Combustion basics. Fuels, basics. Martti Aho VTT Processes, University of Jyväskylä. Composition of dry fuel:

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1 Combustion basics Martti Aho VTT Processes, University of Jyväskylä 2 Fuels, basics Organic part Composition of dry fuel: Ash: Unburnt inorganic residue Si, Ca, Mg, Fe, Na, K... Impurities: S, P, Cl... Nitrogen Hydrogen Oxygen Carbon

2 3 Fuels basics, unit wt% Fuel C O H N S ash Bark ,5 <0,5 <0,5 2-4 Forest resid <0,5 <0,5 2-4 Straw ,5 <0,5 <0,5 3-7 Oth. Agr <0,5-5 <0, Peat , , Coals ,5-5 0,5-2 0, Energy units toe MWh GJ toe = 1 11,63 41,87 MWh = 0, ,6 GJ = 0, , toe = öljyekvivalenttitonni k kilo 10 3 T tera M mega 10 6 P peta G giga 10 9 E eksa MW = MJ/s

3 5 Important calculation basis Heating values (H) of fuel (MJ/kg) Calorific heating value (dry basis) = H cal Effective heating value (dry basis) = H eff H eff = H cal * l 25 * mh 2, where Constant = M H 2 O/ M H 2 = l 25 vaporisation heat of water at 25 o C (2.443 MJ/kg) mh 2 = mass of hydrogen in dry fuel (kg H 2 / kg fuel) Heating value as received H rec H rec = H eff * (1-mH 2 O) - I 25 * mh 2 O, where mh 2 O= mass of water in fuel (kg H 2 O/ kg fuel) For ideal gas: V 2 = V 1 x T 2 / T 1, 1 mol = 22.4 dm 3 NTP Atomic weights: H= 1.01 O= S= C= 12,01, N= Important calculation basis, continued Energy from the fuel during complete combustion P (as MW): P = H rec * mass flow of fuel (as kg/s) Energy content in fuel Q (as MWh): Q = H rec * m / MWh where m is the mass of fuel kg Energy density in the fuel E (as MWh/ m 3 ): E = H rec * D rec / 3600, where D rec is density of the fuel as received (kg/m3) Rough calorimetric heating value from elemental composition: H cal = 0.34* C *(H-O/8) * S MJ/kg (elements in dry ashless fuel)

4 7 Heating values of different fuels Heff MJ/kg dry fuel min max coal lignite peat crushed wood needles bark straw rape seed reed canary grass REF tyre waste pulp sludge black liquour manures Source: Taipale Wood fuel properties. Source: Opet Property Logging residue chips Whole tree chips Chips for delimbed small trees Bark (soft wood) Bark (hard wood) Sawdust Pellets Moisture content,w-% Lower heating value,dry, MJ/kg Lower heating value, as received, MJ/kg Bulk density, kg/m 3 loose Energy density, MWh/ loose m

5 9 Combustion basics Air composition: O , N Ar 0.9 vol% Main stages of combustion: Drying (<100 o C) Pyrolysis 160- ( ) o C (heat destroys fuel structure liberating CO, CO 2 and hydrocarbons as gas and tars) Ignition o C (biomasses) Combustion (250 o C -, volatile combustion, char combustion) Main combustion reactions: C + O 2 -> CO 2 CO +1/2 O 2 -> CO 2 2H + 1/2 O 2 -> H 2 O S + O 2 -> SO 2 10 Combustion, basics Direct oxidation C + O 2 -> CO 2 C +1/2O 2 -> CO, CO+1/2O 2 ->CO 2 Volatile combustion Char combustion Pyrolysis C, H -> CO, CH 4, CxHy... Oxidation: CO + 1/2O 2 -> CO 2, CxHy + n O 2 -> z CO 2 + q H 2 O, glow= hot submicron C particles

6 11 Boiler efficiency: N= boiler efficiency = 100% * (heat + electricity)/ fuel energy N= (qp + qs + qg + qr) % qp = losses from unburnt carbon fuel in the fly and bottom ash gs = losses of heat in the flue gases qg = losses from unburnt gaseous compounds in the flue gas qr = radiation and conduction losses Large power plants > 90%, modern stoves and fireplaces 70-80% Efficiency of electricity production (If CHP boiler) : 100% * electric power/fuel energy Fluidized bed combustion Martti Aho VTT Processes, University of Jyväskylä

