Thermal use of agricultural biomass (PDF) - bioenergybaltic

BOVA course“Energy Crops and Biogas Production”3-7 March 2008 in TartuThermal use of agricultural biomassM. WachendorfDepartment of Grassland Scienceand Renewable Plant ResourcesUniversity of Kasselemail:web:mwach@uni-kassel.dewww.agrar.uni-kassel/gnrDepartment of Grassland Scienceand Renewable Plant Resources

Water content 45-60% in fresh wood (type of tree/wood harvest time); reduction of net energy yield(exeption: furnace utilising condensation energy) Storage losses above 16% through biodegredation and transformation; firehazard with grasslandHeating value Heating value (HV) strongly affected by water content (usually based onwater (and sometimes ash-) free DM Wodden solid fuels are 9% higher in HV than cereals, hay and straw, as theycontain more lignin and extracts Thumb rule: 2.5 kg DM wood ≈ 1 l fuel oil ≈ 10kWh ≈ 36 MJ HV of dry biomass is correlated with contents of oxidisable elements (mainlyC, H and S, N) => approximation of HV by elemental constitution:HV waterfree(MJ/kg) = 34.8 C + 93.9 H + 10.5 S + 6.3 N - 10.8 O (after BOIE)Department of Grassland Scienceand Renewable Plant Resources

Heating value (continued) HV depends on ashcontentType of BiomassHeatvalueH l (wf)MJ/kgHeatvalueH u (wf)MJ/kgAsh Volat.content compon.(wf) (wf)in % DMin % DMAsh softening behaviourHard coalLigniteSpruce woodBeech woodPoplar woodWillow woodBarkRye strawWheat strawTriticale strawBarleyRapeseed strawMaize strawSunflower strawHemp strawRye WCSWheat WCSTriticale WCSRye grainWheat grainTritical grainRapeseed grainMiscanthusNature conservation hayFestuca arundinaceaeIntensive grasslandDitch cutDepartment of Grassland Scienceand Renewable Plant Resourcesa SB: start of sintering; EP: softeing point; FP: flowing pointbwater content of hard coal 5.1%; lignite 50.4%

Heating value (continued)WoodHeating valueKS, Abb. 8.8Grasses etc.(% of DM)Ash content(Kaltschmitt, 2001)Department of Grassland Scienceand Renewable Plant Resources

Upper heating value Upper heating value (HV u) ≈ 8% higher than HV During cooling of flue gases condensation of water vapour adds to theenergy yieldEnergy yield based on H uEnergy yieldbased on H lHeating valuesH lH uEnergy yield(Kaltschmitt, 2001)Water contentWeight of solid fuel at 1 kg DMDepartment of Grassland Scienceand Renewable Plant Resources

Nitrogen Occurs in biomass proteins, amines, and NPN constituents Is totally transformed into the gaseous phase during combustion NO x-formation is correlated with N content of biomass Manipulation through:- Choice of species and developing stage of crops- Addition of O in furnace- Catalysts (NH radical to reduce N to N 2)Department of Grassland Scienceand Renewable Plant Resources

NitrogenConventional combustionAddition of OUse of catalyst (SNCR)StrawGrassMiscanthusRest woodWoodChipboardUse of catalyst (SCR)N content in solid fuel(Kaltschmitt, 2001)Department of Grassland Scienceand Renewable Plant Resources

Properties of biomass as relevant for combustionType Biomass of BiomassHard coalLignite% of DMSpruce woodBeech woodPoplar woodWillow woodBarkRye strawWheat strawTriticale strawBarleyRapeseed strawMaize strawSunflower strawHemp strawRye WCSWheat WCSTriticale WCSRye grainWheat grainTritical grainRapeseed grainMiscanthusNature conservation hayFestuca arundinaceaeIntensive grasslandDitch cutDepartment of Grassland Scienceand Renewable Plant Resources(Kaltschmitt, 2001)

Potassium (Sodium) K mainly in vegetative parts of plants (=> Wheat grain) Variability in straw is due to fertilisation and leaching after harvest (=> greystraw) Both facilitate corrosion of furnace: K => alkaline chlorides => K sulphateduring condensation on heat changer => Free Cl 2=> FeCl on furnace wallSulphur Easily volatile; few knowledge on integration in ash Low contents in cereals => no measures for reduction necessaryChlor HCl concentration during combustion of grass: 20-120 mg/Nm³ (target valueof 30 only possible by means of separation (e.g. washer)Department of Grassland Scienceand Renewable Plant Resources

Pathways of biomass K, Cl und S through the furnacesolidsolid=> Low separation in cyclones, high in fabric filter(Kaltschmitt, 2001)Department of Grassland Scienceand Renewable Plant Resources

Properties of biomass as relevant for combustionBiomass Type of BiomassHard coalLignite% of DMSpruce woodBeech woodPoplar woodWillow woodBarkRye strawWheat strawTriticale strawBarleyRapeseed strawMaize strawSunflower strawHemp strawRye WCSWheat WCSTriticale WCSRye grainWheat grainTritical grainRapeseed grainMiscanthusNature conservation hayFestuca arundinaceaeIntensive grasslandDitch cutDepartment of Grassland Scienceand Renewable Plant Resources(Kaltschmitt, 2001)

