Analysis of Cordwood-Burning RWH Emissions Dynamics

Figure 1 is a generalized illustration showing how particulate and carbon monoxide emission factors (g/kg) change as well as how the burn rate itself changes as one, full cordwood fuel load is burned (i.e., one "burn cycle") and the RWH appliance is operated at one air supply setting. A percentage (relative) scale is used on the "y" axis to show the maximum for each graphed parameter. This illustration demonstrates how each parameter changes relative to the minimums and maximums of the other graphed parameters.

The fuel load is ignited at 0% fuel-load consumption. The burn rate begins slow at this point but increases rapidly to a maximum fuel consumption rate (i.e., kg/hr) at a point when about 50% of the fuel weight has been consumed. This is also the time when the greatest amounts of volatile and semi-volatile materials (i.e., fuel-gases) are being driven from the solid wood fuel. Emission factors (g/kg) for both particulate and CO emissions begin rising right at the point the fuel is ignited. At the beginning of a burn cycle, particulate emission factors (g/kg) increase more rapidly than the emission rate (g/hr), however, as volatile and semi-volatile materials in the fuel load are heated and vaporized by the increasing amount of heat being generated.

Because all RWH appliances, except for pellet stoves, are batch-loaded fuel processors and rely on very weak, naturally drafted air supplies, it is unavoidable that periods of time will occur during a burn cycle when at least a portion of the combustion zone will have un-optimized, imperfect combustion conditions (e.g., not enough oxygen, residence time, or the mixing and/or temperature conditions are not optimum). State-of-the-art, low-emission RWH appliances optimize the average combustion conditions of the burn cycle using combustion-air distribution systems which are powered by natural draft forces. They also use enhanced firebox heat management designs for optimized average thermal performance. But, even with this "new" technology, it is impossible without expensive auxiliary (e.g., electrically) powered control designs to avoid all imperfect combustion conditions that can occur with a batch-loaded process. It is the unburned or incompletely burned volatile and semi-volatile materials resulting from these imperfect combustion conditions that escape the firebox and form particulate emissions as they cool and condense on their way up the chimney.

Analyzing Figure 1 further shows that after the particulate emission factors (g/kg) are maximum at a point when about 30% of the fuel load has been consumed, they decrease due to improving, more vigorous combustion conditions in the firebox (i.e., higher temperatures and more mixing). Particulate emission factors (g/kg) then decrease further after about 60% of the fuel load is consumed due to decreasing volatile and semi-volatile contents of the fuel. The stage of the burn cycle after which the volatile and semi-volatile contents of the fuel have been depleted is sometimes called the charcoal stage of a burn cycle and is characterized by low particulate emissions.

Like particulate emission factors (g/kg), CO emission factors (g/kg) increase in the early stages of the fire due to the increasing amount of fuel being burned with inadequate temperature and/or mixing conditions. CO emissions per kilogram of fuel being burned (g/kg) start decreasing after about 40% of the fuel is consumed which is when combustion conditions begin to improve. CO emission factors (g/kg) decrease slowly until a point at which the burning fuel is dominated by coals (which is characterized by both high carbon and low volatile and semi-volatile material content). At this point, there is usually sufficient temperature for good combustion. However, inadequate air and fuel mixing becomes the dominant combustion imperfection which causes the dramatic increase in the amount of CO produced for every kilogram of fuel burned at this final part of the burn cycle.

Figure 2 has similar axis scales to those in Figure 1 except that Figure 2 shows how particulate and CO emission rates (g/hr) change during a burn cycle and, as in Figure 1, it includes a curve showing how the burn rate itself changes during one full fuel-load burn cycle at a constant air supply setting. Since the emission rates are 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 follow changes in the burn-rate curve very closely. This emission-rate graph (gr/hr) shows clearly that at the same time in the burn cycle when large changes in the amount of emissions being produced for every kilogram of fuel being burned are taking place (as shown in Figure 1), there is little evidence of whether combustion conditions are improving or deteriorating. Figure 2 is useful however, for showing that increasing fuel consumption rates do increase both CO and particulate emissions rates (g/hr).

