Guidelines For Low Emission Stove Concepts

Transcript

Future Bio Tec Guidelines for low emission stove concepts International Workshop Technologies for clean biomass combustion September 20th 2012 Graz, Austria Prof. Jorma Jokiniemi University Of Eastern Finland, Fine Particle and Aerosol Technology Laboratory & Technical Research Centre of Finland (VTT), Fine&Nano Particles Future Bio Tec Introduction ■ This document is based on scientific investigations and test runs ■ Improvement of wood stoves ■ application of air staging ■ primary measures for OGC, PM1 and CO emission reduction ■ Support for stove manufacturers ■ Optimization of their products ■ Development ■ Design Guidelines for low emission stove concepts / Jorma Jokiniemi 2 Future Bio Tec Target group ■ Primarily for stove manufacturers for development of low-emission appliances ■ Researchers ■ Stove users ■ Policy makers Limitations ■ Appliances that have a closed fire box ■ Typical stove models ■ Stoves using the updraft combustion principle ■ NOT applicable to ■ ■ ■ Heat storing appliances, Sauna stoves, Cooking stoves Stoves with water jacket Stoves which apply the downdraft principle Guidelines for low emission stove concepts / Jorma Jokiniemi 3 Future Bio Tec Basic definitions ■ Schematic picture of a chimney stove ■ Main combustion chamber ■ ■ ■ Post combustion chamber ■ ■ Combustion of the gasification products and intermediate products PM1: ■ ■ Combustion gases and particles burn out Secondary combustion ■ ■ Fuel gasifies and the majority of the combustion reactions take place Fuel zone and secondary combustion zone Particulate matter below 1 µm TSP ■ Total suspended particulates 4 Future Bio Tec Parameters affecting the emissions ■ Particle emissions ■ Fine particles (particles <1 µm ) – Soot – Formed in the flame when oxidation of the combustion gases is not complete – Fine fly ash – Formed by vaporization and nucleation/condensation of inorganic vapours released during combustion – Organic matter – Formed in incomplete oxidation of combustion gases – Condenses onto particles ■ Coarse particles (particles >1 µm ) – Unburned fuel particles and ash particles from the fuel bed – Coarse particle emissions affected by the air flow through the grate and lenght and shape of the ducts Guidelines for low emission stove concepts / Jorma Jokiniemi 5 Future Bio Tec Parameters affecting the emissions ■ Gaseous emissions ■ The most important gaseous pollutants are OGC, CO and NOX ■ OGC = organic gaseous carbon compounds – OGC is released from the fuel during combustion – Affected by the completeness of the combustion ■ CO = carbon monoxide – Intermediate product from the oxidation of carbonaceous material – Efficiency of combustion affects also CO emissions – More difficult to control during the burn out phase (after flame extinction) ■ NOX = nitrogen oxides – Emissions from wood combustion are fuel derived – Amount of NOX is determined by the nitrogen content in the fuel Guidelines for low emission stove concepts / Jorma Jokiniemi 6 Future Bio Tec General requirements for low emission chimney stoves ■ Adequate amount of combustion air ■ Especially secondary air ■ Sufficient draft ■ Temperature ■ Oxidization of combustion byproducts ■ Temperature is affected by: – Refractory lining in the combustion chamber – The shape and size of the combustion chamber – Window material & size – Location of air nozzles Guidelines for low emission stove concepts / Jorma Jokiniemi 7 Future Bio Tec General requirements for low emission chimney stoves ■ Mixing ■ Is needed to achieve complete combustion ■ Mixing is affected by – The direction and geometry of the air nozzles – The velocities of the flue gas and combustion air – The distribution of different air flows, such as secondary air and window purge air (air staging) – The geometry of the fire box – The use of baffles in the secondary combustion chamber ■ Leakage air should be avoided by using appropriate materials for the door and sealing ■ Short-circuiting of the flue gases should be avoided ■ No gaps between the plate separating the main combustion chamber from the post combustion chamber Guidelines for low emission stove concepts / Jorma Jokiniemi 8 Future Bio Tec Geometric design concept ■ The stove should consist at least of a main combustion chamber and a post combustion chamber ■ Insulation materials should be used in the main combustion chamber to keep temperatures high ■ For example refractory bricks with heat resistant wool and a small air volume between isolation and the outer stove casting ■ Window in moderate size ■ Glass qualities with with low radiation coefficient ■ Double glazed windows (with an air gap) ■ Combustion chamber should be hot enough but the fuel bed should be kept at moderate temperatures Guidelines