03-j.jokiniemi-guidelines for low emission stove concepts

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Guidelines for low emission stove concepts

Prof. Jorma Jokiniemi

University Of Eastern Finland, Fine Particle and AerosolTechnology Laboratory

&

Technical Research Centre of Finland (VTT), Fine&Nano Particles

International Workshop

Technologies for clean biomass combustion

September 20th 2012

Graz, Austria


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Introduction

This document is based on

scientific investigations and test

runs

Improvement of wood stoves

application of air staging

primary measures for OGC, PM1and CO emission reduction

Support for stove manufacturers

Optimization of their products

Development

Design

Guidelines for low emission stove concepts / Jorma Jokiniemi2


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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


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Basic definitions

Schematic picture of a chimney

stove

Main combustion chamber

Fuel gasifies and the majority of

the combustion reactions take

place

Fuel zone and secondary

combustion zone

Post combustion chamber

Combustion gases and particles

burn out

Secondary combustion

Combustion of the gasificationproducts and intermediate

products

PM1:

Particulate matter below 1 m TSP

Total suspended particulates 4


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Parameters affecting the emissions

Particle emissions

Fine particles (particles 1 m )

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



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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



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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



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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



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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 combustionchamber to keep temperatures high

For example refractory bricks with heat resistant wool and

a small air volume between isolation and the outer stovecasting

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



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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 lastbatch after flame extiction

Combustion of coal briquettes is possible if the stove is equipped

with a grate



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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



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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 alongthe window

Guidelines for low emission stove conce ts / Jorma Jokiniemi

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Primary air

Secondary air

Window purge air

Main combustion chamber Post combustion chamber


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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



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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



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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

furnace temperature based control

The combustion air can be easily controlled by dampers

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 adequatecombustion air

Shorter ignition phase can be achieved

Higher furnace temperatures

Lower gaseous and particulate emissions within a shorter time



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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



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CFD-aided design of wood stoves

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


Basic geometry

(tot = 2.3)Optimised geometry

(tot = 2.0)

window

entrance of

flushing air

flue gas

exit

combustionchamber

post-combustion

chamber

tertiary air

nozzles

wood logs

redirection

baffle

transition

5000

4750

4500

42504000

3750

3500

3250

3000

2750

2500

22502000

1750

1500

1250

1000

750

500

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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 incombustion plants


i


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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 beenperformed 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-

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)


F t Bi T


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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 surfacetemperatures

Heat transfer

Efficiency

Pressure losses



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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


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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


Basic geometry

(

tot = 2.3)

Optimised geometry

(

tot = 2.0)

window

entrance of

flushing air

flue gas

exit

combustion

chamber

post-combustion

chamber

tertiary air

nozzles

wood logs

redirection

baffle

transition

5000

4750

4500

4250

4000

3750

3500

3250

3000

2750

2500

2250

2000

1750

1500

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750

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Further optimization

Additional tertiary airnozzles

Optimization leads to

Better burnout

Reduced excess air

Better efficiency


CFD-aided design of wood stoves: example

Basic geometry

(tot = 2.3)Optimised geometry

(tot = 2.0)

window

entrance of

flushing air

flue gas

exit

combustion

chamber

post-combustiochamber

tertiary air

nozzles

wood logs

redirectionbaffle

transition

5000

4750

4500

4250

4000

3750

3500

3250

3000

27502500

2250

2000

1750

1500

1250

1000

750

500

250

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Guidelines for low emission stove concepts will be

available online!www.bioenergy2020.eu

Thank you for your attention!


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03-j.jokiniemi-guidelines for low emission stove concepts

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