[Stoves] [stove] Comparison of stove testing procedures

Sun Mar 20 09:25:59 CDT 2016

Dear Paul and Philip

The "International WBT" referred to in the article is the 2009 version of
the WBT 4.1.2. Readers are cautioned that there are multiple versions of WBT
4.1.2 with different calculations of output metrics in them, most
significantly the "thermal efficiency", among others.

The article mentions the 2009 version specifically but also refers to the
IWA which references the 2011 version of 4.1.2, which was again altered in
June 2012 while retaining the same version number.

"As previous studies found that the differences in pollutant emissions
between the cold start and hot start of the International WBT was small for
stoves with relatively small thermal mass, an averaged value was calculated
representing a value for high power performance, as specified by the
International Standard Workshop Agreement. tiered stove rating framework
(Carter et al., 2014;Water Boiling Test, WBT Version 4.1.2, 2009;
International Workshop Agreement, IWA, 2012)."

The 2011 version was referenced in the IWA but there is also a provision in
that same IWA that all test methods have to be subjected to an external,
expert review before they can be used. As far as I have heard, that has not
yet happened for that or subsequent versions save discussions of sections of
it here <http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6827753>
and here <http://ujdigispace.uj.ac.za/handle/10210/8188>  (any others?).

The efficiency calculation, which is the one most seriously incorrect in the
WBT, is given as the 

The heat transferred to water in the pot which was heated from room
temperature to boiling point plus the heat transferred to evaporate the
water from the pot, Divided by the heat delivered by the equivalent dry fuel
consumed.

The really mean "Divided by the heat theoretically available from the
equivalent dry fuel consumed" but that is a quibble. 

The WBT uses the heating value of missing fuel, not the fuel needed to
replicate the experiment. The Chinese WBT uses the energy contained in the
fuel needed whether it is burned or not. 

The Chinese standards have never used the dry fuel equivalent of the energy
released from the completely combusted fuel. It is a surprise therefore to
see the claims first, that this is the "Overall Thermal Efficiency" (which
the WBT 4.1.2 clearly does not use, it reports the Thermal Efficiency =
combustion efficiency * heat transfer efficiency) and second, that both
these tests use "the same" equation, which they do not.  They also did not
correct the efficiency calculation error in the Chinese test reported in
Zhang, Y (2014) (first 'here' link above).

Then things get confusing. The Thermal efficiency, having been renamed
Overall Thermal Efficiency is divided by the Modified Combustion Efficiency
[MCE = CO/CO+CO2)] to calculate a proxy Heat Transfer Efficiency. As a proxy
for fuel efficiency, this simply does not work with a stove that is creating
char, in the same manner as using a carbon balance method to determine the
volume of gases in which the emissions are carried. If you don't burn the
carbon you do not know how much energy has been released or when. That is
why the carbon balance method (using a hood and dilution tunnel) often
estimates that the amount of fuel burned is as little as half of what was
actually used in the fire.

The section titled "Differences among three pellet-gasifier stoves" has it
that the heat transfer efficiency (produced as above) is an indicator of the
fuel consumption.  This has never been true for a solid-fuelled stove unless
it makes zero char. The stoves tested are 'semi-gasifiers' meaning they will
inevitably make a lot of char.

The thermal efficiency has never, since the world started testing wood
stoves, indicated the fuel consumption which is why the WBT 3.x and 4.x lab
tests keep getting pwned by the field tests. In the reported tests, a proxy
heat transfer efficiency has been used instead of the thermal efficiency
which makes the error even larger. It is unfortunate that the emissions were
not reported on the basis of MJ of energy delivered to the pot as per (the
same) IWA 2012:11 and as proposed in the Draft ISO 19867: Part 1. It would
have overcome all sorts of problems associated with assessing fuel energy.
They had the number as it is used to calculate the thermal efficiency.

There are 46 citations. If they had included a 47th,
DOI:10.1109/DUE.2014.6827753 and read the bit about stove efficiency I think
the whole set of comparative results would have been substantially different
and the differences between results for different protocols would have all
but disappeared.