for low emission stove concepts / Jorma Jokiniemi 9 Future Bio Tec Geometric design concept ■ The flue gas should have enough time to efficiently cool down downstream of the combustion chamber ■ Sufficient heat exchanging surfaces to maximize the efficiency – Should be associated with mainly post combustion chamber – The heat exchange can be improved by introducing forced ventilation ■ A grate should be used ■ Simple deashing ■ However, air flow through the grate should be able to be shut down completely – Only kept open during the first ingition phase and during the last batch after flame extiction ■ Combustion of coal briquettes is possible if the stove is equipped with a grate Guidelines for low emission stove concepts / Jorma Jokiniemi 10 Future Bio Tec Geometric design concept ■ Firebox geometry: ■ High and slim combustion chamber is usually preferable (compared to wide and low) – This shape improves flame dispersion – Leads to more homogeneous residence pattern for the produced pyrolysis gases in the hot zones – Less danger of short circuit flows to the exhaust pipe Guidelines for low emission stove concepts / Jorma Jokiniemi 11 Future Bio Tec Air supply and staging ■ Different air flows are introduced ■ Facilitate optimized fuel decomposition ■ Char burnout ■ Almost complete gas phase burnout ■ An effective way of reducing the emissions in a chimney stove ■ Combustion air can be supplied as primary, secondary and window purge air ■ Primary air: supplied directly to the fuel bed either from below the grate or at the bottom of the combustion chamber (if there is no grate) ■ Secondary air: supplied to the secondary combustion zone – Where burn out of the combustion gases take place ■ Window purge air: – – – – Mainly creates a flush air for the window Can take part in secondary combustion Can also add to the promary air It is recommended to introduce only at the top of the door so that it flows downwards along the window 12 Guidelines for low emission stove concepts / Jorma Jokiniemi Future Bio Tec Air supply and staging • Primary air • Secondary air • Window purge air • Main combustion chamber • Post combustion chamber Guidelines for low emission stove concepts / Jorma Jokiniemi 13 Future Bio Tec Air supply and staging ■ Minimum requirements ■ ■ Primary air and window purge air ■ Should be separately controllable ■ Manual control should be achived by single control (to avoid false operation) Injection of secondary air is strongly recommended ■ Other points of air staging design ■ ■ ■ ■ ■ Secondary air should be preheated Primary air should not be preheated Even distribution of window purge air Pressure drop should be kept low due to limited draught Secondary air nozzles should be at the correct place – With too low nozzles, secondary can act in primary combustion – If they are too high, no optimized mixing of air and flue gases is achieved Guidelines for low emission stove concepts / Jorma Jokiniemi 14 Future Bio Tec Automatic combustion control ■ Reduces user influence on the combustion process ■ Efficient measure for low emissions combustion and improved combustion efficiency ■ The simplest way is to employ a thermo-mechanical operated primary air flap ■ Electronic sensor driven automatic control by monitoring: ■ Temperature (for example in the secondary combustion zone) ■ Oxygen concentration ■ Incompletely burned compounds Guidelines for low emission stove concepts / Jorma Jokiniemi 15 Future Bio Tec Automatic combustion control ■ Examples of automatic control concepts: ■ Different combustion phases can be indentified by temperature changes – T-sencors are the cheapest sensors available for this purpose Æ furnace temperature based control – The combustion air can be easily controlled by dampers Æ temperature controlled combustion air supply ■ As soon as temperature exceeds a certain level, the primary air damper reduces the air supply to avoid excessive burning rates ■ At the same time secondary air is increased to keep adequate combustion air ■ Shorter ignition phase can be achieved – Higher furnace temperatures – Lower gaseous and particulate emissions within a shorter time Guidelines for low emission stove concepts / Jorma Jokiniemi 16 Future Bio Tec Automatic combustion control: examples • Control strategy: as soon as the furnace temperature drops below a certain value, the amount of window purge air should be reduced to keep the temperature at a reasonably high and nearly constant value over the batch ■ In the burnout phase the air supply should be adjusted – excess oxygen is kept low and too much cooling of the combustion chamber is prevented ■ With combustion air flow control during the main combustion and burnout phase a more stable O2 concentrations in the flue gas can be achieved ■ Generally lower