7 13 Importance of fluidised bed combustion In Finland, thousands of MW s heat and power are produced by fluidised bed technology Boilers with simultaneous heat and power production can convert >90% fuel energy to electricity + heat FB technology has many advantages in combustion of wet and inhomogeneous biomass High mass of hot material balances fluctuations in fuel quality Biomass mixtures are typical fuels Bubbling bed boilers (BFB) are simpler than circulating fluidised bed boilers (CFB), but sulphur capture is more effective in CFB boilers Many manufacturers of FB boilers operate in Finland 14 Bubbling bed combustion Flue gas cleaning Heat transfer 2-3 m/s freeboard Fuel Bubbling sand C C C Secondary air Primary air

8 15 Bubbling bed/ fuel handling, Rauhanlahti power plant Co-firing of wood - Fuel Handling and Co-combustion Source: Fortum 16 Circulating fluidised bed combustion heat 7-10 m/s C Return cyclon sand fuel sek Primary air

9 Grate combustion Martti Aho VTT Processes, University of Jyväskylä 18 Main features of grate combustion Traditional (old) combustion method for wet biomasses (bark etc ) The fuel dries, pyrolyses ignites and burns on the grate There is ONLY FUEL on the grate (in contrast to FB combustion) There is high temperature distribution in the furnace (because balancing sand is missing) Good method in small scale combustion of wet biomass (< 10 MW th ) FB boilers dominate at present in larger scale ( MW) Please note: Many European counties are conservative: Grate combustion still largely used in large boilers and also in incineration plants

10 19 Classification of grate boilers on size basis Boiler type fuel power (kw Single house size Large buildings Regional and district heating Industrial boilers Boilers for urban waste * * = Abundant in Europe Class > kw is dominated by FB 20 Basis of fuel analysis Kosteus Tuhka Water Ash S N O H C Vola tiles Haihtuvat aineet Residual carbon Jaannos hiili Dry ashless Kuiva, tuhkaton (daf) Dry Kuivaaineesta (d) As received Saapumiskosteudessa (ar)

11 21 Grate combustion 1-30 MW th 22 Wood chips: Most important fuel in biomass grate boilers

12 Mechanical step grate 23 Illustration of grate combustion Fuel layer Moist fuel Drying pyrolysis char combustion Radiation Convection Comb. air Source: Poltto&palaminen 2002 (prim) 24 Grate boiler furnace Pre combustion furnace Furnace Boiler Flue gas Water tube boiler Secondary air Primary air Flue gases out EKOPOINT.DSF

13 25 Modern grate furnace for moist fuel (Wärtsilä) To boiler Refractory mass for stabilisation Fuel feeding Source: Wärtsilä 26 Grate heating boiler air Post comb. furnace Water tube boiler Hot water District heat Precombustion furnace air ash Flue gas cleaning Wet wood up to 65 wt% H2O Source: Wärtsilä Ash container

14 Small-scale combustion (< 1 MW) Martti Aho VTT Processes, University of Jyväskylä 28 Small-scale combustion (< 1 MW th ) Stokers (even > 1 MW!), stoves and fireplaces Fuel is fed continuously by screws in the stokers The most advanced stoves and fireplaces can also have continuous feeding (for example wood pellet stoves and fireplaces) However, they operate usually in batch basis Batch basis is very old way to add fuel: The same batch dries, ignites, burns in flame and as char before the next batch Such a continuous changing process is very challenging to control (produces higher emission levels unburnt ib fly ash etc.) The flue gas cleaning techniques cannot as effective as in large boilers (in smallest furnaces there is no flue gas cleaning, the largest can have multicyclons or even ESP s)

