The fuel load is ignited at 0% fuel-load consumption. The burnrate begins slow at this point but increases rapidly to a maximumfuel consumption rate (i.e., kg/hr) at a point when about 50%of the fuel weight has been consumed. This is also the time whenthe greatest amounts of volatile and semi-volatile materials (i.e.,fuel-gases) are being driven from the solid wood fuel. Emissionfactors (g/kg) for both particulate and CO emissions begin risingright at the point the fuel is ignited. At the beginning of aburn cycle, particulate emission factors (g/kg) increase morerapidly than the emission rate (g/hr), however, as volatile andsemi-volatile materials in the fuel load are heated and vaporizedby the increasing amount of heat being generated.
Because all RWH appliances, except for pellet stoves, are batch-loadedfuel processors and rely on very weak, naturally drafted air supplies,it is unavoidable that periods of time will occur during a burncycle when at least a portion of the combustion zone will haveun-optimized, imperfect combustion conditions (e.g., not enoughoxygen, residence time, or the mixing and/or temperature conditionsare not optimum). State-of-the-art, low-emission RWH appliancesoptimize the average combustion conditions of the burn cycle usingcombustion-air distribution systems which are powered by naturaldraft forces. They also use enhanced firebox heat managementdesigns for optimized average thermal performance. But, evenwith this 'new' technology, it is impossible withoutexpensive auxiliary (e.g., electrically) powered control designsto avoid all imperfect combustion conditions that can occur witha batch-loaded process. It is the unburned or incompletely burnedvolatile and semi-volatile materials resulting from these imperfectcombustion conditions that escape the firebox and form particulateemissions as they cool and condense on their way up the chimney.
Analyzing Figure 1 further shows that after the particulate emissionfactors (g/kg) are maximum at a point when about 30% of the fuelload has been consumed, they decrease due to improving, more vigorouscombustion conditions in the firebox (i.e., higher temperaturesand more mixing). Particulate emission factors (g/kg) then decreasefurther after about 60% of the fuel load is consumed due to decreasingvolatile and semi-volatile contents of the fuel. The stage ofthe burn cycle after which the volatile and semi-volatile contentsof the fuel have been depleted is sometimes called the charcoalstage of a burn cycle and is characterized by low particulateemissions.
Like particulate emission factors (g/kg), CO emission factors(g/kg) increase in the early stages of the fire due to the increasingamount of fuel being burned with inadequate temperature and/ormixing conditions. CO emissions per kilogram of fuel being burned(g/kg) start decreasing after about 40% of the fuel is consumedwhich is when combustion conditions begin to improve. CO emissionfactors (g/kg) decrease slowly until a point at which the burningfuel is dominated by coals (which is characterized by both highcarbon and low volatile and semi-volatile material content). At this point, there is usually sufficient temperature for goodcombustion. However, inadequate air and fuel mixing becomes thedominant combustion imperfection which causes the dramatic increasein the amount of CO produced for every kilogram of fuel burnedat this final part of the burn cycle.
Figure 2 has similar axis scales to those in Figure 1 except thatFigure 2 shows how particulate and CO emission rates (g/hr) changeduring a burn cycle and, as in Figure 1, it includes a curve showinghow the burn rate itself changes during one full fuel-load burncycle at a constant air supply setting. Since the emission ratesare a direct function of burn rate,
i.e., burn rate (kg/hr) x emission factor (g/kg) = g/hr,
changes in both the particulate and CO emission-rate curves followchanges in the burn-rate curve very closely. This emission-rategraph (gr/hr) shows clearly that at the same time in the burncycle when large changes in the amount of emissions being producedfor every kilogram of fuel being burned are taking place (as shownin Figure 1), there is little evidence of whether combustion conditionsare improving or deteriorating. Figure 2 is useful however, forshowing that increasing fuel consumption rates do increase bothCO and particulate emissions rates (g/hr).
Figure 2 (the gram per hour curves) also illustrates how goodcombustion conditions, and hence, low emissions per kilogram offuel being burned can be masked by a high burn rate: i.e., lowerg/kg emissions and optimum combustion conditions occur at therelatively higher burn rates but are not indicated by the g/hrcurves. This is ironic because it is at the higher burn ratesthat the batch-loaded cordwood-burning RWH appliances universallyhave the best combustion conditions and the lowest amount of emissionsper kilogram of fuel being burned. This also means that a goodcordwood-burning RWH appliance design can consistently producethe best optimized combustion conditions but because it may haveconsistently high burn rates, and hence, more heat output, itcan be kept from the market because of high g/hr emissions. Withequal overall efficiencies and equal g/hr emission rates, a highburn-rate cordwood-burning RWH appliance would discharge lesspollution to the atmosphere than a low burn-rate cordwood-burningRWH appliance delivering the same total amount of useful heat.
