Teoría del modelo de Adaptación de Callista Roy

5.3.1 Size reduction

Size reduction of the as-received waste biomass samples were needed prior to characterisation and carbonisation in order to ensure that they fit in the quartz reactor used for carbonisation and steam activation.

78

The GW and JKWS samples were oven-dried whereas JKWR samples were first air-dried to enable the soil to dry out and removed by wire brushing the rhizome pieces.

Once dried, JKWS samples were broken by hand into 30-50mm sizes whereas GW samples did not require this step. JKWR samples were more difficult to reduce in size. The large rhizome crowns had to be cut using a band saw into 30-50mm sizes.

All size-reduced samples were crushed in a Glen Creston cross beater mill (Glen Creston Ltd., London, UK) with a 10mm pre-breaker plate.

5.3.2 Moisture content

Moisture content (MC) was determined in triplicate using ASTM Method D4442-07 (ASTM, 2007b).

Samples were oven-dried at 105°C for at least 24 hours or until there was no further weight change.

The oven used was a Gallenkamp Size 2 Hotbox oven (Weiss-Gallenkamp, Loughborough, UK).

Samples were allowed to cool in a desiccator before weighing to avoid re-adsorption of moisture from the atmosphere.

The moisture content was calculated using Equation 5-1:

Equation 5-1:

Where: A= original mass in g and B= oven-dried mass in g.

5.3.3 Lignocellulosic content

The lignocellulosic content of the waste biomass samples were carried out in triplicate according to ASTM Method D1106-96 (2007) for lignin (ASTM, 1996a) and ASTM Method D1109-84 (2007) (ASTM, 1984) for hemicellulose.

In determining lignin content, the waste biomass is hydrolysed with strong acids, leaving the insoluble lignin as a residue. The sample is first extracted by ethanol-toluene using Soxhlet extraction apparatus to remove waxes, fats, resins and some gums. The residue is then hydrolysed

79

using 72% sulphuric acid (Analar grade), washed with distilled water, filtered, dried and weighed to determine lignin content.

Hot alkali extracts low molecular weight carbohydrates consisting mainly of hemicelluloses and degraded cellulose in biomass (ASTM, 1984). The sample is boiled in a beaker of 1% sodium hydroxide (Analar grade), using a water bath to maintain the temperature. The sample is then filtered, washed, dried and weighed. The hemicellulose content is determined according to Equation 5-2:

Equation 5-2:

Where: W1= weight of moisture-free sample and W2= weight of dried sample after treatment with NaOH.

The cellulose content is determined by subtracting the lignin, hemicellulose and ash contents from the total sample weight.

5.3.4 Ash content

The ash content of the waste biomass was determined according to ASTM Method D1102-84(2007) (ASTM, 2007a). The ash content of a precursor is important in determining its suitability as a feedstock for activated carbon (Marsh and Rodríguez-Reinoso, 2006).

Pre-dried samples were weighed and placed in pre-ignited porcelain crucibles with lids. These were then placed into a muffle furnace and heated slowly to 600°C to avoid flaming and lids were used to protect the sample inside the crucible from strong drafts. The muffle furnace used was a Gallenkamp Size 2 muffle furnace.

Samples were left in the furnace until all carbonaceous material was burnt off and only inorganic ash was left. The crucibles were allowed to cool to room temperature in desiccators before being weighed.

The ash content of the sample was calculated using Equation 5-3.

80

Equation 5-3:

Where: W1= weight of ash and W2= weight of oven-dried sample.

5.3.5 Thermal Analysis using TGA

Thermal analysis of the biomass samples and subsequent char samples was conducted using a Rheometric Scientific STA 1500. Thermal analysis involves measuring the weight changes as a sample is heated in an inert/non-inert atmosphere. This is termed thermo-gravimetric analysis (TGA). TGA also refers to the instrument, the thermo-gravimetric analyser.

The plot of weight change versus temperature is known as a thermogravimetric (TG) plot and its derivative is known as a DTG (derivative thermogravimetric) plot. The common procedure involves placing the sample in a crucible and placing the crucible on a high precision microbalance, which records the weight changes during heating of the furnace. This gives a TG plot for the sample and the DTG is determined by the software.

The STA 1500 thermal analyser also makes use of a technique called Differential Scanning Calorimetry (DSC). DSC makes use of a crucible containing an inert reference material in addition to the sample crucible. The difference in temperature between the two crucibles and the amount of energy needed to heat either crucible is measured. This allows the determination of an endothermic or exothermic reaction. A negative heat flow value indicates that the sample temperature has decreased due to an endothermic reaction and that heat was supplied to the sample crucible whereas a positive heat flow indicates that the sample temperature increased due to an exothermic reaction and that heat flow was supplied to the reference crucible (Beshara, 2010).

Alumina crucibles of ca. 4.5mm diameter were used. Nitrogen was used as the inert atmosphere and air was used for a reactive atmosphere. All samples to be tested were ground and sieved to ≤ 500µm to homogenise the samples. For the purposes of this project, the TG analysis was of more importance than the DSC analysis.

The TGA was used to perform initial tests on the thermal degradation behaviour of the samples in an inert atmosphere. Flow rate was set to 50 ml/min. The TGA was used to determine the temperature

81

of the onset of biomass degradation and the temperature at which maximum weight loss occurred.

It was also used to determine the proximate analyses of the samples.

Proximate analysis gives the moisture content, volatile matter, fixed carbon and ash contents of the sample (Robinson et al., 2010). The TGA allows for small samples to be used and the results can be used to design experiments based on the thermal characteristics of the feedstock samples.

The parameters for proximate analysis using the thermal analyser are as follows:

 Heating the sample to 105^oC in an inert atmosphere initially to drive off the moisture,

 The temperature of the furnace is then increased to 950^oC to determine the volatile matter content of the sample,

 The furnace is allowed to cool down to 600^oC where the inert atmosphere is substituted for an oxidising atmosphere (air/oxygen); allowing for combustion and therefore, the determination of the fixed carbon and ash contents.

The method for proximate analysis as described was taken from Robinson et al. (2010), who modified ASTM Method E870-82.

The fixed carbon content, as given by the proximate analysis of biomass, indicates the amount of non-volatiles carbon available that is not converted into bio-oils and syngas, which should end up in the char and finally, the activated carbon. The volatiles content gives us the expected total yield of bio-oil and syngas.

5.3.6 CHNS analysis

CHNS (carbon-hydrogen-nitrogen-sulphur) was performed by an external laboratory, MEDAC Ltd. in Surrey UK. This test was done by combustion analysis where the samples are flash combusted with the help of oxidation catalysts at 900°C and the combustion products are analysed using a chromatographic column where the individual components are separated and eluted as nitrogen, carbon dioxide, water and sulphur dioxide to determine the concentrations of C, H, N & S in the

82

sample (MEDAC Ltd., 2007). The oxygen content is determined by difference, based on the assumption that the sample only contained CHNS and O including incombustible inorganic minerals.