Materials Science and Engineering for Clean Biomass Cookstoves

Materials Science and Engineeringfor Clean Biomass CookstovesÖmer DoğanDon Ferguson and James BennettDOE Biomass Cookstoves Technical Meeting – January 11-12, 2011 – Alexandria, VA.

Materials• Materials are enabling technology for cookstoves.• To make clean, efficient, affordable, widely-used anddurable biomass cooking stove, one has to haveappropriate materials for the cookstovecomponents.2

Clean Cookstove Materials• Structural materials– Combustion chamber– Insulation and envelope– Accessories (grate, skirt, etc.)• Functional materials– Thermoelectric generator– Heat conducting probe– Heat sinkN. MacCarty, J.Cedar,Proceedings of ETHOS 20103

Current Examples of Combustion ChamberMaterials• Refractory ceramics– Prefired refractory shape (refractory fired to final“brick”)– Plastic refractory (refractory shaped to need)– Castable refractory (shape cast)– Ceramic fiber (potential backup material)– Powdered refractory filler• Metallic materials– Steel (mild steel, Cr steel, stainless steel)– Cast iron4

Materials Selection Criteria for CombustionChamber• Is the material low cost?• Can the material be produced locally (in thecountry)?• What type of biomass fuel will the stove use?• Will the stove combust or gasify?• What is the shape of the temperature profile in thestove?• Is heat insulation a requirement?• What is the minimum life expectancy?5

FUELTapack6Sjoerd Nienhuys, Improved Cooking Stove –Metal One Pot, August 2010Treshkin

Fuel Types and Potential Flame Temperatures• In the absence of moisture and assuming adiabatic(no heat loss) conditions:Mass FractionFuels C H O N S AshFlame Temperature(K)Bituminous Coal 75.5 5 4.9 1.2 3.1 10.3 2445Ag Waste (CornStover) 47.04 5.47 0.68 41.1 0.06 5.65 2070Wood (Pine) 50.26 5.98 0.03 42.14 0.01 1.58 2253Switchgrass 47.33 5.61 0.67 41.05 0.07 5.27 2150Charcoal 92.04 2.45 2.96 0.53 1 1.02 1300• Addition of moisture and heat loss will reduce flame temperatures7

Emissions from Biomass Fuel Flames• Carbon Monoxide• PM: Volatile Organic Compounds (VOC), Poly-Aromatic Hydrocarbons (PAH)• Sulfur Dioxide, Hydrogen Sulfide, Aldehydes, Etc.• Most of these can be reduced through improvedcombustion– Longer residence times– Increased temperatures8

Select Biomass Ash Chemistry, Quantity, andMelting TemperatureProperty (Wt %)MaterialRiceStrawWheatStrawSwitchGrassSugar CaneTrashDouglas FirWood% Cl (dry wt biomass) 0.7 2.0 0.1 0.2 0.01% Ash (dry wt biomass) 19.5 13.0 9.0 5.0 0.5Oxide (in Ash) - Al 2O 31.4 2.5 4.5 NL 2.8CaO 1.6 4.7 5.6 13.1 37.1Fe 2O 30.7 1.0 2.0 1.7 4.2K 2O 11.3 18.3 11.6 13.4 17.0MgO 1.9 2.5 3.0 4.3 5.9Na 2O 1.9 10.5 0.6 0.3 3.2P 2O 52.7 1.5 4.5 2.3 1.9SiO 274.3 35.8 65.2 57.4 12.3TiO 20.02 0.2 0.2 NL 0.1SO 30.8 5.5 0.4 7.3 11.2Unknown 3.4 17.6 2.3 0.3 4.4Melting Temperature ( o C) 1494 1294 1468 1300 17679NL = Not Listed Source: B. Jenkins, R. Bakker, and J. Wei; On the Properties of Washed Straw; Biomass and Bioenergy; vol. 10(1996), no. 4, pp 177-200.

High temperatures•600°C-900°C (Up to 1100°C reported)•Large temperature fluctuationsCombustion gas andash•Major gas constituents: CO, CO 2 , H 2 O•Minor gas constituents: H 2 S, alkaline vapors•Ash constituents: Fe 2 O 3 , SiO 2 , Al 2 O 3 , CaO, MgO, K 2 O, Na 2 O, SO 3Thermal cycling•3-10 cycles per day between room temperature and 600-900°CLarge thermalgradient•Near room temperature outside surface to 600-900°C inside surfaceHeating / cooling rate•Accidental spills and overflowsMechanical stresses•Dropping stove while carrying or dropping hard objects on it•Fuel handling/inserting•Loading with heavy pots10

HightemperaturecorrosionThermalfatigueThermalshockCreepWear11

High-Temperature Corrosion• Metallic materials– High temperature oxidation (scale formation)– Scale spallation due to thermal cycling– Hot corrosion• Ceramic materials– Slagging conditions12

Thermal FatigueSjoerd Nienhuys, Improved Cooking Stove – Metal OnePot, August 2010• Cyclic temperature / cyclicstress• Constrains to expansion /contraction• Adjacent components atdifferent temperatures and / ormade of different materials• Phase transformations• Fatigue damage (crackinitiation and growth)13

• Low StressCreep• Intermediate temperatures• Stove can be designed so that the load bearingcomponents are not located in the hot zone.14

Wear and Fracture• Fuel handling / inserting• Dropping stove or dropping hard objects on stove• Loss of toughness (temper embrittlement)15

How can we simulate thecookstove operatingconditions that influencematerials?• Computer modeling• Laboratory tests• Field tests16

Location• Fuel type• Fuel chemistry• User habits• Cultural aspectswill determine the operating conditions!!!We have to come upeither with construction materials that are not sensitive to these variablesor with different materials for different locations.17

Materials Production / Fabrication• Sheet steel– Integrated steel plant– Mini-mill (arc melting)• Cast irons– Foundry• Ceramics– Refractory plant18

Summary• Due to local nature of fuel, there will be more thanone solution to the materials problems.• High temperatures and cyclical nature of operationmay cause thermal fatigue in both metallic andceramic materials.• Combustion gases combined with high temperaturesare likely to cause corrosion on metallic materials.• Integration between combustion research andmaterials research is essential.19