Figure 3 is a laboratory data graph that shows how both the PM10-particulateemission rate (g/hr) and emission factor (g/kg) values changeas full fuel-load burn-cycle burn rates change in a typical non-catalyticRWH appliance. Although the data used in Figure 3 are from non-certifiedstoves, the patterns shown are characteristic of all cordwood-burningnon-catalytic RWH appliances with adjustable air supplies andhence, adjustable burn rates. Each data point in each curve representsa whole fuel-load burn cycle at one air supply setting. Therefore,it takes many tests to gather the data for these curves. The size of any particular stove (and hence its fuel-load size) willshift the kilogram-per-hour burn rate scale right or left butthe resultant emission rate and emission factor patterns willstay the same. Even poorly designed woodstoves would have thesame patterns but the scale for emissions rates and emission factorson the 'y' axis would increase to show higher emissionsat any given burn rate.
The emission rate (g/hr) curve in Figure 3 shows rapidly increasingemissions as burn rates increase in the very lowest burn-raterange below 0.4 kg/hr, followed by a continuing, although lower-slopeincrease to the 1.0 kg burn-rate level. The g/hr then shows adecrease as the burn rate increases in the mid-ranges to 2.0 kg/hr. The rapidly rising g/hr emissions that occur when the burn rateincreases in the lowest burn-rate range below 0.4 kg/hr, are dueto large relative increases in burn rate with concurrently increasingemissions reaching the atmosphere for each kilogram of fuel beingburned. There is an increase in emissions discharged to the atmosphereas burn rates increase at these very low burn rates in spite ofthe fact that air/fuel mixing is improving and higher temperaturesare being generated. This is because at the very lowest burnrates (i.e., less than 0.4 kg/hr on this graph) where the worstcombustion conditions occur and the maximum amount of emissionsare produced in the combustion zone for every kilogram of fuelburned, some of the emissions condense and get deposited on fireboxand flue pipe walls before they can be discharged to the atmosphere. This phenomenon actually results in lower emissions to the atmospherebut a higher rate of creosote deposition in the chimney. Althoughalways present in these low burn-rate ranges, the effect of flue-pipecreosote deposition on emissions discharged to the atmospheredecreases as the burn rates increase from about 0.5 kg/hour.
The emission-factor (g/kg) curve also increases in the burn raterange below 0.4 kg/hr. Since g/kg does not have a direct mathematicalrelation with burn rate like the g/hr units, the increase in emissionsin this burn rate range is due only to the decreasing effect ofcreosote deposition as the burn rate increases.
The g/kg curve decreases after the 0.4 kg/hr burn rate becausethe effect of better air/fuel mixing and higher temperatures decreasethe amount of emissions being produced for every kilogram of fuelbeing burned. Although the amount of emissions per kilogram offuel being burned decreases, the g/hr curve continues to increaseabove the 0.4 kg/hr burn rate due to the fact that the large relativeincrease in burn rate offsets the relative decrease in the amountof emissions produced for each kg of fuel burned. For example,if there is a doubling of the burn rate from 0.5 kg/hr to 1.0kg/hr and at the same time there is a 35% decrease in emissionsproduced by each kilogram of fuel being burned, the g/hr emissionrate still increases 30%. That is,
0.5 kg/hr burn rate x 30 g/kg emission factor = 15 g/hr emissionrate,
then doubling the burn rate and decreasing the emission factorby 35% gives:
1.0 kg/hr burn rate x 19.5 g/kg emission factor = 19.5 g/hr emissionrate,
which is a ((19.5-15.0)/15.0) x 100 = 30% increase in the emissionsrate when combustion and heat delivery conditions are actuallyimproving.
y definition and by their direct mathematical relationship, theg/hr- and g/kg-curves cross at the 1.0 kg/hr burn rate. Afterthese curves cross, the decreasing g/kg emissions overcome therelative increases in burn rate which then effect a decrease inthe g/hr-curve. As the burn rate approaches 2.0 kg/hr the g/kgemissions decrease to a minimum due to optimized combustion conditionsin the firebox. The height of the g/kg-curve at the point thatcombustion (or more appropriately, the quality of the burn) isoptimized is a function of firebox/stove design. Better designswill have lower g/kg-curves in the combustion-optimization range.
