two fuels burning independently within a furnace. In this instance, the rate of burning (flame velocity) may not match the rate of material feed, leading to the flame either blowing out or flashing back [56]. Thermogravimetric analysis (TGA) is one method originally developed to compare and evaluate fuel burning characteristics using the first derivative thermogravimetric (DTG) curve [52]. When undertaking this test, five key characteristic temperatures are taken, which were developed from the Babcock and Wilcox TGA method for coal and adapted to biomass. The first temperature is the volatile initiation temperature, where the weight loss begins. The second temperature is peak volatile burn, where you ge<sup>t</sup> the highest rate of mass loss during devolatilisation. The third temperature is the char initiation temperature, where the rate of combustion changes due to the onset of char combustion. The fourth temperature is peak char burn temperature. The fifth temperature is the burn-out temperature where the weight is constant, indicating the completion of combustion [52].

Figure 2 shows the DTG combustion profiles of the bio-coals following di fferent treatments. For the 120 ◦C treatment, the acids reduce the first initiation temperature, with weight loss starting at 160 ◦C for the three acid treatments but 200 ◦C for the water and alkali treatment. For the water, acetic acid, and formic acid treatments, there appears to be an initial peak at 300 ◦C, which can be constant with the presence of a fibrous component, such as hemi-cellulose, followed by a larger second peak at 325 ◦C, which is typically associated with cellulose in lignocellulosic biomass [29,30]. This peak is the peak volatile burn. For the sulphuric acid 120 ◦C sample, there does not appear to be a distinct peak where the "hemi-cellulose-like" material decomposes, but the presence of a shoulder at 300 ◦C. A higher mass loss is observed between 160 ◦C and 300 ◦C than any other sample, which may sugges<sup>t</sup> that this component is still present but partially hydrolysed, resulting in it thermally decomposing earlier. The sodium hydroxide treatment has only one distinct volatile peak, peaking at about 300 ◦C. This result is most likely a consequence of the strong basic conditions degrading the "hemi-cellulose-like" material observed with the strong acid conditions, but the high sodium and potassium contents of the fuel (see Table 4) could also be catalysing the volatile burn, giving a di fferent combustion profile [59]. Both the sodium hydroxide and the sulphuric acid treatments have higher char burn temperatures with temperatures of 500 ◦C, as opposed to 450 ◦C for the other treatments. The burnout temperature is similar at 580 ◦C for all treatments.

As the process severity increases, the profiles for 170 ◦C, 200 ◦C, and 250 ◦C retain the first initiation temperature of 160 ◦C, and a new initial peak begins to arise at 235 ◦C for the water and acids, initially starting in the 170 ◦C profile but becoming increasingly pronounced in the 250 ◦C profile. This peak represents potentially hydrolysed or repolymerised structures chemisorbed to oxygen functional groups on the char surface, ye<sup>t</sup> to dehydrate to form the stable ether or pyrone functional groups required to fix the carbon in the fixed carbon [41]. This peak does not appear in the sodium hydroxide profiles, which has a di fferent combustion profile to that of the water and acid samples, particularly with regard to the volatile burn in the 170 ◦C and 200 ◦C profiles, potentially due to the influence of sodium (alkali metals) on catalysing devolatilisation and combustion [59–61]. For all the 170 ◦C and 200 ◦C combustion profiles, the main volatile peak becomes increasingly dominant at 325 ◦C, although, upon reducing its dominance in the 200 ◦C profiles, the volatile content within the fuel decreases and the fixed matter increases (see Table 5). The increasing dominance of this peak is most likely due to the removal of hydroxyl, carboxyl, and carbonyl groups present within the biomass, along with any structural components that are hydrolysed at lower temperatures such as hemi-cellulose in any cellulosic material present [3,54]. The peak at 325 ◦C is constant with the presence of residual cellulose and the breaking of its glyosidic linkages [30]. By the 250 ◦C treatments, the samples adopt a "coal-like" single-stage combustion profile [29], whereby the transition between the volatile release and initiation of char burn (char initiation temperature) is marked more by a "shoulder" as opposed to a distinct peak. This shoulder becomes less distinct with lower pH and correlates with a modest reduction in volatile matter.

The benefit of creating a more "coal-like" burning profile is that it aids flame stability. As previously discussed, when there is a two-stage burn, as seen in the burning profiles for the 120 ◦C treatments in Figure 2, you ge<sup>t</sup> homogeneous combustion, whereby the volatile burn and char burn occur in isolation. In this case, upon drying and devolatilisation, the fuel particle can become entrained in the gas stream and move higher in the furnace while still burning, drawing the flame upward and promoting flame instability [52]. With the single-stage "coal-like" profiles seen in the 250 ◦C treatments in Figure 2, the devolatilisation, ignition, and combustion of the volatiles do not occur in isolation; instead, the char should oxidise/combust at the same time (heterogeneous reaction), promoting a simultaneous combustion of the fuel mass and a stable flame [57].

