Houtkachel in laboratorium “schoner” dan in praktijk

REAL-WORLD PARTICULATE EMISSIONS FROM WOOD HEATERS: A REVIEW

John J. Todd  

kachel pelletDirector, Eco-Energy Options Pty Ltd, Hobart;

Adjunct Prof Edith Cowan University, Faculty of Computing, Health and Science


<BRON PDF MET TEKENINGEN>

 

VERTAALD GEDEELTE: Onderzoek naar fijn stof uitstoot van houtkachels in de praktijk van John J. Todd, september 2013

figuur 1

figuur 1

Todd heeft onderzoek gedaan naar PM 10 en 2,5 uitstoot van houtkachels in de praktijk en die vergeleken met de uitkomsten van laboratorium tests.
Hij heeft daarbij gebruikt gemaakt van onderzoeksgegevens uit Australië, Nieuw Zeeland en Noord Amerika.

De uitstoot van fijn stof in de dagelijkse praktijk worden gekenmerkt door grote variaties tussen huishoudens, in sommige gevallen betrof het een factor 13.
Maar ook zijn er van dag tot dag binnen één enkele huishouding verschillen waargenomen oplopend tot een factor 8.
De grote spreiding in uitstoot doet vermoeden dat de huidige generatie kachels sterk onderhevig is aan de wijze van gebruik.
Houtkachels moeten worden voorzien van gecontroleerde automatische toevoer van zuurstof om een aanmerkelijke afname in de uitstoot van fijn stof te bewerkstelligen, aldus Todd.

In figuur 1 van de publicatie wordt aan de hand van onderzoek van Scott (2005) duidelijk gemaakt hoe zeer de uitstoot van houtkachels verschillen in die van laboratorium tests en bij gebruik in de praktijk.
Zoals valt af te lezen kunnen de verschillen oplopen tot een factor 12.
Daarmee wordt duidelijk dat lab waarden voor houtkachels “schone schijn” zijn.

 

Abstract Originele tekst vervolg 

 

Published papers and reports dealing with real-world PM10 and PM2.5 emissions (as distinct from standard emission tests in laboratories) from residential wood-burning are reviewed.   Research from Australia, New Zealand and North America is included.  Real-world emissions of particles are characterised by large variations from household to household (by a factor of 13 in results reviewed in this study) and from day-to-day in a single household (typically by a factor of around 8).  Laboratory simulations of real- world heater operation give some indication of why emission factors vary so much.  The large spread of emission factors suggests the current generation of wood heaters is sensitive to the way the heater is operated in the home. Wood heaters may require automatic control of combustion air in order to significantly improve real-world emission factors.   A review of the PM10 emission factor for controlled combustion wood heaters suggested in the Australian NPI Emissions Estimation Technique Manual for Aggregated Emissions from Domestic Solid Fuel Burning is desirable.

Keywords: wood heater; real-world emissions; particulate matter

 

 1.  Introduction

This paper reviews and discusses the implications of real-world measurements of wood smoke from controlled combustion wood heaters.   Real-world (sometimes referred to as real-life) emission measurements are made in people’s homes with householders operating the heater ‘normally’ with their own firewood.

 1.1.   Wood combustion

Wood combustion is not uniform; it is a complex physical and chemical process, especially in batch- fed, manually operated wood heaters.  When logs are added to a hot fire, free moisture is driven off; then, as wood temperature increases, pyrolysis releases a complex mix of combustible gases; and finally the charcoal residue burns through surface oxidation.  Smoke is generated through incomplete burning of the combustible gases.   These gases may  fail  to  burn  because  of  insufficient  oxygen, poor mixing of gases and oxygen, lack of ignition points or flame quenching.   The ignition point for wood gases is around 600⁰C.   This means for combustion   there   must   be   flame   or   glowing charcoal to ignite these gases.

Good  combustion  requires:  (a)  a  well  designed wood heater, (b) relatively dry firewood and (c) careful, informed operation of the heater. Control of combustion air, fuel loading geometry and refuelling intervals appear to be major factors influencing emissions.

