Leaching procedure at elevated temperatures

In order to investigate the effect of elevated temperatures during biomass leaching runs at 150 °C were carried out in a modified Gallenkamp bomb calorimeter; displayed in Figure 5-3. A gas inlet and outlet, pressure sensor, internal thermocouple, and magnetic stirrer were added to the bomb. The saturated vapour pressure for water and acetic acid were calculated up to 150 °C, with results shown in Figure 5-4. These results indicate the vapour pressure for acetic acid leaching solutions reaches 0.47 MPa at 150 °C. Therefore, system was pressure tested to a maximum of 5 MPa, and was equipped with a pressure relief valve in case of higher pressures. Biomass was heated using a magnetic stirring plate, with a concentric aluminium block placed on top to aid heat transfer. After leachings were completed, samples were neutralised with DI water, analogous to leachings at 30 °C. Three duplicates were averaged for each reagent.

Figure 5-3: Modified Gallenkamp bomb calorimeter for high temperature leachings

Figure 5-4: Vapour pressure system for high temperature leachings

0 0.1 0.2 0.3 0.4 0.5 100 110 120 130 140 150 160 Press ure (MP a) Temperature (°C) Water Acetic acid Total

108

5.2 Comparison between different leaching reagents

The efficiency of leaching with the two main organic acids identified in the solution produced during torrefaction, namely acetic and formic acid, were compared to leaching with water and three mineral acids: hydrochloric, sulfuric, and nitric acid. The total yields and structural analysis of biomass samples following leachings with different reagents are given in Table 5-1. The lignin composition of 28±1% for raw P. radiata was comparable to that reported previously of 26-29% by Evans et al [5], Green [6], Iiyama and Willis [7], and Johnsson and Packer [8]. The acid soluble lignin was 1.0±0.2%, this was slightly higher than typical values of <1% for softwoods [9, 10][7]. The cellulose composition was measured in the present study as 43±1%, which is comparable to reported values of 42 and 42.5% by Uprichard and Lloyd [11] and Berrocal et al [12], respectively. The hemicellulose content was measured in the present study as 26±1%, which is also within the reported range of 22.9% [12] to 29% [11]. The hemicellulose content varies significantly between studies, possibly due to the tree’s age as the wood chemical composition varies with age: the cellulose content increases and hemicellulose content decreases [11]. Boonstra and Tjeerdsma [13] analysed P. radiata and found that the raw acetyl content was 1.48%, which was close to the 1.51±0.03% measured in this work. Overall, the hydrolysis procedure used to determine the biomass’s chemical composition was considered reliable.

During the leachings, biomass sugar polymers depolymerised if conditions were severe enough for hydrolysis to proceed. The high mass yields indicate that hydrolysis was minimal for all leachings; the slight decreased in the acetyl content for sulfuric and hydrochloric acid leached biomass represent mild hydrolyses but the polymer composition did not vary.

The inorganic fraction in biomass was reduced from 0.45 to 0.27 wt% with water washing alone. This indicates that approximately 40% of the inorganics in biomass are present as soluble metal salts, such as alkali metal chlorides, sulfates, and carbonates [14]. Reduction of the acid soluble salts, such as alkaline earth carbonates and sulfates, was accomplished through dilute acid leachings. Inorganics remaining after acid leaching were inaccessible within the wood matrix and could not be removed even with severe leaching procedures for

P. radiata. These could be either as organically bound salts, that require oxidation or dissolution of the organic

matter for removal (using ammonium acetate or sodium hydroxide leaching solutions), or present as silicates which have limited removal during acid leachings [14]. Further reductions in the inorganic content have been reported by other researchers using different feedstocks [3, 15].

The reduction in biomass inorganics during leachings at 30 °C was slightly more effective with mineral acids than organic acids; this may be correlated with the pH of the leaching solution, as shown in Figure 5-5. From the figure, it was observed that the inorganic content in leached biomass decreased as the pH of the leaching solution was decreased, regardless of the actual acid type. This confirmed that organic acids can be used for

109 leaching biomass rather than mineral acids. Figure 5-5 can be used to predict the inorganic content after leaching for any given acid leaching reagent under the same leaching conditions.

Table 5-1: Biomass composition after leaching with different leaching reagents Leaching yield (%) Inorganic content (%) Acetyl content (%) Lignin (%) Cellulose (%) Hemicellulose (%) Raw wood - 0.41±0.04 1.51±0.03 28±1 43±1 26±1 DI water 99.3±0.2 0.27±0.03 1.53±0.17 30±1 42±2 26±1 1% acetic acid 99.3±0.1 0.16±0.02 1.48±0.30 28±4 43±1 27±1 1% formic acid 99.0±0.3 0.14±0.02 1.53±0.15 28±1 42±1 26±1 1% sulfuric acid 99.3±0.9 0.11±0.02 1.36±0.05 29±2 42±2 27±1 1% hydrochloric acid 98.4±0.8 0.11±0.02 1.38±0.17 29±1 42±1 27±1 1% nitric acid 99.0±0.4 0.12±0.02 1.48±0.05 28±2 44±3 27±4

