This is an except of a much longer, and more technical paper by Prof. Gael Ulrich -“BioCombustion Institute Bulletin #3.” Prof. Ulrich calculated the efficiency of six
popular pellet stoves, finding a wide difference. The highest, the Italian made Piazzetta
Sabrina was 76% efficient and the lowest was the Enviro M55 Insert at 51%
efficient. In between were the Ravelli
RV80 (62%), Englander PDCV55 (63%), Quadrafire Mt Vernon AE (64%) and Harman
Accentra 52i (71%).
He did this by using performance data produced by the
Alliance for Green Heat, who tested these 6 stoves over a 30-day period. The Alliance operated the stoves, often for
24 hours a day, testing them almost every day at various heat output settings
and averaging the results. All the stoves were purchased new, without the
knowledge of the manufacturers and operated with the same PFI certified
pellets. The Alliance produced an in-depth report about the findings, but we did not report the efficiency values
because the instrument we used was a Testo 320, which produces a proprietary
European (LHV) number, not the kind of efficiency values that are used and
reported in North America.
Gael’s full paper can be downloaded as a PDF here,
which is quite technical. We reproduced
the less technical parts which are accessible to a wider audience.
One conclusion is that many
pellet stoves lack a very simple solution to increasing their efficiencies –
larger heat exchangers. Gael found that
“All [the stoves], except the Enviro and Quadrafire, appear capable of adding
another 5 to 10 percentage points by increasing heat exchange area to reduce
the flue gas temperature.” This solution
may only add $100 - $200 to the price of a stove but would save consumers far
more in fuel costs.
One thing is clear: more expensive stoves do not
necessarily provide consumers with higher efficiency. The Englander is sold by
big box hardware stoves for $1,100, and is on par or better in efficiency
than stoves that sell for $3,000 or $4,000.
This is significant because the big pellet stove manufactures do not release the actual efficiency of their stoves to consumers and consumers have
virtually no way to tell which models are lower or higher efficiency. The EPA contributed to a myth that pellet
stoves have high efficiencies by giving them a default efficiency of 78%. Emerging data shows the average pellet stove
is likely around 70% efficiency, but many big name brands make pellet stoves
that have efficiencies in 50s and 60s. This analysis begins to dismantle the
lack of transparency in efficiency values that manufacturers have tried to
maintain for many years.
Biomass Combustor Efficiency
BioCombustion Institute Bulletin #3
(Gael Ulrich: 16
March 2016)
Abstract
 |
Gael Ulrich was a professor of
Chemical Engineering at the
University of New Hampshire |
If flue gas temperature and composition are known, one can calculate the
efficiency of a biomass combustor using the so-call "stack loss"
technique. This paper explains in detail
why that is possible and how to do it.
Fortuitously, during the preparation of this bulletin, the Alliance for
Green Heat published data from their testing of six pellet stoves this past
September. Test
equipment used in the AGH study delivered composition, temperature, and
efficiency numbers. Investigators
declined to report the efficiency numbers for various reasons, although they do
mention a range of 60 to 75%.
Using the AGH temperature and concentration data, I made independent
calculations as described in detail herein.
I find one of the six stoves operating at 51% efficiency, three in the
low 60s, and the remaining two operating at 71 and 76%. I also conclude from my analysis that some of
these units use "dilution as the solution to pollution." If we consider actual emissions in grams per
hour or milligrams per MegaJoule of heat delivered instead of parts per million
in flue gas, the rating is rearranged with one stove deemed second dirtiest
becoming the cleanest and that ranked third cleanest becoming the
dirtiest. Factors that influence
efficiency and cleanliness and how to improve these important performance
properties are also discussed herein.
Introduction
As pointed out in BCI Bulletins #1 (Units)
and #2 (Emissions), biomass is
intrinsically a clean fuel composed primarily of carbon, hydrogen, oxygen, and
ash. If burned properly, with ash un-entrained,
flue gases from biomass can be as clean as those from natural gas and perhaps
even cleaner than from oil. The problem,
of course, is that biomass is neither a liquid nor a gas like these fossil
fuels. Burning a solid cleanly and
efficiently is much more difficult.
Bulletin #2 dealt with cleanliness and the standards expected. This one focuses on efficiency and how it can
be measured for a biomass burner.
Instruments and software are available to
deliver efficiency ratings and other data even to an ignorant user with enough
money to buy them. But to use these tools intelligently, one must know how they
function and should be able to calculate efficiency separately and from
scratch. This bulletin describes how to
do that.
Fundamentals
Efficiency is a concept that everyone understands, but different
people often define it differently. Let's
solve that problem first. For
simplicity, visualize a biomass combustor as a black box with fuel and air
flowing in; flue gases and ash flowing out. ... As
defined by logic, efficiency is the ratio of useful heat released to fuel
energy provided. Fuel Energy is the Higher Heating Value, a quantity
that has been carefully measured over the last couple centuries by scientists
for all common fuels.
...
For highest
efficiency,
1. Burn with the least amount of excess air
possible.
2. Operate with the lowest feasible flue gas
temperature.