In practice, some combustible gases always remain unburnt,   they   condense   to   form   sub-micron particles (droplets of oils and tars) or, those with higher  boiling  points, remain  gaseous.    The  fine particles scatter light giving wood smoke its characteristic  white  appearance.    There  is  little doubt   that   exposure   to   wood   smoke   causes adverse health effects, even at smoke concentrations  experienced  in  some  cities  and towns in Australia and New Zealand.    Two particularly relevant studies reinforcing this health impact are a recent study by Johnston et al. (2013) which showed significant reductions in mortality in Launceston, Tasmania after implementation of a successful wood smoke reduction program, and a study in Libby Montana which showed significant reductions in children’s respiratory symptoms when exposure to wood smoke was reduced (Noonan et al. 2012).   Reviews by Naeher et al. (2007) and Bølling   et   al.   (2009)   provide   more   general discussion   of   why   wood   smoke   presents   a significant health risk.  Clearly, it is very desirable to reduce exposure to residential wood smoke where possible.

Particulate  emissions  are usually  reported  as  an emission factor expressed as grams of particles per oven-dry kilogram of wood burnt (g/kg).

 

 2.  Measuring Real-World PM Emissions

 Measurement of wood heater particulate emissions at people’s homes requires  moveable  equipment that is  not  overly intrusive  and  does not  require significant modification of the flue or heater.  In the research covered in this review three methods have been used to measure particle emissions.   Two involve sampling directly from the flue within the house and one samples from a device fitted at the top of the flue.  In all studies reviewed the firewood supply available at the house is burnt and the heaters are operated ‘normally’ by the householder.

 

2.1.   Measurement Methods

 

2.1.1. CSIRO sampling method

The real-world emission study discussed below by Meyer et al. (2008) used the CSIRO sampler.  This equipment is fitted on top of the flue, extending flue height by 1.2m (which increases draft in the flue), but this is off-set by an orifice plate (to measure flue velocity) which decreases natural draft.  A sample of exhaust gas and particles is drawn through a sample probe where it is cooled and diluted with outside air.   Particle mass is determined using an aerosol monitor (DustTrak) with parallel filtration (to calibrate the monitor).   A second sample stream passes through a gas analyser measuring CO and CO2 (Meyer et al. 2008).

 

2.1.2. Condar/Oregon Method 41

Several of the New Zealand real-world emission surveys were carried out using Oregon Method 41 (also referred to as the Condar Method).  A sample is drawn directly from the flue and diluted with room air to cool it before passing through a filter.   CO2 concentrations are monitored simultaneously in the flue and the diluted sample to allow calculation of the dilution factor.  Good correlations are achieved with the AS/NZS4013 method when tested in the laboratory (Wilton and Smith 2006).

 

2.1.3. VPI Sampler

The VPI Sampler method provides a measure of particles and CO by drawing a sample from the flue through filters by means of an evacuated tank.  The system can be left unattended for a week.  Analysis of the gases collected in the tank allow calculation of the mass of fuel burnt.   The method has been calibrated against US Method 5G (Jaasma et al. 1990).

 

2.2.   Laboratory Simulation

Several studies have attempted to simulate real- world PM emissions in the laboratory.   From a practical perspective this is desirable because it allows analysis of various operating practices that lead to high emissions.   However, it seems more


needs to be known about how people actually operate their heaters before this approach can be considered a meaningful alternative to field studies.

Scott (2005) carried out an interesting set of measurements  in  New  Zealand  where  several wood heater models were tested in the laboratory using the standard operating and refuelling method (as specified in AS/NZS4012:1999) and a simulated real-world operating and refuelling method. The same models were tested in a private home using the same simulated real-world operating and refuelling method; and finally the householder operated the heater as they normally do with their own fuel.