Figure 2 (the gram per hour curves) also illustrates how good combustion conditions, and hence, low emissions per kilogram of fuel being burned can be masked by a high burn rate: i.e., lower g/kg emissions and optimum combustion conditions occur at the relatively higher burn rates but are not indicated by the g/hr curves. This is ironic because it is at the higher burn rates that the batch-loaded cordwood-burning RWH appliances universally have the best combustion conditions and the lowest amount of emissions per kilogram of fuel being burned. This also means that a good cordwood-burning RWH appliance design can consistently produce the best optimized combustion conditions but because it may have consistently high burn rates, and hence, more heat output, it can be kept from the market because of high g/hr emissions. With equal overall efficiencies and equal g/hr emission rates, a high burn-rate cordwood-burning RWH appliance would discharge less pollution to the atmosphere than a low burn-rate cordwood-burning RWH appliance delivering the same total amount of useful heat.

Figure 3 is a laboratory data graph that shows how both the PM₁₀-particulate emission rate (g/hr) and emission factor (g/kg) values change as full fuel-load burn-cycle burn rates change in a typical non-catalytic RWH appliance. Although the data used in Figure 3 are from non-certified stoves, the patterns shown are characteristic of all cordwood-burning non-catalytic RWH appliances with adjustable air supplies and hence, adjustable burn rates. Each data point in each curve represents a 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) will shift the kilogram-per-hour burn rate scale right or left but the resultant emission rate and emission factor patterns will stay the same. Even poorly designed woodstoves would have the same patterns but the scale for emissions rates and emission factors on the "y" axis would increase to show higher emissions at any given burn rate.

The emission rate (g/hr) curve in Figure 3 shows rapidly increasing emissions as burn rates increase in the very lowest burn-rate range below 0.4 kg/hr, followed by a continuing, although lower-slope increase to the 1.0 kg burn-rate level. The g/hr then shows a decrease 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 rate increases in the lowest burn-rate range below 0.4 kg/hr, are due to large relative increases in burn rate with concurrently increasing emissions reaching the atmosphere for each kilogram of fuel being burned. There is an increase in emissions discharged to the atmosphere as burn rates increase at these very low burn rates in spite of the fact that air/fuel mixing is improving and higher temperatures are being generated. This is because at the very lowest burn rates (i.e., less than 0.4 kg/hr on this graph) where the worst combustion conditions occur and the maximum amount of emissions are produced in the combustion zone for every kilogram of fuel burned, some of the emissions condense and get deposited on firebox and flue pipe walls before they can be discharged to the atmosphere. This phenomenon actually results in lower emissions to the atmosphere but a higher rate of creosote deposition in the chimney. Although always present in these low burn-rate ranges, the effect of flue-pipe creosote deposition on emissions discharged to the atmosphere decreases as the burn rates increase from about 0.5 kg/hour.

The emission-factor (g/kg) curve also increases in the burn rate range below 0.4 kg/hr. Since g/kg does not have a direct mathematical relation with burn rate like the g/hr units, the increase in emissions in this burn rate range is due only to the decreasing effect of creosote deposition as the burn rate increases.

The g/kg curve decreases after the 0.4 kg/hr burn rate because the effect of better air/fuel mixing and higher temperatures decrease the amount of emissions being produced for every kilogram of fuel being burned. Although the amount of emissions per kilogram of fuel being burned decreases, the g/hr curve continues to increase above the 0.4 kg/hr burn rate due to the fact that the large relative increase in burn rate offsets the relative decrease in the amount of 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 1.0 kg/hr and at the same time there is a 35% decrease in emissions produced by each kilogram of fuel being burned, the g/hr emission rate still increases 30%. That is,

0.5 kg/hr burn rate x 30 g/kg emission factor = 15 g/hr emission rate,

then doubling the burn rate and decreasing the emission factor by 35% gives:

1.0 kg/hr burn rate x 19.5 g/kg emission factor = 19.5 g/hr emission rate,

which is a ((19.5-15.0)/15.0) x 100 = 30% increase in the emissions rate when combustion and heat delivery conditions are actually improving.

y definition and by their direct mathematical relationship, the g/hr- and g/kg-curves cross at the 1.0 kg/hr burn rate. After these curves cross, the decreasing g/kg emissions overcome the relative increases in burn rate which then effect a decrease in the g/hr-curve. As the burn rate approaches 2.0 kg/hr the g/kg emissions decrease to a minimum due to optimized combustion conditions in the firebox. The height of the g/kg-curve at the point that combustion (or more appropriately, the quality of the burn) is optimized is a function of firebox/stove design. Better designs will have lower g/kg-curves in the combustion-optimization range.