3. Use dry fuel.
Alliance for Green Heat Data
The AGH
study ran for a period of 30 days. Investigators found results that showed little
drift with time. Five of the six stoves
operated with more than 200% excess air; beyond maxima considered in Figure 7. One could derive additional curves for these
high air rates just as was done for the lower percentages of excess air, but I
chose to extrapolate instead, creating the dashed lines in Figure 9.
Efficiencies
for the six AGH pellet stoves as read from Figure 9b are listed in Table
6.
Table 6.
Calculated efficiencies of pellet stoves studied in the September 2015
Alliance for Green Heat test series.
Stove O2 % X's Air Flue
Gas Efficiency e
Brand Conc. (Figure 8) Temp. (oC) (Figure 9b)
Enviro 18.7% 800% 150 51 %
Ravelli 16.8% 400% 195 62 %
Englander 16.0% 315% 222 63 %
Quad 17.4% 480%
160 64 %
Harman 15.0% 245%
205 71 %
Piazzetta 13.5% 175% 203 76 %
One of the six operated at 51% efficiency, three in the low 60s, and the
remaining two operated at 71 and 76%.
These numbers are consistent with the range mentioned in the AGH
report.
Piazzetta
achieves superiority through low excess air rate. Enviro, at the other end of
the spectrum, would have an even lower efficiency if its flue gas temperature
were as high as the others. All, except
Enviro and Quad, appear capable of adding another 5 to 10 percentage points by
increasing heat exchange area to reduce the flue gas temperature.
Emissions
What about
Pollution? The AGH data demonstrate an
interesting application of using "dilution as a solution to
pollution." The Harman emitted flue gases containing
about 820 ppm CO while the Enviro emitted 534 ppm. But the Harman operated with about 240%
excess air; the Enviro with 800%. And,
the Harman was 22% more efficient.
At the same
pellet burning rate, the Enviro produces roughly 900/340 or 2.6 times as much
flue gas as the Harmon, and its useful heat delivery rate is only 82 percent as
great. Thus, in terms of mass of CO per
kJ of delivered heat, a better measure of actual pollution, the Enviro is
(2.6/0.82)*(534/820) = 2.1 or about twice as bad as the Harmon. Based on the data provided, I calculated mg
of CO per MJ of useful heat delivered for the six pellet stoves. Results are listed in Table 7.
Table 7.
Calculated CO emissions of pellet stoves studied in the September 2015
Alliance for Green Heat test series.
CO
emissions
Stove (mg/MJ
of (ppm)
Brand % X's Air Efficiency e (ppm) heat
normalized**
Enviro 800% 51% 534 3000 850
(2.7)
Ravelli 400% 62% 428 1100 365
(1.2)
Englander 320% 62% 542 1200 387
(1.2)
Quad 480% 64% 318
930 318
(1.0)**
Harman 240% 71% 821 1300 487
(1.5)
Piazzetta 170% 76% 648 780 370 (1.2)
*Normalized to Quad as the
reference.
**Normalized to Quad as the
reference using Wikipedia formula.
In terms of mass per unit of useful heat, the Enviro emits about four times
as much CO as the Piazzetta (3000 versus 780 mg/MJ or roughly 130 versus 35
milligrams per hour).
What about non-steady-state? My analysis assumes the appliance operates at
steady state with feed rates and temperatures invariant with time. This is valid for automatic-feed pellet
stoves but not for wood stoves that are fed batch-wise. There, the burn mode migrates from
de-volatilization and combustion of light organics, gradually progressing to
char or carbon burn-out. Fortunately,
stage changes are slow relative to combustion kinetics. At any given time, the analysis described
herein can be used to analyze the appliance at that instant. To more accurately reflect the performance of
a batch-fired wood burner, one must record data over a complete firing cycle
and then integrate results to obtain an average. This is further complicated by the fact that
heat of combustion changes with time.
That for carbon, for instance (near burn-out), is about 30,000
kJ/kg. Since the overall HHV for biomass
is 20,000 kJ/kg, that for the volatiles must be lower than this.
What about moisture condensation? Mark Knaebe
advocates improving efficiency by increasing heat exchange surface to the
extent that water in the flue gas is condensed, adding its latent heat to the
useful Q. This
requires dropping flue gas temperature below the dew point. With low
amounts of excess air, the dew point might be as high as 60 deg-C, but with
400% excess air, where many pellet stoves operate, the dew point is nearer 30
deg-C.
As cleaner appliances develop,
the prospect of taking advantage of this extra heat becomes more intriguing
because the condensate will be purer and non-fouling. The added heat transfer surface and increased
capital cost, however, may not be practical.
Ray Albrecht
suggests that temperatures in the range of 1000oC or greater are
needed to achieve good burnout of flue
gases. He stresses the importance of preserving flame temperature by
insulating the combustion chamber to make sure reaction is complete before gases
enter the heat exchanger.
Staging the air feed can promote gasification and partial
combustion at low excess air conditions where temperatures are higher. Preheat can almost deliver a one-to-one
increase of flame temperature with increased feed air temperature. Staging and preheat are common in newer
biomass burners.
Catalysts are another important way to promote oxidation at
lower temperatures than those needed otherwise.
