The effect of particle size and pyrolysis on the performance of low-smoke fuels in coal stoves

Transcription

1 The effect of particle size and pyrolysis on the performance of low-smoke fuels in coal stoves Frans B Waanders 1, Lu Sumbane 1, John R Bunt 1, Hein WJP Neomagus 1, Stuart J Piketh 2 1 School of Chemical and Minerals Engineering, NWU Potchefstroom, South Africa, 2 School of Geo- and Spatial Sciences, NWU, Potchefstroom, South Africa 1,2 CoE in C-based fuels

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3 Overview 1. Introduction 2. Aims & Objectives 3. Performance test theory 4. Experimental overview: Phase Characterisation results 6. Effects of pyrolysis on char properties 7. TGA measurements 8. Burning rates and bed core temperatures 9. Combustion test results Performance assessments Pollution reduction assessments Selection of optimum fuel 10. Conclusions 11. Acknowledgement

4 Introduction Over 1.7 million homes use coal as a domestic source of energy [1,2] 3 million tons /year burnt domestically 3-5% of national coal consumption 20% of country s particulate emission Electrification of townships did not curb domestic coal burning due to: [2, 3,4] Cost and convenience over electricity Societal perception Coal is burnt in inefficient stoves leading to [5] Air pollution Health risk to users

5 Aim and Objectives AIM: Evaluate the technical viability of a low-smoke fuel (LSF) prepared from lump coal of different size and produced via pyrolysis OBJECTIVES: Produce a LSF from coal lumps of various sizes (15mm, 20mm, 30mm, and 40mm) with pyrolysis conducted at T = 450 C C Determine the effects of pyrolysis temperature on the properties of the subsequent chars Assess the performance and pollution reduction abilities of the LSF during combustion tests Compare results with fuels produced at different pyrolysis temperatures Select the best performing fuel

6 Performance Tests theory Stove power output: LHV f (MJ.kg -1, (d.a.f) = lower heating value, M f (g) fuel mass, Dt (s) = time System efficiency : S(%) = Siegert efficiency, other symbols as above Thermal efficiency: M w, = mass (g) H 2 O in cooking pot, M e = H 2 O evaporated (= 0), M f = fuel mass (g), M r = residue mass, Cp w and H vap = spec. heat cap. and latent heat of vaporisation of H 2 O, D T = H 2 O temp change in o C Heating efficiency: symbols as above Combustion efficiency: 32.8 MJ.kg-1 and CV f (MJ.kg -1 = gross cal. value of carbon and fuel

7 Experimental Coal sample procured from the Kwadela township network similar as residents do. Coal comes from the Mpumalanga Highveld, South Africa. Experimental Phases Parent Coal Time Programmed Pyrolysis Chars Characterisation Benchmark Anthracite Combustion Tests Characterisation analyses subcontracted to Bureau Veritas Testing and Inspections South Africa Combustion test done in collaboration with NWU Climatology group. Fuels tested: Coal (15, 20, 30 and 40 mm ), anthracite, 450-char, 550-char, 650-char, 750-char.

8 Experimental: Phase 1 Time Programmed Pyrolysis 450, 550, 650, 750 C 5 C/min 3 L/min -3 sweep N 2 2 hr holding time Tube Furnace

9 Experimental: Phase 2 Stove and Temperature Probes Combustion Tests 2 hour runs in Union no 7 stove Performance Assessments Pollution Reduction Assessments Ignition time Time taken for bottom and core temperature to reach 700 C and 500 C respectively Maximum stove top temperature Measured 20 cm from the bottom of the coal heap Water boiling time Time taken for 1L of water to boil from room temperature Space heating time Measured how long the fuel is able to provide space heating Combustion efficiency

10 Experimental: Phase 3 Chimney and Pollution Analysis Probes Gaseous emissions CO, CO 2, NO and SO 2 Particulate matter emission Only Total PMs Measured Volatile Organic Compounds

11 Characterisation Results Rank Classification from %RoV analyses Rank Classification improves with pyrolysis temperature

12 Characterisation Results

13 Effects of Pyrolysis on Char Properties Chars produced at increasing pyrolysis temperature showed increases in: 1. Mass reduction during pyrolysis 450 C = 10.7% 550 C = 17.1% 650 C = 20.0% 750 C = 23.8% 2. Rank classification* * ASTM:D388 Method Parent Coal = Medium C rank bituminous 750-char = High A rank Anthracite 3. Fixed and elemental carbon content Parent Coal = ~70% 750-char = ~95% (d.a.f) 4. Gross calorific value Parent Coal = 29 MJ/kg 750-char = 34 MJ/kg (d.a.f) Chars produced at increasing pyrolysis temperature showed decreases in: 1. Volatile Content Parent Coal = 33% 750-char = 3.9% (d.a.f) Consistent with increase in rank 2. Elemental nitrogen, hydrogen and sulphur content

14 TGA measurements Gross calorivic value 22 MJ/kg for coal, different particle sized coal and LSF chars Increase in pyrolysis temperature to produce LSF results in: increase in: rank, fixed carbon content, gross calorific value decrease in volatile matter yield, hydrogen, nitrogen and total sulphur content Particle size little effect TGA experiments

15 TGA measurements Different coal particle TGA results Particle size not really influencing results Different LSF particle TGA results

16 Burning rates and bed core temperatures Mass consumption rates Bed core T Ignition point Particle size not really influencing results Cooking stage

17 Performance Assessments Water boiling time But why water boiling time? Staple food in RSA is mieleipap and water has to be boiled Boiling times do not differ much for most fuels - 15 mm coal an exception => cause is the low burning rate during cooking time Drop in water boiling time from coals to LSF's. Ave boiling times = reported literature values. Boiling times are only dependent on the rate at which energy is supplied.