Seven  heater  models  were  included  in  Scott’s study, although all models did not undergo the full set of tests.  The simulated real-world tests resulted in higher emissions than the standard laboratory tests (on  average by a factor of about 2.5), the simulated real-world tests in the laboratory and in people’s homes gave similar results, but the emissions from tests in people’s homes with the occupant operating the heater gave much higher emissions (on average by a factor of 4 compared to the simulated real-world tests and by a factor of 12 compared to standard laboratory tests).   Figure 1 shows results for a typical heater model. 

Figure 1. Emission results (g/kg) for one of Scott’s (2005) wood heater models (SRW = simulated real-world operation in the laboratory or a home; RW = real-world with householder operating heater as they normally do).

The number of heater models tested was too small to  draw  firm  conclusions,  but  the  consistency across the models tested suggests:

•  The standard emission test gives significantly lower emissions than real-world,

•  Simulating real-world operation in the laboratory or in the home is difficult, and

•  When householders operate a wood heater much higher emissions may occur.

Todd and Greenwood (2006) also carried out emission measurements in the laboratory using simulated  real-world  operating  conditions.    Four

 

heater models were tested.   Simulated real-world emission results were 4 to 6.5 times greater than emissions measured using operating conditions specified in AS/NZS4012:1999.  The heater models were not tested in people’s homes.

 

3.  Real-World Emission Studies

 

Real-world emission measurements presented here are divided into those from Australia and New Zealand and the United States. In some studies

the 24-hour wood use and PM emission is measured allowing calculation of an average daily emission factor. In other cases only weekly fuel and PM emission totals are available.

 

3.1.   Australia and New Zealand

Four published papers and reports providing wood heater particle emission measurements made in people’s homes are reviewed (three from NZ and one  from  Australia).    The  test  method  used  to certify new wood heaters in Australia and New Zealand is the same (AS/NZS4012/4013) although maximum allowable emission limits in the two countries are different (see Todd 2013).

 

3.1.1. Daily average emission factors

The three studies from NZ provide daily emission factor measurements from 27 households. Figure 2 shows all measurements ordered from lowest to highest.  They range from 0.92 g/kg to 90.6 g/kg, a spread of almost two orders of magnitude.

 

 

 

Figure 2. Daily emission factor measurements (g/kg) from measurements  at  27  households  in  NZ  (Scott  2005; Wilton and Smith 2006; Kelly, Mues and Webley 2007)

 

Some households seem to produce very high emission factors on some days and much lower on other  days  (see  Figure  3).     Other  households exhibit  much  lower  variation  in  emission  factors from day-to-day.    The results do not allow conclusions to be drawn about why some heaters show such large daily variation – it might be due to heater design with some designs very sensitive to


relatively small changes in operating practices, it might be due to large day-to-day differences in the way the heater is operated, the fuel used, or a combination all three.

 Figure  3:  Daily  emission  factors  from  four  houses. Houses A and B are examples of houses with large daily variation (maximum/minimum ratios of 39 and 16 respectively)  and  houses  C  and  D  are  examples  of houses  with  relatively  small  daily  variation  (max/min ratios of 1.9 and 1.6 respectively).

 

Of the 27 houses, six had a ratio of maximum to minimum daily emission factors greater than 10; the median ratio was 5.2.

 

3.1.2. Weekly average emission factors

The only field study providing real-world emission factors in Australia was carried out in Launceston, Tasmania by the CSIRO (Meyer et al. 2008).  The published report provides weekly average emission factors for 18 households.

Figure  4  shows the spread  of average  emission factors for 45 houses in Australia and New Zealand.

As noted by Meyer et al. the Australian and New Zealand emission results show a similar range and have similar average values (9.4 and 10.9g/kg respectively).  This is interesting as softwoods are mainly burnt in NZ and hardwoods in Australia.

 

3.1.3. Emission factors for ‘low emission’ heaters

One of the NZ real-world emission studies selected only heaters that were certified to the newer NZ emission  limit  of  1.5g/kg  (reduced  from  4g/kg) (Kelly et al. 2007).  This study, which is included in the overall average emission factors discussed above, resulted in an average real-world emission

 

factor for 9 houses of 4.7g/kg. This was significantly lower than the average emission factor for non- certified wood heaters: 14g/kg (Wilton and Smith

2006).