An important fact about this part of the g/kg-curve is that thecombustion-optimization segment of the curve covers a relativelylarge area of the mid- to high-burn-rate range and not the lowburn- rate ranges. All EPA certified non-catalytic cordwood-burningRWHs must burn a large portion of each fuel loading within thisrange or they will not have a low emissions rate (i.e., g/hr). It is important to note again that even if the quality of thecombustion process (i.e., g/kg) stays the same from one burn rateto the next in these stoves, just increasing the burn rate wouldincrease their g/hr emissions rate. It is also important to notethis low emission factor part of the g/kg curve because this isthe burn-rate range where Colorado- approved masonry heaters alwaysoperate. This is also true for pellet-fired RWHs, however, insteadof having one (high) burn rate in the optimized g/kg burn-raterange, pellet-fired RWHs adjust their fueling and combustion-airdelivery rates to maintain the same relative fuel-load (burnpot)consumption rate.
Masonry heaters are all designed to burn fuel at one burn ratein the mid- to high-combustion-optimized range to obtain the mostheat production and lowest emissions possible. The whole curvefor a masonry heater would be one point or would only cover asmall segment in the combustion-optimized segment of the burn-raterange. This is because masonry heaters are only designed to haveone burn rate. If a masonry heater firebox is designed poorly,the g/kg-curve (point) would be higher in this combustion-optimizedpart of the curve and the curve (i.e., point) would be lower ina well designed masonry heater. Well designed pellet stoves onthe other had would have a constant, flat, no-slope, curve allthe way across the whole range of burn rates.
Again, by definition and because of the direct mathematical relationshipbetween g/hr and g/kg, the g/hr-curve increases throughout thecombustion-optimized segment of the burn-rate range. This isdue to the fact that although the g/kg curve is constant (flat)showing no change in the quality and efficiency of the combustionprocess taking place in this burn-rate range, merely increasingthe burn rate causes the g/hr-curve to increase. For example,keeping the g/kg emission factor constant while the burn ratechanges from 2.5 to 4.0 kg/hr will increase the g/hr emissionrate by 60%. That is,
5.5 g/kg emission factor x 2.5 kg/hr burn rate = 13.75 g/hr emissionrate,
then increasing the burn rate from 2.5 to 4.0 kg/hr gives;
5.5 g/kg emission factor x 4.0 kg/hr burn rate = 22.00 g/hr emissionrate,
which is a ((22.00-13.75)/13.75) x 100 = 60% increase in the emissionrate.
Therefore, it can be very misleading to assess the pollution characteristicsof a cordwood-
burning RWH appliance, by only using a g/hr value. Any cleanburning appliance design can have high g/hr emission rates justbecause it can be made to burn fuel fast. Even though they canbe producing more heat with lower total emissions to the atmosphere,single, high burn-rate appliances such as masonry heaters areunfairly viewed by some regulators as high polluters when g/hrvalues are used for comparison to other types of appliances likeadjustable burn-rate RWHs.
To conclude the g/hr- and g/kg-curve analysis, the increase ing/kg emissions at burn rates above 4.0 kg/hr is due to decreasingcombustion efficiency which is caused, in most cases, by excesscombustion-air cooling or by dilution of the combustible fuel-gasesgiven off by the heated wood before they can burn. Dependingon the firebox design, the fire can also become too fuel-gas richbecause too much of the fuel load is being heated to high temperaturestoo quickly which creates large amounts of combustible fuel-gaseswithout enough air for efficient combustion. In either of thesecases the amount of pollution created by each kilogram of fuelincreases and hence, the slope of the g/hr-curve increases evenmore. The g/hr-curve increase progresses at a steeper slope thanthe g/kg-curve because it is compounded by both an increasingburn rate and an increasing emission factor.
To get around the problems presented by the variable and constantlychanging cordwood-burning RWH combustion and emissions parameters,the EPA and the Oregon and Colorado state certification testingprograms, required that regulated RWH appliances be tested foremissions at four different burn rates ranging from low to high. Since each certification test-run emissions sample is taken/collectedover an entire burn cycle, each test represents the average pollutantdischarge that took place during the burning period for each ofthe four whole fuel loads. The results from each of the fourseparate tests are then weight-averaged together using weighingfactors derived from the expected average annual residential heatdemand of the average house in an average heat demand locationin the U.S. (i.e., about 17,000 Btu/hour). Therefore, at theend of this certification process there is a single emission rate(in g/hr as required by EPA) for each model of regulated RWH appliance. This emission rate indicates the average mass of pollution thatcan be expected to be discharged on an hourly basis when the applianceis in operation.