O2 levels as well as sufficiently high temperatures can be achieved ■ Control of secondary air injection: ■ When high combustion temperatures are reached at the end of the ignition phase, secondary air should be supplied to improve mixing of the combustion air and flue gases released from the logs to improve burnout ■ Control strategy: the ratio of window purge air and secondary air is recommended to be fixed ■ During charcoal burnout the secondary air should be closed again and only primary air should be injected in order to expedite char burnout Guidelines for low emission stove concepts / Jorma Jokiniemi 17 Future Bio Tec CFD-aided design of wood stoves 5000 4750 4500 4250 4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 Basic geometry (λtot = 2.3) Optimised geometry (λtot = 2.0) transition flue gas exit combustion chamber wood logs post-combustion chamber entrance of flushing air redirection baffle window tertiary air nozzles ■ Iso-surfaces of CO concentrations [ppmv w.b.] in the flue gas in the vertical symmetry plane of a stove ■ Modifications: closure of opening in the redirection baffle; additional tertiary air nozzles; larger transition to the chimney and insulation of the post-combustion chamber Guidelines for low emission stove concepts / Jorma Jokiniemi 18 Future Bio Tec CFD-aided design of wood stoves ■ CFD model developed by BIOS BIOENERGIESYSTEME, Graz University of Technology and BIOENERGY 2020+ ■ Empirical fixed-bed model ■ Can be applied to wood log combustion ■ CFD model inplemented in ANSYS/Fluent ■ Adapted and validated for turbulent reactive flowe in combustion plants Guidelines for low emission stove concepts / Jorma Jokiniemi 19 Future Bio Tec CFD-aided design of wood stoves ■ Because unsteady state simulation of the whole batch is impossible, virtual steady-state operating conditions have been defined ■ An energy balance around the stove as a function of time has been performed based on test run data ■ To reduce possible falsifications by the heat storage ■ Two virtual steady-state operating cases with a heat storage of the stove can be estimated ■ Gas phase simulation ■ Realized k-ε Model for turbulence ■ Discrete Ordinates Model fro radiation ■ Eddy Dissipation Model in combination of with a Methane 3-step mechanism (CH4, CO, H2, CO2, H2O, O2) Guidelines for low emission stove concepts / Jorma Jokiniemi 20 Future Bio Tec CFD-aided design of wood stoves ■ With CFD model for stoves, relevant processes can be analyzed ■ ■ ■ ■ ■ The flow of combustion air The flue gas in stove The flow of the convective air in the double air jacket of the stove Gas phase combustion in the stove Heat transfer between gas phase and stove material ■ Several factors can be simulated ■ Combustion air, convective air and flue gas: – Velocities & temperatures – Path lines – Concentrations of gases ■ ■ ■ ■ Material and surface´temperatures Heat transfer Efficiency Pressure losses Guidelines for low emission stove concepts / Jorma Jokiniemi 21 Future Bio Tec CFD-aided design of wood stoves ■ The CFD-aided development and optimization ■ ■ ■ ■ Can lead to reduced stove emissions (CO and PM) Better utilizations of the stove volume Enhanced efficiency Reduced development times – Less tests – Better security in plant development Guidelines for low emission stove concepts / Jorma Jokiniemi 22 Future Bio Tec CFD-aided design of wood stoves: example ■ CO concentrations before and after optimization ■ Before ■ ■ ■ ■ High emissions Bypass flow Post combustion chamber not insulated Optimized ■ ■ ■ ■ ■ Closure of bypass flow Insulation of the post combustion chamber Higher T in the post combustion chamber Better CO burnout Larger heating surface & better efficiency 5000 4750 4500 4250 4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 Basic geometry (λtot = 2.3) Optimised geometry (λtot = 2.0) transition flue gas exit combustion chamber wood logs Guidelines for low emission stove concepts / Jorma Jokiniemi post-combustion chamber entrance of flushing air redirection baffle window tertiary air nozzles 23 Future Bio Tec CFD-aided design of wood stoves: example ■ Further optimization ■ Additional tertiary air nozzles ■ Optimization leads to ■ Better burnout ■ Reduced excess air ■ Better efficiency 5000 4750 4500 4250 4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 Basic geometry (λtot = 2.3) Optimised geometry (λtot = 2.0) transition flue gas exit combustion chamber wood logs Guidelines for low emission stove concepts / Jorma Jokiniemi post-combustion chamber entrance of flushing air redirection baffle window tertiary air nozzles 24 Future Bio Tec Guidelines for low emission stove concepts will be available online! www.bioenergy2020.eu Thank you for your attention! 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