15 29 Fire wood use in small scale combustion in Europe PJ year 1995 Polttopuun pienkäyttö PJ/a Ranska * Saksa * Italia * Kreikka ** Ruotsi * Romania * Suomi * Itävalta * * lähde: ABF-net V, Export & Import and fuel prices ** lähde: EUROSTAT-tilastot vuodelta 1995 Portugali * Puola * Viro * Espanja * Latvia * Alankomaat * Iso-Britannia ** Slovenia * Tanska * Irlanti * Slovakia * Luxemburg ** Belgia * 30 Use of firewood in small scale combustion per capita in EU 14 Polttopuun pienkäyttö asukasta kohti (keskim. 1,4 GJ/a) GJ/a Viro * Suomi * Slovenia * Ruotsi * Kreikka ** Latvia * Itävalta * Portugali * * lähde: ABF-net V, Export & Import and fuel prices ** lähde: EUROSTAT-tilastot vuodelta 1995 Ranska * Romania * Luxemburg ** Saksa * Italia * Irlanti * Tanska * Alankomaat * Slovakia * Puola * Espanja * Iso-Britannia ** Belgia *

16 31 Importance of firewood as a heating source of houses in Finland Portion of individual houses Osuus kiinteistöstä, % Puu päälämmönlähde Puu lisälämmönlähde as a main heating source as an completing heatinmg source Lähde: Työteho seura 15/2002 (658) Maatila farm houses Vapaa-ajan asunto freetime houses Omakotitalo single houses Kaikki pientalot houses, total 32 Chip stoker equipped with pre-furnace ( kw) Fuel storage (wood chips) Unloader * Continous feed Pre-furnace Post-furnace Source: Biomass handbook 2002 Feeder

17 33 Simpler chipstoker without pre-furnace ( kw) * Continuous feed Tmax about 1000 C Source: Biomass handbook Combustion of wood in batch type stoves: continously changing process 1. Drying 2. Pyrolysis 3. Gas comb 4. Char comb 1. Puu kuivuu. Vesihöyry vapautuu. Esimerkki puun panospoltosta 2. Kaasumaiset palavat ainekset vapautuvat kuivasta puusta. 3. Kaasut syttyvät ja palavat. O 2 CO 2, H 2 O 4. Jäännöshiili palaa ja jäljelle jää tuhka O CO 2 2 flame combustion NO, ppm Haihtuvien aineiden palaminen Aika Jäännöshiilen palaminen NO 4 CO up again! CO :00 2:59 6:00 9:00 11:59 14:59 18:00 20:59 23:59 26:59 0 CO ja CH, x y %, char combustion Aika, min:s Source: H. Oravainen/ E. Alakangas VTT Pal_vaih.cdr/EAA/1003

18 35 Two sequential batches Esimerkki puun panospoltosta Puupanos 3 kg Kokonaishiilivedyt, mg/m 3 CO-pitoisuus, % 9000 Nopea pyrolyysikaasun vapautuminen Nopea pyrolyysikaasun vapautuminen batch batch CO Jäännöshiilen Jäännöshiilen Jäännöshiilen 1500 palaminen palaminen palaminen 0.5 CH x y :46 11:49 11:52 11:56 11:59 12:02 12:06 12:09 12:12 12:16 12:19 12:22 12:26 12:29 12:32 12:36 12:39 12:42 Aika Source: Heikki Oravainen/Eija Alakanjas VTT PanoPoltto.cdr/EAA/ Traditional co- or counter current stoves Co-current (only batch) Counter current (continuos feeding possible!) Prim. air Batch: continu ously changing process! Prim. Air! Source: Poltto&Palaminen 2002

19 37 Ready batch (5 kg) + ignition source Source: Altener/E. Alakangas 38 Hot gas circulation in stoves Source: Tulikivi

20 39 Wood pellets: promising fuel for small-scale combustion: Competitive price, low ash content, constant composition, high energy density, continuous feed => better combustion control Origin: timber residue (sawdust) 40 Pellet storage for a single house: Dimensions about 2.5 x2x2 m source: Biowatti

21 41 Wood pellets: unloading and small hot water boiler Pneumatic unload from truck Continuous feed* Hot water Flame Tmax about 1000 C * Continous feed and constant fuel quality results in stable combustion 42 Stages in single pellet combustion Direct oxidation C + O 2 -> CO 2 C +1/2O 2 -> CO, CO+1/2O 2 ->CO 2 Volatile combustion Char combustion Pyrolysis C, H -> CO, CH 4, CxHy... Volatile oxidation

22 43 Types of pellet burners Vertical Blue arrows indicate combustion air Horizontal Source: Tullin/Oravainen Cup 44 Source: H. Oravainen