An important fact about this part of the g/kg-curve is that the combustion-optimization segment of the curve covers a relatively large area of the mid- to high-burn-rate range and not the low burn- rate ranges. All EPA certified non-catalytic cordwood-burning RWHs must burn a large portion of each fuel loading within this range 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 the combustion process (i.e., g/kg) stays the same from one burn rate to the next in these stoves, just increasing the burn rate would increase their g/hr emissions rate. It is also important to note this low emission factor part of the g/kg curve because this is the burn-rate range where Colorado- approved masonry heaters always operate. This is also true for pellet-fired RWHs, however, instead of having one (high) burn rate in the optimized g/kg burn-rate range, pellet-fired RWHs adjust their fueling and combustion-air delivery rates to maintain the same relative fuel-load (burnpot) consumption rate.

Masonry heaters are all designed to burn fuel at one burn rate in the mid- to high-combustion-optimized range to obtain the most heat production and lowest emissions possible. The whole curve for a masonry heater would be one point or would only cover a small segment in the combustion-optimized segment of the burn-rate range. This is because masonry heaters are only designed to have one burn rate. If a masonry heater firebox is designed poorly, the g/kg-curve (point) would be higher in this combustion-optimized part of the curve and the curve (i.e., point) would be lower in a well designed masonry heater. Well designed pellet stoves on the other had would have a constant, flat, no-slope, curve all the way across the whole range of burn rates.

Again, by definition and because of the direct mathematical relationship between g/hr and g/kg, the g/hr-curve increases throughout the combustion-optimized segment of the burn-rate range. This is due to the fact that although the g/kg curve is constant (flat) showing no change in the quality and efficiency of the combustion process taking place in this burn-rate range, merely increasing the burn rate causes the g/hr-curve to increase. For example, keeping the g/kg emission factor constant while the burn rate changes from 2.5 to 4.0 kg/hr will increase the g/hr emission rate by 60%. That is,

5.5 g/kg emission factor x 2.5 kg/hr burn rate = 13.75 g/hr emission rate,

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 emission rate,

which is a ((22.00-13.75)/13.75) x 100 = 60% increase in the emission rate.

Therefore, it can be very misleading to assess the pollution characteristics of a cordwood-

burning RWH appliance, by only using a g/hr value. Any clean burning appliance design can have high g/hr emission rates just because it can be made to burn fuel fast. Even though they can be producing more heat with lower total emissions to the atmosphere, single, high burn-rate appliances such as masonry heaters are unfairly viewed by some regulators as high polluters when g/hr values are used for comparison to other types of appliances like adjustable burn-rate RWHs.

To conclude the g/hr- and g/kg-curve analysis, the increase in g/kg emissions at burn rates above 4.0 kg/hr is due to decreasing combustion efficiency which is caused, in most cases, by excess combustion-air cooling or by dilution of the combustible fuel-gases given off by the heated wood before they can burn. Depending on the firebox design, the fire can also become too fuel-gas rich because too much of the fuel load is being heated to high temperatures too quickly which creates large amounts of combustible fuel-gases without enough air for efficient combustion. In either of these cases the amount of pollution created by each kilogram of fuel increases and hence, the slope of the g/hr-curve increases even more. The g/hr-curve increase progresses at a steeper slope than the g/kg-curve because it is compounded by both an increasing burn rate and an increasing emission factor.

To get around the problems presented by the variable and constantly changing cordwood-burning RWH combustion and emissions parameters, the EPA and the Oregon and Colorado state certification testing programs, required that regulated RWH appliances be tested for emissions at four different burn rates ranging from low to high. Since each certification test-run emissions sample is taken/collected over an entire burn cycle, each test represents the average pollutant discharge that took place during the burning period for each of the four whole fuel loads. The results from each of the four separate tests are then weight-averaged together using weighing factors derived from the expected average annual residential heat demand of the average house in an average heat demand location in the U.S. (i.e., about 17,000 Btu/hour). Therefore, at the end 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 that can be expected to be discharged on an hourly basis when the appliance is in operation.