18 Pollution Reduction Assessments NO x emissions factor Expected: NO x should increase with decreasing particle size =>No correlation to particle size observed NO x emissions factor ranges between 0.25 and 0.55 g.mj -1

19 Pollution Reduction Assessments SO 2 emissions SO 2 emissions are linked to the S-content and combustion conditions => LSF's can be altered in pyrolysis S-content range from % d.a.f in coal to % d.a.f. in LSF's Point where SO 2 emissions peak = further in the cooking cycle for coal and at ignition point for the LSF's => gives an indication of the S-origins

20 Pollution Reduction Assessments Co and CO 2 emissions factor CO:CO 2 ratio speaks to efficiency of combustion CO 2 is a greenhouse gas and a reduction in emissions is desired

21 Pollution Reduction Assessments Particulate emissions Coals reported a higher PM than the LSF's with a large drop in the CompLSF PM's The higher the volatiles the higher the PM emissions Coals used contribute to the organic component of PM emissions

22 Pollution Reduction Assessments Volatile compounds Treated fuels (LSF's) show a decrease in aromatic and polyatomic aromatic volatile compounds LSF's achieved higher combustion temperatures => improving complete combustion

23 Selection of Optimum Production Temperature A decision matrix was used to determine the fuel with the best overall performance Preferred fuel

24 Conclusions 1. Pyrolysis Chars prepared at increasing pyrolysis temperatures display Increase in rank, fixed carbon content, CV decrease in volatile yield 2. Comparative Performance and Pollution Reduction Assessments Low-volatile fuels (650-char,750-char) perform better in pollution (PM, SO 2 and VOC) reduction High-volatile fuels (450-char, 550-char) were best performing in terms of efficiency, boiling and ignition time. The high fixed carbon content of the high-temperature chars resulted in longer space heating times 3. Selection of Optimum Production Temperature A decision Matrix taking into account all the assessed parameters showed that the 550-char (i.e 550 C) had the best average performance. 4. The way forward Address shortcomings in ignition time and temperature methods Use various stoves/combustion vessels in order to increase efficiency Pilot Tests on best performing fuel Ascertain the economic implication and possibility of mass production Design the facility to carry out the devolatisation and make LSF available to public

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26 Acknowledgements A special extension of gratitude to: 1. Mrs L Sumbane for collating all data and results. 2. Prof. JR Bunt as main study leader in this project. 3. Mr MJ Kuhn for his invaluable involvement in the work. 4. Mr WJ Smit for producing the 650/750-chars, his participation during the combustion tests, and the discussions. 5. Prof SJ Piketh and the NWU climatology group for the equipment and assistance during the combustion tests. 6. Mr J Kroeze and the team at the workshop for their inexhaustible effort in creating the bits and pieces we needed. Literature [1] Sowazi, S and Maake R Clean Energy Initiatives in South Africa: Qalabotja Micro-Scale Experiments as a Case Study. In Domestic Energy Conference. Minerals and Energy Policy Centre. [2] Mduli, T The Societal Dimensions of Domestic Coal Combustion: People s Perception and Indoor Aerosol Monitoring. PhD Thesis, University of Johannesburg. [3]Lloyd, P Challenges in Household Energisation and the Poor. Journal of Energy in Southern Africa. Vol 25(2). [4] Balmer, M Household Coal Use in an Urban Township in South Africa. Journal of Energy in Southern Africa. Vol 25(2). [5] Franz, L Low-smoke Fuel to Strike Sparks. Engineering News. [6] Le Roux, L., Cilliers, K. & van Vuuren D Low-smoke fuels: Standard Testing and Verification. CSIR report.

27 The research presented in this paper was hosted by the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and National Research Foundation of South Africa (Coal Research Chair Grant No ). Any opinion, finding or conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard.

28 In 2018, the Centre of Excellence (CoE) in C-based fuels was launched This centre is built around 3 research chairs: NRF SARChI Chair in Coal Research Eskom Specialisation Centre for Emissions Control DST/NRF Research Chair in Biofuels and Other Clean Alternative Fuels Coal / Biomass conversion (gasification, pyrolysis, tar formation) Emissions monitoring, quantification and control (PM, SO X, NO X, CO 2 ) Mineral matter transformational behaviour and Ash mineralogy Fine coal re-use (briquetting / extrusion) Dry coal beneficiation Biomass and waste utilisation Water management and chemistry relevant to coal fired power plant operation