Figure 5-5: Inorganic content in biomass after leaching with varying reagent type and concentrations

The ICP-OES results given in Table 5-2 indicate that only S, P, Na (and Zn for formic acid) were reduced to the same degree by organic acids as by mineral acids for leachings at 30 °C. The higher concentration of alkaline earth metals (Mg and Ca) in the organic acid leached biomass could be beneficial as alkaline earth metals have been used as catalysts for deoxygenation during pyrolysis [16, 17], by favouring depolymerisation reactions over dehydration reactions [18]. Sulfuric acid leaching caused a large increase in S, indicating that either some of the acid was not washed out during the neutralising step or S became incorporated into the biomass, and thus could not be removed through water washing alone. Organic acids contain no inorganics, which is a potential benefit of leaching with organic acids compared to mineral acids.

A mass balance for the elements in biomass leached with 1% acetic acid at 30 °C is given in Table 5-3. The discrepancy between the calculated mass balance for the elements removed from the biomass and the experimentally measured values in the leachate indicate that additional ions were present in the leachate which

0.0% 0.1% 0.2% 0.3% 0 1 2 3 4 5 6 7 Inorga nic cont ent in biom ass a fte r lea chi ng (%) pH of leaching solution Acetic Formic HCL Nitric Sulphuric DI water

110 were not generated from the biomass. These could originate from the acetic acid or DI water, but both would be minimal based on the assay of the raw solutions. The most likely introduction of additional ions would be from either the vessel in which the leaching solution was collected or stored. For example, the Na and Si content can increase due to glassware leaching [19].

Table 5-2: ICP-OES results for biomass leached at 30 °C

Element1 (ppm) Al B Ca Fe K Mg Mn Na P S Zn Si2 Raw wood 70.9 3.1 756.0 81.5 524.9 204.0 51.4 60.3 146.6 63.0 6.8 2126.7 DI water 48.3 2.2 647.6 44.6 142.1 173.1 45.6 50.7 107.2 40.5 6.1 1389.4 1% acetic acid 58.2 2.1 198.6 76.9 18.1 38.6 9.1 42.7 111.8 41.2 2.7 998.2 1% formic acid 54.6 1.9 133.8 63.8 19.2 18.5 3.4 44.8 114.5 43.7 1.4 898.7 1% sulfuric acid 42.1 1.2 52.8 44.3 9.5 12.3 1.4 42.4 112.0 82.8 2.4 677.7 1% nitric acid 48.9 0.8 47.5 51.3 10.5 11.9 1.3 42.4 111.3 45.6 1.2 825.1 1% hydrochloric acid 62.8 0.9 84.3 68.4 11.6 18.4 1.8 43.4 115.2 46.3 0.8 644.9

1_{Elements below 2 ppm are not displayed, these include Ba, Cd, Cr, Cu, Li, Ni and V. Elements tested and not detected were As, Co and Pd.} 2_{Si was}

calculated as the difference between the total inorganic content and the sum of the total ions measured

Table 5-3: Mass balance of elements for 1% acetic acid leached biomass at 30 °C

Element(mg) Al B Ca Fe K Mg Mn Na P S Zn

In 70 g of raw biomass 5.0 0.2 52.9 5.7 36.7 14.3 3.6 4.2 10.3 4.4 0.5 In 70 g of 1% acetic acid leached biomass 4.1 0.1 13.9 5.4 1.3 2.7 0.6 3.0 7.8 2.9 0.2 Calculated mass balance for ion removal 0.9 0.1 39.0 0.3 35.5 11.6 3.0 1.2 2.4 1.5 0.3 Measured amount in the leachate 1.7 2.0 44.9 0.6 46.6 12.6 3.0 9.6 3.3 1.9 0.5

Figure 5-6 displays the ash of leached biomass produced after combustion at 620 °C. The colour and density of the ash were different for samples leached with different reagents. Raw biomass produced a darker and denser ash, water washed biomass produced a slightly lighter and less dense ash, while biomass leached with an acid produced the very pale and light ash. Wood ash is a complex mixture of oxides (-Oi), hydroxides (-OH),

carbonates (-COi), and silicates (-SiO4). For example, Etiégni and Campbell [20] proposed that calcium was

present as mainly lime (CaO), calcite (CaCO3), portlandite (Ca(OH)2), and calcium silicate (CaSiO4). They also

observed that average particle diameter of ash was 230 µm. Ash can contain irregularly shaped inorganic particles as well as porous carbon particles. Therefore, the difference ash characteristics were due to complex mixture of compounds formed from the inorganics in biomass. For example, compounds such as MnO are black; therefore a reduction in Mn would alter the ash’s colour.

111 Figure 5-6: 1.5 g of biomass ash after acid leaching, from the left: raw wood, water washed, 1% acetic acid leached, and 1%

formic acid leached