Fundamentals of Charcoal Production

Transcription

1 Fundamentals of Charcoal Production Stefan Czernik National Bioenergy Center IBI Conference on Biochar, Sustainability and Security in a Changing Climate September 8-10, Newcastle, U.K.

2 Introduction Charcoal Biomass Pyrolysis Charcoal formation Outline Stoichiometric and thermodynamic potential Biomass pyrolysis pathways Optimum process conditions for charcoal production Technologies for producing charcoal Conclusions

3 What is Charcoal? Charcoal is a solid product of pyrolysis of biomass carried out at temperature above 300ºC. Is black in color, retains morphology of original feedstock, burns without flame. Is not a pure carbon or a single compound. Elemental composition: C, H, O, N, S, ash Proximate analysis: fixed carbon >70%, volatiles, ash Picture from Wikipedia Has been produced for thousands years.

4 Charcoal Global charcoal consumption: 45 Mton/year Africa 23 Mton/year South America 17 Mton/year WEC 2007 Survey of Energy Resources FAOSTAT-Forestry (Ethanol global production: 60 Mton/year) Cost of charcoal: $ /ton Applications: fuel, metallurgy, activated carbon Emerging use as a soil amendment and a carbon sequestrating material. 5.5 Gton carbon released annually by combustion of fossil fuels can be offset by 7.5 Gton of charcoal used as soil amendment

5 Biomass Biomass is a plant matter, renewable product of photosynthesis. Includes trees, grasses, agricultural crops and residues, animal wastes and municipal solid wastes. World terrestrial biomass resources 120 Gton/year. Field, C. B. et al., (1998) Science 281, Potential for 30 Gton/year charcoal. Starch, fat, and protein rich biomass Lignocellulosic biomass: trees, grasses, agricultural residues Food supplies <20% of total biomass. Non food >80% of total biomass.

6 Lignocellulosic Biomass Cellulose: 38% - 50% Extractives: 1% - 5% Average elemental composition: CH 1.4 O 0.6 Most abundant form of carbon in biosphere Polymer of glucose Hemicellulose: 23% - 32% Polymer of 5- and 6-carbon sugars Xylose is the second most abundant sugar in the biosphere Lignin: 15% - 25% Complex aromatic structure p-hydroxyphenylpropene building blocks

7 Pyrolysis of Biomass Thermal decomposition occurring in the absence of oxygen. At temperature above 300ºC biomass polymeric building blocks undergo crosslinking as well as partial depolymerization and fragmentation to form smaller molecules which are released as gases and vapors that can react with residual solids producing more condensed structures. Pyrolysis always produces solid (charcoal), liquid (water and organics), and gaseous (CO, CO 2, CH 4, H 2 ) products at proportions and composition dependent on feedstock and on process conditions.

8 Slow heating of biomass Temperature Solid Phase Gas Phase <200ºC Drying H 2 O 230ºC-250ºC Retification Acetic acid, MeOH 250ºC-280ºC Torrefaction Extractives 300ºC-500ºC Devolatilization Organics, H 2 O, gas >500ºC Carbonization Tars, H 2 O, gas

9 Stoichiometric: Charcoal Yields CH 1.4 O 0.6 CH H 2 O 53.0% (100% C) Thermodynamic: Cellulose: C 6 H 10 O C H 2 O CO CH % (62.4% C) Antal, M,J. and Gronli, M, Ind.Eng.Chem.Res 2003, 42, Practical: CH 1.4 O 0.6 charcoal + gas + liquid 10-35% (15-60% C)

10 Biomass Pyrolysis Pathways Primary Processes Secondary Processes Tertiary Processes Vapor Phase CO, CO 2, H 2 O Primary Vapors Light HCs, Aromatics, & Oxygenates Olefins, Aromatics CO, H 2, CO 2, H 2 O PNA s, CO, H 2, CO 2, H 2 O, CH 4 CO, H 2, CO 2, H 2 O Liquid Phase Low P Low P Primary Liquids High P Condensed Oils (phenols, aromatics) Tars High P Solid Phase Biomass Charcoal Coke Soot Pyrolysis Severity Evans, R.J. and Milne, T.A., Energy & Fuels 1987, 1,