It is important to note in this discussion that the g/hr and g/kg units are both resultant data from certification testing of RWH appliances. No additional testing is required to obtain either reporting unit. It's only a quirk of history that of the three options for reporting units, the EPA, and the states that have had certification programs, chose the g/hr units to establish regulatory emission limits for RWH appliances.

The use of g/hr units started in Oregon and then was adopted by Colorado and finally by EPA. During the NSPS negotiations, there was EPA resistance to change from the units used by Oregon and Colorado even with solid technical arguments supporting change. The record of EPA's New Source Performance Standard (NSPS) negotiations with the RWH appliance manufacturing industry clearly shows that the choice for g/hr was not made without challenging comments or good alternative recommendations. EPA argued that since their goal was only to develop a reliable ranking system for comparing regulated RWH appliances to one another, the already-used g/hr units would be chosen.

Clearly the most useful reporting units would have been in grams of pollutants discharged to the airshed per unit of useful heat output 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 appliance models had equal emissions, the more efficient model would burn less fuel to heat the same space, and hence, emit less pollution to the airshed. As mentioned above, the heat output-based emission-rate units were not used since they would require the measurement of overall thermal efficiency and EPA felt the thermal efficiency measurement methods available at the time the NSPS was being negotiated were costly and not verified enough to use in EPA's certification program.

A very important point to note is that in all of the codified test methods for determining grams per hour (g/hr) reporting units for regulating RWH appliances, it is not required that the appliances being tested provide any useful space heating. All that is needed to determine g/hr is the measurement of total exhaust-gas flow rate and the pollutant concentration; i.e.,

g/m³ x m³/hr = g/hr.

Where: g/m³ = grams of emissions per cubic meter of flue gas.

m³/hr = cubic meters of flue gas flow per hour.

Neither the concept of g/hr itself nor the test method protocols to measure g/hr emissions, require the production of any useful heat, only that the appliance be able to burn specified fuel loads within 4 prescribed burn rate categories. To emphasize: The test methods do not use heat output categories, just burn rate categories.

During the New Source Performance Standards (NSPS) negotiations in 1986, EPA decided to assume standard thermal efficiency levels for all regulated RWH appliances. Considering their objectives, this approach to efficiency is somewhat reasonable since the definition of the appliances being regulated (EPA calls them "affected facilities") imposes physical and operating specifications like air-to-fuel ratio, weight, and firebox volume limitations which when used in combination with the required burn rate categories and the emission limits, result in the approximate EPA-assumed efficiency levels. This is not just coincidental but an engineering fact that if all the affected-facility definition criteria, test-protocol requirements, and emission limits are met, the overall thermal efficiency levels of the regulated RWHs will be close to EPA's assumed levels.

Most importantly, when discussing units of measure, the unique features of the EPA regulated RWH appliances (e.g., not including masonry heaters) that make the g/hr units useable in the regulation of these emissions are:

1) The heat output (burn rate) of the appliance is adjustable on a real time basis. If the user desires more heat, the air supply and/or fuel load is increased to increase the combustion process and if less heat is desired the air and/or fuel load is decreased, and

2) The production of heat in the firebox by the fuel-burning combustion process and the release of that heat to the surrounding space occurs at virtually the same time. There is no, or only a very small delay between heat production by the fuel-burning process and the transfer of that heat to the space being heated. RWH appliance firebox shells are virtually all made from either sheet metal or cast iron to accommodate this heat transfer. In either case, these high heat-conducting materials are used because they "transfer" heat from the firebox to the surrounding space as quickly as possible. Because the regulated RWHs are not designed for heat storage, there is no (or only very little) storage of heat in the mass of the appliance. Regulated RWH appliances make heat in the firebox and transfer it to the space being heated as soon as possible.

These features allow the g/hr unit of measure to be applied to regulated RWHs, but applicable only because these features are unique to the regulated RWHs. This does not mean that g/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 one of these reporting units could be used with at least as much and definitely more useful information being provided about the quality and efficiency of the combustion process taking place. No other appliances or EPA-regulated source types burning any other fuel for any other purposes can reasonably use the g/hr unit alone. No matter what the source, without production or process throughput data there is always serious potential for communicating incorrect information.