In all the profiles, with increasing reaction severity, lower pH increases char burnout temperature. This is particularly notable at 250 ◦C as the peak temperature (char burn) increases from 400 ◦C for the sodium hydroxide (pH 13) to 500 ◦C for the sulphuric acid (pH 1). This result would sugges<sup>t</sup> that pH influences reactivity of the char. There is strong consensus in the literature that the alkali

metals, potassium and sodium, catalyse char reactivity [59,60,62,63], although the concentrations of alkali metals appears similar for the 250 ◦C bio-coals (see Table 4). The alkaline earth metals, calcium and magnesium, are understood to catalyse char reactivity but to a lesser extent than the alkali metals [63–65], with iron known to behave using similar mechanisms [66]. Given that pH appears to more strongly influence alkaline earth metal content, with higher calcium and magnesium contents, along with iron, associated with higher pH (see Table 4), it is possible that the higher concentrations at higher pH catalyse the thermal decomposition.

Figure 3 displays the ash transition temperatures obtained from the ash fusion test for the four di fferent temperatures and five di fferent pH values. Table 6 gives the transition temperatures for all samples, along with the unprocessed sample, and their standard errors. The results sugges<sup>t</sup> that the unprocessed swine manure has a reasonably high deformation temperature of 1320 ◦C, compared to between 980 ◦C and 1140 ◦C for the conventional *Miscanthus* [26,29]. Despite this result, the shagging and fouling indices sugges<sup>t</sup> almost certain flagging and fouling for these samples. The results of the ash fusion test certainly would sugges<sup>t</sup> a low slagging fuel, possibly due to the high calcium and phosphorus content of the fuel. This would strongly sugges<sup>t</sup> that calcium potassium phosphate complexes and calcium sodium phosphate complexes remove the potassium and sodium available to form low-melting-temperature potassium and sodium silicates [31,32]. A high ash melting temperature is usually indicative of a low alkali metal content and, thus, low fouling propensity temperatures [16], although caution is required here. The results of the 120 ◦C treatment with water would sugges<sup>t</sup> that around 90% of the sodium and potassium in the fuel is in the form of free ionic salts. Potassium and sodium, when in the form of free ionic salts, are more readily released into the vapour phase and likely to bring about issues with fouling [16,67,68]. The high calcium and phosphorus content of the fuel may, however, prevent this release, instead forming the stable calcium potassium phosphate complexes and calcium sodium phosphate complexes [69]. The slagging and fouling indices used in this paper would not consider such mechanisms when predicting the fuel propensity to slag and foul.

**Figure 3.** Ash transition temperatures from the ash fusion test for the di fferent treatments at di ffering hydrothermal treatment temperatures.


**Table 6.** Ash transition temperatures from the ash fusion test.

For the hydrothermally treated samples, the results show that ash shrinkage temperature is reduced; however, this is believed to be due to formation of carbonates during the hydrothermal process [26,28,29]. The deformation and hemisphere temperatures, however, appear to change reasonably little, indicating limited change to slagging propensity. The exception to this is the low-temperature sodium hydroxide-treated samples and the 250 ◦C treated sulphuric acid sample. The 120 ◦C, 170 ◦C, and 200 ◦C sodium hydroxide samples are suppressing, given that these are the only hydrothermal samples where the alkali index (given in Table 5) suggests almost certain slagging and fouling. Sodium and potassium contents are, however reduced, when compared to the starting feedstock, which may explain this increase.

In the 250 ◦C treated sulphuric acid sample, there is the greatest improvement in ash behaviour with the sample not undergoing deformation within the test conditions (test limit 1570 ◦C). This is predominantly due to the ash becoming a highly stable magnesium calcium phosphate silicate complex. It should be noted that the sulphuric acid samples acquire sulphur from the acid, which makes up 3.4% (db) of the fuel. Sulphur can play both a positive and a negative role during combustion in large combustion plants. During combustion, sulphur is predominantly oxidised to sulphur dioxide (SO2) (>95%), but some sulphur trioxide (SO3) is also formed [16]. While the sulphur dioxide plays an undesirable role in terms of corrosion, active oxidation of furnace components, and emissions of sulphur dioxide to the atmosphere (unless abated using flue gas desulphurisation) [17], sulphur trioxide plays an important role in the abatement of particulate emissions. This is due to the thermally derived sulphur trioxide forming sulphuric acid in the flue gas, which then adsorbs onto the fly ash particulates [70]. This affects the surface electrical conductivity of the particulate, greatly increasing the efficiency of the electrostatic pacificators [71]. This could be particularly advantageous for the 250 ◦C treated sulphuric acid sample, given that potential to emit (PTE) metals (strontium, copper, and zinc) are present within the swine manure and the derived hydrothermal fuels. Precipitation of these in the fly ash would be required to avoid issues with emissions.

When combusting fuel in pulverised applications, fuel sulphur is desirable as, while sulphur emissions are largely in the form of sulphur dioxide, when a fuel with high sulphur content is combusted, there is generally enough sulphur trioxide formed to bring the electrical resistivity of the fly ash into a range which results in good precipitator operation [72]. This can be a particular issue with biomass, which is typically low in sulphur and results in a very low collection e fficiency of electrostatic pacificators [72]; consequently, biomass-fuelled furnaces, such as Drax (UK), add sulphur to overcome this. The sulphur content of the 250 ◦C treated sulphuric acid sample is typically too high for single-fuel combustion; however, if blended with low sulphur biofuels or coal, this sulphur content could be brought within "design fuel" specification. Moreover, the magnesium calcium phosphate silicate ash of the fuel would have an additive influence and could improve the slagging and fouling propensity of the blended biomass and coal. Consequently, with blending, the 250 ◦C treated sulphuric acid fuel could be safely combusted within the pulverised fuel plant if appropriately blended.
