2.3.1. Inorganic Analysis

For analysis, dried samples were ground and homogenised to below 100 μm. Samples were ashed in a mu ffle furnace (Nabertherm, Lilenthal, Germany) to a final temperature of 550 ◦C, with a hold at 250 ◦C to minimise volatile metal loss, as directed in BS EN ISO 18122-2. The ash was then mixed with a lithowax binder at a 10:1 ratio and palletised using a laboratory press (Spex, Stanmore, UK). The elemental composition of the ash was then determined using wavelength-dispersive X-ray fluorescence (WD-XRF) (Rigaku, Tokyo, Japan) using a metal oxide method. To correct for residual carbon within the ash, the carbon content was determined using a CHNS analyser (Thermo Scientific, Waltham, MA, USA), and carbon content was manually input to the XRF component list.

### 2.3.2. Organic Analysis and Determination of Combustion Properties

The carbon, hydrogen, nitrogen, and oxygen contents were determined using a Flash 2000 CHNS-0 analyser (Thermo Scientific, Waltham, MA, USA), calibrated using both chemical standards and certified biomass reference materials (Elemental Microanalysis, Okehampton, UK). Hydrogen and oxygen values were corrected to account for residual moisture, and figures are given on a dry free basis, in accordance with ASTM D3180-15. The error stated is based on the calculated standard error. The higher heating value (HHV) was calculated by bomb calorimetry (Parr, Moline, IL, USA). Proximate analysis was undertaken using a thermogravimetric analyser (TGA) (Mettler Toledo, Columbus, OH, USA). To obtain the residual moisture, 10 μg of homogenised sample was heated under nitrogen to 105 ◦C, where the temperature was held for 10 min before heating at 25 ◦C·min−<sup>1</sup> to 900 ◦C to determine the volatile carbon content. Fixed carbon was determined by holding the temperature at 900 ◦C and switching to air. Burning profiles, ignition, flame stability, and burnout temperature were obtained by temperature-programmed oxidation (TPO) in a TGA (Mettler Toledo, Columbus, OH, USA). Then, 10 μg of homogenised sample was heated at a rate of 10 ◦C·min−<sup>1</sup> in air to 900 ◦C, and the first derivative of the weight loss was calculated.

### 2.3.3. Prediction of Slagging and Fouling Propensity

The propensity of the fuels to slag and foul was assessed using both predictive slagging and fouling indices in the ash fusion test (AFT). Slagging and fouling indices are numerical indices based on the ash composition, as determined in Section 2.3.1. The equations for the alkali index (AI), bed agglomeration index (BAI), and acid base ratio (R*<sup>b</sup> a* ) are given in Table 1, along with the key values indicative of the onset of issues. The AFT is a qualitative method of assessing the propensity of a fuel to slag, and it works by heating an ash test piece and analysing the transitions in the ash chemistry in accordance with DD CEN/TS 15370-1:2006. Cylindrical test pieces are formed using 550 ◦C ash and a dextrin binder (Sigma-Aldrich, USA). The sample is then heated from 550 ◦C to 1570 ◦C in an ash fusion furnace (Carbolite, UK) using an airflow of 50 mL·min−<sup>1</sup> to give an oxidising atmosphere. The key transitions are as follows: (i) shrinkage, which predominantly represents the decomposition of carbonates in hydrothermally derived chars, (ii) deformation temperature, essentially representing the onset point at which the powdery ash starts to agglomerate and starts to stick to surfaces, (iii) hemisphere, whereby ash is agglomerating and is sticky, and (v) flow, whereby the ash melts [29]. The temperature for each transition is given to the nearest 10 ◦C in accordance with the standard. For most power stations, slagging becomes problematic between the deformation and hemisphere temperature [29]; thus, the deformation temperature is taken as the onset temperature for slag related issues.


**Table 1.** Predictive slagging and fouling indices.