Heavy metals Heavy metals (HM) are tranformed into the gaseous phase withtemperatures of 800 to 1100 °C in the furnace During cooling-down of flue gas vapours condensate on fine particles=> Furnace ashes are low, separator ashes are high in heavy metalconcentrationHeavy metal concentration(mg/kg DM)straw whole crops bark chopped Higher contents in wood caused by:- increased dry and wet deposition of HM in forests- growth periods of 70 to 120 years facilitate HM accumulation in wood- soil pH in forests lower than on arable land => increased solubility of HMDepartment of Grassland Scienceand Renewable Plant Resources

Ash content and softening pointT ep= 1172 - 53.9 * K + 252.7 * Ca - 788.4 * Mg0% Mg 0.4% Mg 0.8% Mg200020002000160016001600120012001200TepTepTep8008008004004002.44002.42.41.81.81.8000.81.62.4K3.244.85.601.20.6Ca000.81.62.4K3.244.85.601.20.6Ca000.81.62.4K3.244.85.601.20.6Ca Increase of T epthrough the addition of- calcium containing amendments (lime)- alkaline-binding amendments (Kaolin)- leaching during field phase (straw, hay)Department of Grassland Scienceand Renewable Plant Resources

Mineral composition of grassland speciesg kg -1 TMmg kg -1 TMPflanzengruppeCaPMgNaKFeMnCuZnGrasses Holz4,3 3,03,0 0,51,8 0,5o.N. 0,120,0 1,5o.N. 60o.N. 60o.N. 7o.N. 25Legumes Gräser17,0 4,34,5 3,04,5 1,80,4 0,135,0 20,0110 6055 6012 735 25Herbs15,05,54,10,322,090409,025(Kaltschmitt, 2001; Haiger et al., 1988)Department of Grassland Scienceand Renewable Plant Resources

Fine particulatesFine particulate emmissionK, Cl, S concentration in the fuel (% of DM)GrassStrawGrassStrawMiscanthusMiscanthus(Kaltschmitt, 2001) Target value of 150 mg/Nm³ is often exceeded=> Separation of fine particulates is mandatory (Cyclone, fabric andelectrostatic precipitators, washerDepartment of Grassland Scienceand Renewable Plant Resources

λλλλCereal crop compatible incinerators Due to problematic properties of crops- complex technique necessary- not a common fuel for small private furnaces Controlled supply of air into the furnace= m Luft,total/ m Luft,min= air excess valuem air,total: Total quantum of air fed into the furnacem air,total: Minimum quantum of air necessary for complete oxidation( >1: full oxidation (usually 1.5 to 2.5); 0<

λCereal crop compatible incineratorsprobethermistorsuction fancombustionzoneflue gasheat changersolid fuelashautomaticignitionflue gascombustion gasash pusherfancombustionmouldIncinerator for bulk goods (e.g. straw pellets, chopped material, residues fromseed production, broken grains, milling residues; 49 kW)Department of Grassland Scienceand Renewable Plant Resources

Cereal crop compatible incineratorsfeeding gateheat changersecondaryair supplywater-cooledwallfanIncinerator for straw bales (continuously and discontinuously; above 85 kW)German emission regulations do not allow such furnaces (CO, fine particles)Department of Grassland Scienceand Renewable Plant Resources

Flue gas cleaning by cyclonesRotational motion creates centrifugal force Separation of particles of 2-5mThe grade of separation can be improved by installing several cycloneswithin one casingScheme of a cyclone and multi cycloneraw gas(tangentialentry)piperaw gas(tangentialentry)clean gasclean gascasecentralupstream vortexconeto the bunkersrew conveyerDepartment of Grassland Scienceand Renewable Plant Resources(Kaltschmitt, 2001)

Flue gas cleaning by washersWetting of particles by water dropletsDownstream separator e.g. cycloneHigh energy demand depending on the levelof cleaningPrinciple ofVenturi washer1. raw gas entry2. clean gas exit3. feeding of washliquid(Kaltschmitt, 2001)Department of Grassland Scienceand Renewable Plant Resources

Flue gas cleaning by electrostatic precipitatorParticle motion is caused bynegative charging in a strongelectromagnetic fieldCleaning of precipitationelectrode (anode) throughbeatingHighest degrees of precipitationeven for fine particulatesspraying electrodeOperation of electrostatic precipitatorselectroneparticulatecharged precipitatedprecipitationelectrode(anode)neutral ionizedgas moleculesDepartment of Grassland Scienceand Renewable Plant Resources(Kaltschmitt, 2001)

Flue gas cleaning by fabric filtersfiltering through sieving effectsand mutual particle adhesionmade of screen cloths,ceramics, metallsnote flying sparksCross section of a fabric filterelectronedusting downprocedurefilteringproceduresupportingcagefiltermediumDepartment of Grassland Scienceand Renewable Plant Resources(Kaltschmitt, 2001)

Degree of flue gas cleaningFabric filterDegree of flue gas cleaningElectromagneticfilterVarioustypesofwasherCycloneParticle size(Kaltschmitt, 2001)Department of Grassland Scienceand Renewable Plant Resources