 

 

Figure 4. Weekly average emission factors (g/kg) for Aust

(light colour bars) and NZ (dark coloured bars) homes

 

3.2.   United States

In the United States a number of field studies to determine real-world emissions of wood heaters have been conducted commencing in the mid-

1980s. The results of these studies have been used to develop average emission factors for various classes of wood burning heater (Table 1).

Type  of  wood  burning heater

Emission Factor g/kg
Pre-Phase I

12.9

Phase I

10

Phase II

7.3

Open fireplaces

17.3

 

Table  1.  Wood  heater  and  fireplace  emission  factors(PM10) in AP-42 for non-catalytic heaters (US EPA 1996)

 

Overall, for non-catalytic wood heaters the US EPA recommends an emission factor of 9.8g/kg.   The US EPA also publishes a table of emission factors for various gases (e.g. CO), PAHs, other organic compounds  and  selected  trace  metals  (US  EPA

1996).

The US real-world emission measurements are similar to those observed in Australia and New Zealand as illustrated by examining data published by  Correll  et  al.  (1997).    The  data  (Figure  5) includes catalytic (7 heaters) and non-catalytic (6 heaters) wood heaters.   Interestingly, the catalytic heaters had much higher emissions than the non- catalytic  heaters  (22.8  and  9.8g/kg  respectively). The   authors   attribute   high   emissions   for   the catalytic heaters to degradation or blocking of the catalysts.    The non-catalytic heaters showed average   emission   figures   very   similar   to   the


Australian and New Zealand figures.   This is discussed further in the following section.

 

 

 

Figure 5. Average weekly emission factors (g/kg) for 13 households in Crested Butte, Colorado.

 

3.3.   Impact of high emission households

From Figure 2, if it is assumed that each household burnt the same quantity of wood on the day of the measurement, then the worst 5% of the houses would contribute 22% of the PM emissions, and the worst 10% of the houses would contribute 34% of the PM emissions.   The cleanest 10% contribute just 1% of the PM emissions.

This observation that a relatively small proportion of wood heaters is responsible for a large fraction of particles emitted is supported by observations of visible smoke (Figure 6).  Thick or very thick smoke was observed from 5% of households.  Thus, it is more effective for pollution control authorities to target households with high smoke emissions.

 

 

Figure 6. Visibility of smoke from observations of over

4000 operating wood heaters using a scale of 0 to 5 (adapted from Todd 2004)

 

4.  Discussion

 

4.1.   Choosing an average PM emission factor

The choice of a realistic average emission factor for the whole population of wood heaters in use in a region is important for planning and air quality management.  The average PM emission factor is used in emission inventories as a means of determining the significance of the various sources of air pollution for a region.

 

In Australia, the National Pollutant Inventory (NPI) Emissions Estimation Technique Manual for Aggregated Emissions from Domestic Solid Fuel Burning suggests a PM emission factor of 5.5g/kg for PM10 from controlled combustion heaters.   It appears that this 5.5g/kg figure refers to the mass of PM per kg of ‘as-used’ firewood rather than the oven-dry mass of the firewood.  Adjusting the value to the conventional oven-dry basis gives increases the value to 6.4g/kg (assuming 16% moisture for the firewood).  This might still be too low by about

35% based on the limited data available.  Meyer et al.’s study indicates an average emission factor of

9.4g/kg.   US and NZ field studies, of which there have been many, also suggest average emission

factors   for   the   whole  population   of  controlled

combustion wood heaters of around 10g/kg.   It seems difficult to justify the lower value suggested in the NPI.

If the 5.5 g/kg (or 6.4g/kg) figure is increased it could make significant changes to estimates of the contribution of wood heaters to PM10 and PM2.5 concentrations.  Using the 5.5g/kg emission factor, residential wood-smoke is estimated in some regional NPI studies to contribute 53% of annual PM10 (Todd 2008).