It is worth mentioning, because it is topical, that the emissions up the
chimney were compared with the indoor air quality 'guidelines' of the WHO
which is intended for stoves that vent directly into the room, (or fugitive
emissions from chimney stoves, which they say was not measured).

"However, the ERs of CO (18-210 mg/min) and PM2.5 (0.12-7.1 g/min) did not
meet the ER targets for PM2.5 and CO suggested by the WHO IAQGs, being
particularly too high for PM2.5. This implies that much additional
improvement of these pellet stoves will be needed if they are to meet WHO ER
targets."

As pointed out in an earlier thread, if the emissions are vented outside,
there is no need to compare them with indoor emission rates. This somewhat
undermines the conclusion,

"This implies that much additional improvement of these pellet stoves will
be needed if they are to meet WHO ER targets."

Those IAQG targets are for stoves venting into the room, not those fitted
with chimneys.

"A total of 29% and 58% of CO ERs exceeded the [time-based] intermediate and
final ER targets limits of WHO IAQGs, respectively, and all of PM2.5 ERs
exceeded both ER targets by approximate one order of magnitude."

So, unless the stoves were leaking >10% of their total emissions into the
room, all three stoves met the WHO IAQG final targets. That is good news. 

This proves the first thing to do to improve human health around the world
is to install good chimneys in every kitchen. Kudos to all those who are
already doing that.

Regards

Crispin

+++++++++

Unfortunately the reference is behind a paywall.

Prof Philip Lloyd

Energy Institute, CPUT

SARETEC, Sachs Circle

Bellville

Tel 021 959 4323

Cell 083 441 5247

PA Nadia 021 959 4330

On 3/17/2016 1:08 PM, Kirk R. Smith wrote:

Can be downloaded from the website below/k

Efficiencies and pollutant emissions from forced-draft biomass-pellet
semi-gasifier stoves: Comparison of International and Chinese water boiling
test protocols

Yuanchen Chen, Guofeng Shen, Shu Su, Wei Du, Yibo Huangfu, Guangqing Liu,
Xilong Wang, Baoshan Xing, Kirk R. Smith, Shu Tao

Energy for Sustainable Development 32 (2016) 22-30

Ab s t r a c t

Biomass fuels are widely combusted in rural China, producing numerous air
pollutants with great adverse

impacts on human health. Some improved cookstoves and pellet fuels have been
promoted. To evaluate the

performance of pellet-gasifier stoves, efficiencies and pollutant emissions
were measured following International

and Chinese water boiling tests (WBTs). Compared with traditional stoves and
unprocessed biomass fuels,

increased efficiencies and lower emissions of pollutants including carbon
monoxide (CO), particulate matter

(PM), parent and derivative polycyclic aromatic hydrocarbons (PAHs) were
revealed for pellet-gasifier stoves.

However, the calculated emission rates (ERs) of CO and PM2.5 cannot meet the
ER targets recently suggested

by WHO indoor air quality guidelines (IAQGs). Better control of air mixing
ratio and gross flow rates of primary

and secondary air supply greatly reduced emissions and increased
efficiencies. Differences among testing protocols

are the key factors affecting the evaluation of stove performance. With
longer burning duration and higher

power, the Chinese WBT had statistically higher efficiencies, gas
temperature, and lower pollutant emissions

(p b 0.10) compared to those obtained through the International WBT.
Statistically significant differences

between the two protocols indicate the need for further efforts in emission
tests and methodology development

before the release of a well-accepted international testing protocol

---------------------------
Kirk R. Smith, MPH, PhD
Professor of Global Environmental Health

Chair, Graduate Group in Environmental Health Sciences
Director of the Global Health and Environment Program
School of Public Health
747 University Hall
University of California
Berkeley, California, 94720-7360
phone 1-510-643-0793; fax 642-5815
krksmith at berkeley.edu <mailto:krksmith at berkeley.edu> 
http://www.kirkrsmith.org/

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