11 Biomass Pyrolysis Processes Char Liquid Gas CARBONISATION 35% 30% 35% low temperature long residence time FAST PYROLYSIS 12% 75% 13% moderate temperature short residence time GASIFICATION 10% 5% 85% high temperature long residence time

12 How to Enhance Charcoal Formation? Charcoal is a product of both primary (char) and secondary (coke) reactions Increasing charcoal yields requires minimizing the carbon losses in the form of gases and liquids and promoting the desired pathways: primary solid-phase dehydration, decarboxylation, and decarbonylation reactions secondary conversion of pyrolysis vapors to solids

13 Charcoal Yields Charcoal yields depend on feeedstock and on process conditions: Cellulose, hemicellulose, lignin and ash content Pyrolysis temperature Process pressure Vapor residence time Particle size Heating rate Heat integration (biomass burn off).

14 TGA of Biomass Polymers heating at 10ºC/min to 600ºC in nitrogen Beech wood xylan Aspen lignin Cellulose (cotton linters)

15 TGA of Poplar Wood heating at 10ºC/min to 600ºC in nitrogen Aspen lignin Beechwood xylan Cellulose (cotton linters)

16 Increasing Charcoal Yield The charcoal yields will increase with: High hemicellulose, lignin and ash content in biomass Low pyrolysis temperature (<400ºC) (but also lower fixed carbon content) High process pressure (1 MPa) (higher concentration of pyrolysis vapor increases rate of secondary reactions) Long vapor residence time (extended vapor/solid contact promotes secondary coke forming reactions) Low heating rate (slower formation and escape of organic vapors) Large biomass particle size (low thermal conductivity of biomass results in slow heat and mass transfer rate within particles) Optimized heat integration (minimized biomass burn off)

17 Heat for Charcoal Production <280ºC 300ºC-500ºC >500ºC Endothermic (drying, depolymerization and devolatilization) Exothermic (char formation) Endothermic (char carbonization) Heat for the process can be provided: directly as the heat of reaction by flue gases from combustion of by-product and/or feedstock directly to the reactor by flue gases through the reactor wall

18 Charcoal Production Batch processes: Yield Earth pits and mounds >10% Brick, concrete, and metal kilns 20-25% Retorts 30% Continuous processes: Retorts (Lambiotte) 30-35% Multiple hearth reactors (Herreshoff) 25-30% Novel processes: Flash carbonization 40-50%

19 Earth Mound Charcoal Production A - fuelwood B - carbonisation zone C - charcoal zone Release of pyrolysis gas and vapor to atmosphere No heat recovery; significant wood burn off for process energy Low yield; environmental pollution Swedish earth kiln with chimney Simple technologies for charcoal making FAO Forestry Paper 41, Rome 1987.

20 Lambiotte Retort Continuous operation Wood moves down in countercurrent with hot flue gases from combustion of pyrolysis gas Cool charcoal is retrieved at the bottom through a lock mechanism Pyrolysis liquid are condensed and used as byproduct Pyrolysis gas is heated in the bottom section then burned FAO Forestry Paper 63, Rome 1985

21 Flash Carbonization Batch operation; 10 tons/day charcoal. Biomass loaded to a canister then heated up to 350ºC at 0.7 MPa for min. Charcoal yield 40-50% (70-80% fixed carbon). Catalytic afterburner for tars eliminates smoke from reactor effluents. Capital cost $200,000. HNEI Flash Carbonization Demonstration Reactor

22 Conclusions Charcoal is a product of primary and secondary reactions occurring during pyrolysis of biomass. High yields of charcoal are favored by: high lignin content and large particle size of biomass, low temperature and high process pressure low heating rate and long vapor residence time in the reactor, heat integration of the process Production processes have to include by-product recovery or use for process energy to decrease detrimental environmental impact of traditional methods.