It is important to note in this discussion that the g/hr and g/kgunits are both resultant data from certification testing of RWHappliances. No additional testing is required to obtain eitherreporting unit. It's only a quirk of history that of the threeoptions for reporting units, the EPA, and the states that havehad certification programs, chose the g/hr units to establishregulatory emission limits for RWH appliances.
The use of g/hr units started in Oregon and then was adopted byColorado and finally by EPA. During the NSPS negotiations, therewas EPA resistance to change from the units used by Oregon andColorado even with solid technical arguments supporting change. The record of EPA's New Source Performance Standard (NSPS) negotiationswith the RWH appliance manufacturing industry clearly shows thatthe choice for g/hr was not made without challenging commentsor good alternative recommendations. EPA argued that since theirgoal was only to develop a reliable ranking system for comparingregulated RWH appliances to one another, the already-used g/hrunits would be chosen.
Clearly the most useful reporting units would have been in gramsof pollutants discharged to the airshed per unit of useful heatoutput from the RWH appliance. The real advantage of this unit-of-measure is that it would take into account the overall thermal(both combustion and heat transfer) efficiency of the appliance. If the g/hr and/or g/kg test results indicated that two RWH appliancemodels had equal emissions, the more efficient model would burnless fuel to heat the same space, and hence, emit less pollutionto the airshed. As mentioned above, the heat output-based emission-rateunits were not used since they would require the measurement ofoverall thermal efficiency and EPA felt the thermal efficiencymeasurement methods available at the time the NSPS was being negotiatedwere costly and not verified enough to use in EPA's certificationprogram.
A very important point to note is that in all of the codifiedtest methods for determining grams per hour (g/hr) reporting unitsfor regulating RWH appliances, it is not required that the appliancesbeing tested provide any useful space heating. All that is neededto determine g/hr is the measurement of total exhaust-gas flowrate and the pollutant concentration; i.e.,
g/m3 x m3/hr = g/hr.
Where: g/m3 = grams of emissions per cubic meter offlue gas.
m3/hr = cubic meters of flue gas flow per hour.
Neither the concept of g/hr itself nor the test method protocolsto measure g/hr emissions, require the production of any usefulheat, only that the appliance be able to burn specified fuel loadswithin 4 prescribed burn rate categories. To emphasize: Thetest methods do not use heat output categories, just burn ratecategories.
During the New Source Performance Standards (NSPS) negotiationsin 1986, EPA decided to assume standard thermal efficiency levelsfor all regulated RWH appliances. Considering their objectives,this approach to efficiency is somewhat reasonable since the definitionof the appliances being regulated (EPA calls them 'affectedfacilities') imposes physical and operating specificationslike air-to-fuel ratio, weight, and firebox volume limitationswhich when used in combination with the required burn rate categoriesand the emission limits, result in the approximate EPA-assumedefficiency levels. This is not just coincidental but an engineeringfact that if all the affected-facility definition criteria, test-protocolrequirements, and emission limits are met, the overall thermalefficiency levels of the regulated RWHs will be close to EPA'sassumed levels.
Most importantly, when discussing units of measure, the uniquefeatures of the EPA regulated RWH appliances (e.g., not includingmasonry heaters) that make the g/hr units useable in the regulationof these emissions are:
1) The heat output (burn rate) of the appliance is adjustableon a real time basis. If the user desires more heat, the airsupply and/or fuel load is increased to increase the combustionprocess and if less heat is desired the air and/or fuel load isdecreased, and
2) The production of heat in the firebox by the fuel-burning combustionprocess and the release of that heat to the surrounding spaceoccurs at virtually the same time. There is no, or only a verysmall delay between heat production by the fuel-burning processand the transfer of that heat to the space being heated. RWHappliance firebox shells are virtually all made from either sheetmetal or cast iron to accommodate this heat transfer. In eithercase, these high heat-conducting materials are used because they'transfer' heat from the firebox to the surroundingspace as quickly as possible. Because the regulated RWHs arenot designed for heat storage, there is no (or only very little)storage of heat in the mass of the appliance. Regulated RWH appliancesmake heat in the firebox and transfer it to the space being heatedas soon as possible.
