Efficiency of Popular Pellet Stoves

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.[1] 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.[2] ... As defined by logic, efficiency is the ratio of useful heat released to fuel energy provided.  Fuel Energy is the Higher Heating Value,[3] 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."[4]  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 1000^oC 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.  

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[1]Alliance for Green Heat press release [Oct. 27, 2015]  http://www.forgreenheat.org/decathlon/intro.html

[2]For simplicity, assume it is burning at "steady-state" where flow rates and temperatures are constant; not changing with time.  This is true of many pellet stoves and large-scale furnaces.  Small batch-fed systems do experience cycles and are more complex to analyze, but the steady-state analysis gives a useful result even for these systems.

[3]Unfortunately, fuel energy can be expressed in multiple ways, depending on how it was measured--giving different numbers for the same fuel.  As argued in BCI Bulletin #1, HHV or the Higher Heating Value is preferred and will be the only one considered here.

[4]A phrase attributed to the 20th century comic-strip character "Pogo."