It is only the design specifications imposed by EPA's NSPS woodstove (affected-facility) definition in combination with the emission limits imposed on the regulated RWHs that allow the use of g/hr units. In addition, because of the definition and the emission limitations, regulated RWHs are, actually by default, regulated based on the amount of emissions produced per unit of useful heat output. This is because of the assumed efficiencies and therefore the assumed amount of useful heat output generated during the burning of test fuel at the rates required by the test protocols: e.g., virtually all regulated non-catalytic stoves burning 1 kg/hr will produce approximately 12,500 Btu/hr of useful heat to the surrounding room, and virtually all catalytic stoves will produce approximately 14,500 But/hr when burning 1.0 kg/hr of fuel. This is because the efficiencies for all of the regulated RWH appliances within each category (i.e., non-catalytic or catalytic), are nearly the same.

Masonry heaters were intentionally excluded from EPA's NSPS by EPA's specified weight criteria (affected facilities have to be less than 800 kg). EPA rationale was that masonry heaters would require time- and money-consuming development of new test method and operating protocols and most importantly because of their designed, consistent high burn-rate, would be clean burning anyway and would not present problems in local airsheds. In addition, if the EPA was to regulate masonry heaters, the reporting units for emissions would have to have been changed first.

The g/hr emission rate is not useful or appropriate for masonry heaters since masonry heaters only burn fuel during a very short part of their useful heat output cycle. In addition, if masonry heaters are to be ranked or compared to other RWHs, EPA's NSPS test-method operating protocol (Method 28, 40 CFR Part 60, Appendix A) would have to be changed. Since the primary burn-cycle mode of operation for masonry heaters is one fuel load burned at the full-high burn rate until all the fuel is gone, the test cycle would need to include the whole cycle including start-up and complete burn down (i.e., a "cold-to-cold" test-burn cycle). The test-method operating protocol would have to take into account the fact that useful heat output is produced by masonry heaters long after the fire has gone out.

Method 28 for woodstoves is a hot-to-hot test-burn cycle: a hot coal bed is established in a hot stove; a specified fuel load is then added to begin the test; and completion of the test is at the point in time when the added fuel is totally consumed back to the original, hot coal bed. This cycle is conducted at 4 different, and specified burn rates to make a complete certification test series.

It is important to realize the difference between testing an RWH appliance using a hot-to-hot test cycle as opposed to using a cold-to-cold test cycle. In 1986, Jay Shelton of Shelton Research in Santa Fe, New Mexico (personal communication) did a woodstove research project for the State of Colorado and found that emissions discharged during the cold start up phase of a woodstove equals 50 percent of the emissions discharged during a whole hot-to-hot test cycle. This means that the standard EPA test method misses up to 33 percent of the total emissions actually discharged by a regulated RWH appliance during cold startup operations.

This is not a criticism of the method, if it is kept in mind that the method was designed and adopted to rank stoves against one another and not to simulate actual and absolute in-consumer-use emission rates. It was felt by almost all of the regulators participating in the NSPS regulation negotiations, that ranking of stoves with an indicated relative emissions reduction was more important than trying to establish absolute or "real world" emission rates for certified stoves. That is, an NSPS limit of 7.5 g/hr for non-catalytic RWHs was a 75 percent reduction from the 30 g/hr which was considered by the regulators to be the emissions rate for the common "conventional" stoves in use at the time of the NSPS negotiations. The idea was that the 75 percent reduction indicated by the laboratory test method would translate to a 75 percent reduction in actual home-heating-use emissions to the atmosphere, regardless of what the actual or absolute emissions rates were. The objective was to get the 75 percent reduction. There was never any attempt or wish expressed by EPA in the NSPS negotiations to use RWH certification emissions values for estimating or modeling airshed emissions loading rates⁽³⁾.

On the other hand, therefore, it should be kept in mind that contrary to the woodstove test protocol of hot-to-hot test periods, all standardized masonry heater testing performed by OMNI to date has sampled the whole burn cycle on a cold-to-cold basis (defined by flue-gas temperatures of less than 100°F). The Automated Emission Sampler (AES) used by OMNI Environmental Services in performing 'in-situ' field and laboratory sampling of masonry heaters, collects emissions samples during all phases of the burn cycle, including start-up from cold (i.e., flue-gas temperature above 100°F) to cold, total-fuel burn-down. All fuel loads burned during the total sample period of one week or more, 24 hours a day, are sampled. Thus OMNI's data on masonry heaters is a "real world" emission rate that is not directly comparable to RWH certification data.