These features allow the g/hr unit of measure to be applied toregulated RWHs, but applicable only because these features areunique to the regulated RWHs. This does not mean thatg/kg or the mass of emissions per unit of useful heat output (e.g.,g/MJ) could not be used or even be useful. In fact, either oneof these reporting units could be used with at least as much anddefinitely more useful information being provided about the qualityand efficiency of the combustion process taking place. No otherappliances or EPA-regulated source types burning any other fuelfor any other purposes can reasonably use the g/hr unit alone. No matter what the source, without production or process throughputdata there is always serious potential for communicating incorrectinformation.
It is only the design specifications imposed by EPA's NSPS woodstove(affected-facility) definition in combination with the emissionlimits imposed on the regulated RWHs that allow the use of g/hrunits. In addition, because of the definition and the emissionlimitations, regulated RWHs are, actually by default, regulatedbased on the amount of emissions produced per unit of useful heatoutput. This is because of the assumed efficiencies and thereforethe assumed amount of useful heat output generated during theburning of test fuel at the rates required by the test protocols: e.g., virtually all regulated non-catalytic stoves burning 1kg/hr will produce approximately 12,500 Btu/hr of useful heatto the surrounding room, and virtually all catalytic stoves willproduce approximately 14,500 But/hr when burning 1.0 kg/hr offuel. This is because the efficiencies for all of the regulatedRWH appliances within each category (i.e., non-catalytic or catalytic),are nearly the same.
Masonry heaters were intentionally excluded from EPA's NSPS byEPA's specified weight criteria (affected facilities have to beless than 800 kg). EPA rationale was that masonry heaters wouldrequire time- and money-consuming development of new test methodand operating protocols and most importantly because of theirdesigned, consistent high burn-rate, would be clean burning anywayand would not present problems in local airsheds. In addition,if the EPA was to regulate masonry heaters, the reporting unitsfor emissions would have to have been changed first.
The g/hr emission rate is not useful or appropriate for masonryheaters since masonry heaters only burn fuel during a very shortpart of their useful heat output cycle. In addition, if masonryheaters are to be ranked or compared to other RWHs, EPA's NSPStest-method operating protocol (Method 28, 40 CFR Part 60, AppendixA) would have to be changed. Since the primary burn-cycle modeof operation for masonry heaters is one fuel load burned at thefull-high burn rate until all the fuel is gone, the test cyclewould need to include the whole cycle including start-up and completeburn down (i.e., a 'cold-to-cold' test-burn cycle). The test-method operating protocol would have to take into accountthe fact that useful heat output is produced by masonry heaterslong after the fire has gone out.
Method 28 for woodstoves is a hot-to-hot test-burn cycle: a hotcoal bed is established in a hot stove; a specified fuel loadis then added to begin the test; and completion of the test isat the point in time when the added fuel is totally consumed backto the original, hot coal bed. This cycle is conducted at 4 different,and specified burn rates to make a complete certification testseries.
It is important to realize the difference between testing an RWHappliance using a hot-to-hot test cycle as opposed to using acold-to-cold test cycle. In 1986, Jay Shelton of Shelton Researchin Santa Fe, New Mexico (personal communication) did a woodstoveresearch project for the State of Colorado and found that emissionsdischarged during the cold start up phase of a woodstove equals50 percent of the emissions discharged during a whole hot-to-hottest cycle. This means that the standard EPA test method missesup to 33 percent of the total emissions actually discharged bya regulated RWH appliance during cold startup operations.
This is not a criticism of the method, if it is kept in mind thatthe method was designed and adopted to rank stoves against oneanother and not to simulate actual and absolute in-consumer-useemission rates. It was felt by almost all of the regulators participatingin the NSPS regulation negotiations, that ranking of stoves withan indicated relative emissions reduction was more important thantrying to establish absolute or 'real world' emissionrates for certified stoves. That is, an NSPS limit of 7.5 g/hrfor non-catalytic RWHs was a 75 percent reduction from the 30g/hr which was considered by the regulators to be the emissionsrate for the common 'conventional' stoves in use atthe time of the NSPS negotiations. The idea was that the 75 percentreduction indicated by the laboratory test method would translateto a 75 percent reduction in actual home-heating-use emissionsto the atmosphere, regardless of what the actual or absolute emissionsrates were. The objective was to get the 75 percent reduction. There was never any attempt or wish expressed by EPA in the NSPSnegotiations to use RWH certification emissions values for estimatingor modeling airshed emissions loading rates(3).
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