Chapter 3: Methodology and Materials
3.5 Characterisation of biochar
3.5.1.1 Biochar from LTSR
The purpose of this test is to determine the elemental percentage of carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) in the biochar (Refer section 3.4.3). A sample of extracted biochar was made by using an extraction method with methanol on a 10-20 mg crude biochar (unextracted) sample using ASE 200 Accelerated solvent extractor equipment. The unextracted char elemental analysis was performed with LECO True Spec CHN equipment (GmbH, Germany) by a standard method DIN 51721. The C and H contents were analysed by infrared detector and N by thermal conductivity detector. A sample mass of 100 mg was combusted at a temperature of 950 0C and a True Spec software program was used for analysis of the results.
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The extracted char was analysed for elemental analysis of CHN by an elementalAnalyser (Analyse automat Leco TRU SPEC) using the same procedure as in section 3.4.3
3.5.1.2 Biochar from BFBR
The biochar was analysed for C, S, N and H at Stellenbosch University by a Eurovector EA elemental analyser in duplicate (see section 3.4.3)
3.5.2 Heating value
The purpose of this test is to determine the heating value of the biochar. In this study, the heat of combustion was determined by burning a weighed biochar sample in an oxygen bomb calorimeter under controlled conditions according to procedures in section 3.4.2.
3.5.3 Ash content
3.5.3.1 Biochar from LTSR
The extracted biochar sample was used to determine ash content of the biochar. The ash content for the biomasses was determined by Analyse automat MAC-500 equipment according to a standard method (DIN CEN/TS 14775:2004-11) using a LECO TGA7 equipment at 575±25 oC.
3.5.3.2 Biochar from BFBR
The ash contents of the biochar were analysed in a muffle furnace (Gallenkamp, Muffle Furnace Size 2) at 575±25 oC according to a procedure described in section 3.5.2. ASTM E1755-01 was used for this analysis. An electronic balance (Mettler AE 200) sensitive to 0.1mg was used for weighing the samples. The ash was also determined from TGA analysis of the biochar using the same method as in section 3.4.1. For each experimental run, samples were held at room temperature for 1 hour. At this stage, the sample mass would have stabilised at a constant dried weight and was then heated to 700 °C at a heating rate 10°C/min. The purge gas, nitrogen, was set to a flow rate of 15 ml min-1. Subsequent to heating to 700 °C the purge gas was switched to oxygen at the same flow rate of 15 ml min-1 and the biomass maintained at 700 °C for a further 30minutes, to allow combustion of the remaining biochar for the subsequent determination of ash content.
3.5.4 Surface area and total pore volume
Nitrogen adsorption experiments were conducted to determine the specific surface area and pore volume of the biochars using an ASAP 2010, Micromeritics USA, multipoint Brunauer-
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Emmet-Teller (BET) surface area instrument. The BET surface area and total pore volume were obtained by measuring their nitrogen adsorption-desorption isotherms at 77K. The analytical method consisted of three steps, namely dehydration of samples, degassing of sample under low pressure and nitrogen gas adsorption at –196 °C as described in the following section. The BET surface area was only analysed on biochar from BFBR. Dehydration of the biochar sample was conducted overnight in a furnace at a temperature of 105 °C. This procedure is done to remove any traces of contaminants such as oil and water on the surface or within the pores of the biochar particles. The biochar was put in a sample holder and placed at the bottom prior to the degassing step.
The solid biochar in the sample holder and Degas system (Micromeritics Vac Prep 061, Sample Degas System) was heated to a temperature of 90 °C under a vacuum for one hour in order to remove volatiles for one hour. The degassing was continued at 250°C, under vacuum conditions for an extended period of time, usually 2 to 3 days to ensure effective removal of volatiles from the sample before analysis. Degassed samples were directed to the analysis port almost immediately after degassing to prevent any exposure to the atmosphere. The biochar sample was analysed by using adsorptive nitrogen gas which was added in incremental dosages. For the initial experiment, adsorption isotherm, BET surface area (Brunauer et al.,1938), t-plot (De Boer et
al.,1966) and Barret-Joyner-Halenda (BJH) desorption were chosen because these methods were
suitable for microporous material such as biochar.
Total pore volumes (V) were estimated from the amount of nitrogen adsorbed at the highest relative pressure( ) . This involved the selection of a suitable relative pressure range in accordance to the type of material. Relative pressure,( ), which is the actual gas pressure, divided by the vapour pressure of the adsorbing gas at the temperature at which the test was conducted. Data were automatically collected, displayed and analysed by computer. The determination of the pore size distribution and surface area by the machine was based on the relative pressure applied to effect penetration of the nitrogen into the pores. The resulting isotherm was analysed using BET method, while pore size distributions were carried out by BJH desorption method.
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3.5.5 Particle size distribution 3.5.5.1 Biochar from LTSR
The LTSR produced biochar in a mixture with bio-oil and in order to determine the particle size distribution, slurry of 34 wt. % solids was made. A special 1.5kW colloid mixer (MAT, Mischanlagentechnik, Type: sc-05-M) well known for the preparation of a very homogeneous cement mortar was used to prepare a pumpable biochar/bio-oil mixture for both CC and CS (Henrich, 2007). This colloid mixer has high shearing stress (> 104 s-1) to completely destroy the solid biochar agglomerates and form a stable paste of slurry without adding additives. The slurry mixtures were milled in a colloid mill at 50-60Hz frequency (Model DMTT 02 ATEX E 026) to study the effect of biochar particle size on the homogeneity and stability of slurries. The mixed and milled samples were analysed for particle size distributionby an XPT-C Particle Analyser. The analyser was installed with a standard gross camera taking photos of the solid particles. A 10mg sample of the solids is put in a cylinder and methanol was topped up to three quarter full. A stirring rod was used to stir the solution as the measurement progresses. The slurry viscosity measurements (within a small temperature range on the mixed and milled biochar slurries) were studied by a Brookfield R/S Rheometer. The viscometer agitator or vane was immersed in a sample of a depth twice that of the vane height and the vane-container diameter ratios was lower than 0.75 to obtain best results.
3.5.5.2 Biochar from BFBR
The biochar from BFBR was dry due to separation of the gas and solid particles in the dual cyclone system prior to condensation of the organic vapours. A Retsch Model AS 200 was used for particle size sieving on a sample volume of 200 ml, amplitude of 1 mm and analysis time of 10 min.