For planning purposes, it is also important to know whether  or  not  tighter  emission  limits  for  new models of wood heater lead to lower real-world PM emissions. The Australian real-world emission measurements throw no light on this question. However, one NZ study does suggest new heater models subject to lower emission limits do, in fact, perform better in the field (see 3.1.3 above).

 

4.2.   Significance of emission differences day- to-day in a single house

Day-to-day differences in emission factors for a single house have been observed to vary by up to a factor of 39.   It is most unlikely this is caused by sampling uncertainty or by changes in firewood quality (since the firewood is from one wood-pile). It is not heater model.  It is almost certainly day-to- day changes in the way the heater is operated that cause these large fluctuations.

A laboratory study of the effect of some common operating practices on emissions (Todd and Greenwood 2006) showed:

•  Setting the combustion air control to slow burn immediately after refuelling increased average emissions by a factor of 1.8 on medium burn rates and 3.4 on slow burn rates;

•  Operating  a  convection  fan  when  running  a heater at its minimum burn rate increased emission factors by a factor of 2.7;

•  Adding logs to a relatively cool coal bed without re-igniting  the  fire  with  newspaper  resulted  in


very   large   increases   in   emissions,   highly variable results meant it was not possible to quantify this effect;

•  Placing logs poorly so that combustion air was diverted away from the base of the fire resulted in large but unquantified increases in emissions;

•  Attempting to burn a single large log (rather than

2 or 3) also resulted in large but unquantified increases in emissions.

Thus, it is relatively easy to explain why emissions from one household vary so much from day-to-day.

Wood heaters need to be less sensitive to these common   operating   practices   is   average   PM emission  factors  are  to  be  significantly  reduced. This might require automatic controls and improved combustion chamber design.

 

4.3.   Significance of emission differences from house to house

All studies of real-world emission factors show a large   spread   of   values   across   the   houses monitored.  The differences between houses seem too large to be accounted for by different heater models.    This suggests that the way the householders operate their heaters is a significant factor.   In some cases households have relatively consistent (day-to-day) emission factors.   This suggests  consistent  good,  bad  or  average operating practices.  In other cases one or two very high emission days in the week can lead to high average emission factors, suggesting inconsistent operating practices.

 

4.4.   Suggestions for further research

It  is  desirable  to  have  more  real-world  emission tests carried out in Australia.  The research should collect information on:

•  Flue height (because this has a large influence on emissions at slow burn rates but does not appear to be recorded in published work to date)

•  Heater   model  (important  for   comparison   of certified emission factors and real-world emissions)

•  Firewood moisture

• Operating practices including combustion air control settings and refuelling intervals

•  Daily variation in emission factors

With this information it (a) allows assessment of the ranking of appliances based on current standard test methods i.e. is the standard laboratory test method effective; and (b) assists in determining which fuel, operation and installation parameters lead to higher or lower emissions.

5.  Conclusions

Measurements of wood heater particle emission factors  made  in  people’s  homes  show:  (a)  real- world   emission   factors   are   much   larger,   on average, than emission factors measured in standard laboratory tests; (b) large differences in emission factors may occur from one day to the next in a single household; and (c) large differences in emissions between households occur when averaged over periods of a week or more.

In   many   houses,   the   day-to-day   changes   in emission factors are too large to be explained by sampling uncertainties.   The changes are unlikely to be caused by differences in fuel quality since wood from the same firewood stack is used.   It is reasonable to assume that the differences are due to  operating  practices,  perhaps  even  relatively small   changes,   from   one   day   to   the   next. Occasional very high emission days push the average emissions well above what might be possible.   This suggests the current generation of wood heaters are too sensitive to operating practices.

Standard test methods and wood heater designs should be changed to reduce this sensitivity to poor operation.

The recommended PM10 emission factor for controlled combustion wood heaters in the Emissions Estimation Manual (Environment Australia 1999) should be reviewed 

References

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Field performance of woodburning stoves in Colorado during the 1995-96 heating season’ Report EPA-600/R-97-112, US EPA Washington

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