How do I make a basic combustion characterisation of biofuel?

Transcription

1 Combustion File No: 24 Version No: 1 How do I make a basic combustion characterisation of biofuel? Date: Author(s): Source(s): Referee(s): Status: Sponsor: 5-Nov-1 Johan Hustad/Maria Barrio Authors Peter Roberts Approved NUST Norwegian University of Science and Technology 1. General The general concept of fuel characterisation is outlined in Combustion File 48. The following describes the procedures to undertake a first order or basic combustion characterisation. A biofuel is primarily described by its proximate analysis and ultimate analysis. The proximate analysis can be obtained by means of thermogravimetric analysis (TGA). From the ultimate analysis the calorific value of the fuel can be calculated. This technique can also give information about the rate of devolatilisation and the rate of char oxidation. 2. Thermogravimetric Analysis (TGA) The instrument measures the weight of a biofuel sample inside an oven under controlled temperature. Figure 1 shows how the sample weight decreases as the temperature rises in the oven. The sample first dries and then looses its volatile components (devolatilisation or pyrolysis) at around 3-6 C. The solid residue after devolatilisation is called char and contains mainly carbon (fixed carbon) and ash. The char oxidises at 8-12 C in presence of oxygen. 1 8 m d Drying (1 C) m/m (%) 6 4 Devolatilization (3-6 C) 2 m c m f Char oxidation (8-12 C) Ash Figure 1: Result of the thermogravimetric analysis.

2 3. Proximate Analysis The proximate analysis tells us about the moisture content of the fuel, the amount of volatile components, the fixed carbon and the ash content. The data are usually presented on dry basis. Moisture content (%) = 1 - m d Volatile matter (%, moisture free (mf)*) = (m d - m c )/ m d x 1 Ash content (%, mf) = m f /m d x 1 Fixed carbon (%, mf) = 1 volatile matter (%, mf) ash content (%, mf) The weight loss during devolatilisation depends on the temperature and heating rate and therefore this additional information is usually given. *Often expresses as dry. 4. Ultimate Analysis and Calorific Value This analysis gives the elemental composition of the biofuel, usually in moisture and ash free basis (dry ash free-daf). The basic components are carbon, hydrogen, oxygen, nitrogen and a negligible amount of sulphur. The higher calorific value (HCV) can be calculated from the ultimate analysis as: HCV (MJ/kg dry fuel)= (34.1 C + 12 H N S 9.85 O)/1 where C, H, N, S and O represent the weight(%, mf) of carbon, hydrogen, nitrogen, sulphur and oxygen in the fuel. The higher calorific value can also be measured in a bomb calorimeter by reacting the fuel with oxygen and measuring the heat released to a known quantity of water. The effective calorific value (EHV) is also useful, and for wood fuels is expressed as: EHV (kwh/kg wet fuel)= (moisture content (%) ) EHV (kwh/kg dry fuel)= (moisture content (%) )/(1-moisture content (%) )

3 5. Rate of Devolatilisation It is necessary in many situations to quantify how reactive is the fuel during devolatilisation. The devolatilisation rate is the slope of the TGA curve, as shown in Figure 2. Devolatilisation rate (%/min) = (m 1 (%)-m 2 (%))/(t 2 (min)-t 1 (min)). 1 m/m(%) Devolatilization (3-6 C) m 1 (t 1 ) m 2 (t 2 ) 6. Rate of Char Oxidation Figure 2: Devolatilisation rate. This value quantifies the reactivity of the solid char during the combustion. In this case, the mass of char is not referred to the initial mass of fuel, m, but to the initial mass of char, m c. Char oxidation rate (%/min)= (m 3 (%)-m 4 (%))/(t 4 (min)-t 3 (min)). m/m c (%) Char oxidation (8-12 C) m 3 (t 3 ) m 4 (t 4 ) 2 Figure 3: Char oxidation rate.

4 Glossary terms Proximate analysis - Analysis that gives the content of moisture, volatile components, fixed carbon and ash in the fuel. Daf abbreviation dry ash free. Ultimate analysis - Analysis that gives the elemental composition of the fuel. Higher calorific value (HCV) - Maximum amount of energy that can be obtained from the combustion of a fuel, including the energy released when the steam produced during combustion is condensed. Lower calorific value (LCV) - Same as the higher calorific value, but without including the energy contained in the steam. Effective calorific value (EHV) - Lower calorific value minus the energy required to vaporise the water contained in the fuel as moisture. Thermogravimetric analysis (TGA) - Instrumental technique that measures the weight of a sample and how this weight diminishes as the sample reacts. The sample temperature is controlled. Devolatilisation (or pyrolysis) - Thermochemical process where the volatile components of the biofuel are released. Char oxidation - Thermochemical process where the solid residue after devolatilisation (char) is combusted. Keywords Biofuel; biomass; calorific value; char oxidation; devolatilisation; pyrolysis proximate analysis; reactivity; ultimate analysis. Related Combustion Files 23 - What is biomass? 48 What is fuel characterisation? 12 - What is biomass pyrolysis? 15 - How do I estimate the higher calorific value of biomass fuels? 16 - What general characterisation data is available on solid biomass fuels? 17 - What data is available on main solid wood fuels' properties? Sources Authors Acknowledgements None. File Placing [Power generation]; [Fuels]; [Characterisation]

5 Access Domain [Open Domain] The information contained in this Combustion File is derived from the IFRF Combustion Handbook ( IFRF