2010: (Diagrams of 2 Stage Gasification and Combustion v. OWB “gasification”) NYSERDA on Differences Between European & U.S. OWBs and Between 2 stage biomass gasifiers and OWBs (one-stage)http://www.nyserda.org/publications/10_19_staged_combustion_biomass_boilers_acc.pdf
The story of direct use of thermal energy from combustion of biomass presents a long history, increased
future potential, and well-known problems. The historical absence of emission and efficiency requirements
based on rigorous regulatory test methods for wood-fired hydronic heaters is rapidly changing.
Forthcoming U.S. Environmental Protection Agency (EPA) rules for all biomass combustion appliances are
anticipated to result in technologies with greatly improved emissions performance. EPA’s previous
efforts on wood stoves in 1988 resulted in a 70% reduction in emissions. The current regulatory effort will
have broad impacts on the industry and create new opportunities. A critical aspect of these changes hinges
on the test methods for certification, and understanding how the test method links to the combustion
technologies and integrated heating-system concepts such as thermal storage. Europe instituted tight
efficiency and emission standards along with test method EN 303-5 over a twenty-five year span which
provided impetus for the growth of new technology firms manufacturing biomass combustion technologies
with thermal efficiencies similar to modern oil heating systems with substantial decreases in fine particulate
emissions. European test methods provide a valuable experience base to the EPA, which for some years
past has relied on a voluntary program to qualify model lines using Test Method 28-OWHH seeking to
clean up emissions from Outdoor Wood Boilers (OWBs). This voluntary program has been successful in
motivating some manufacturers of OWBs to improve emissions performance as evidenced by the number
of OWBs obtaining “Orange Tag” and now “White Tag” qualification, in what will necessarily be an
iterative process of technology forcing limits via the voluntary program and New Source Performance
Standard (NSPS). However, another class of combustion design, staged combustion, is not well-suited for
the evaluation methodology in Method 28-OWHH.
Two-stage combustion systems that are both lower in fine particle emissions and demonstrate high thermal
efficiency are sufficiently distinct from typical OWBs to require examination of test methods applied to this
technology class. Upgrading the underlying test methods will be a critical aspect of EPA rulemaking in
setting the stage for technology evolution in the marketplace applied more broadly than to just conventional
OWBs. The New York State Energy Research and Development Authority (NYSERDA) convened a oneday
workshop among manufacturers and importers of two-stage wood combustion equipment to review test
method impacts on their products and make recommendations for improvements in Method 28-OWHH.
The major finding from the workshop is that Method 28-OWHH, developed by EPA with input from the
conventional OWB manufacturers and the Northeast States to serve the needs of a voluntary OWB program
and State regulations, needs relatively minor, yet significant modifications for application to the broader
array of two-stage combustion technologies which are distinct in fundamental design and operation as
compared to typical OWBs. Among these are:
1. A revised Method 28-OWHH should be in the public domain and published in the Code of Federal
2. Revisions to Method 28-OWHH should be on a 5-year periodic basis in recognition of changes in:
measurement instrumentation, health-based emission standards, combustion technology
innovation, and the need for short-tests to parallel full certification data.
3. EPA should place specific attention on the burn-rate modes of the certification test (Categories III-
III-IV). A wide range of combustion technologies for which Method 28-OWHH must now
apply under the new NSPS includes conventional OWBs, two-stage combustion technologies,
automated pellet burners, and systems with and without thermal storage. This variety of
combustion and thermal storage concepts should lead EPA to review the appropriateness of the
very low burn rate called for in testing Category I. The recommendation is that EPA follows the
European test method standard and/or allows exact testing according to manufacturer
specifications for how the device is to be used by consumers.
4. EPA should consider a review of crib wood moisture testing, and provide a data review of how
much moisture can be found in air-dried crib wood.
5. EPA should consider modifying Method 28-OWHH to conduct the test with crib or cord wood
loaded into the firebox according to the written specifications of the manufacturer as in the
European EN 303 test. This is especially important for determining nominal load as fireboxes can
hold more fuel than currently prescribed in Method 28-OWHH
6. Method 28-OWHH should be modified to add requirements for the temperature of the cold water
and hot water supply as there are none presently and these temperatures impact firebox water
jacket temperatures which in turn impact emissions and efficiency measurements. European
specifications for inlet and outlet water temperature in EN 303-5 are suggested.
7. Piping and control systems for cold water return and hot water supply lines external to the boiler
under test must follow manufacturer specification with respect to pipe sizes, circulator capacities,
tempering and mixing valves as these have great effect on reported efficiencies and measured
8. It is recommended that EPA and industry establish an inter-lab “round robin” testing protocol that
would serve as the benchmark for certification test variability as is currently required in the NSPS
for wood stoves.
9. It is highly recommended that EPA add a simulation of thermal storage to upgrade Method 28-
OWHH for the needs of the new NSPS.
10. EPA should consider allowing the use of the indirect method for documenting thermal efficiencies
by requiring CO and CO
measurements as part of certification testing, especially for pellet-fired
boilers, which can operate with steady firing conditions.
Significant differences exist in the performance of residential biomass boiler technologies in the U.S. and
Europe. These are primarily due to the evolution of staged combustion designs in European heating
equipment over the past 30 years (NYSERDA, 2008; 2010). Figure 1 illustrates the
efficiency required in Europe. Note that Classes 1 and 2 will soon be retired as allowed efficiency standards
effectively establishing Class 3 as the performance floor. Classes 4 and 5 are the proposed new
requirements. Even in advance of the Class 5 efficiency standards, 25% of the new European residential
boiler technologies on the market average 87% thermal efficiency (based on the high-heating value of
wood) and thus exceed proposed Class 5 requirements. Some of these achieve even higher thermal
efficiencies called for in the “Blue Angel” and “Nordic Swan” eco-label targets set by individual European
countries [analogous to the “Energy Star” labels in the US]. Several brands of European residential wood
boilers have been imported to the U.S. in the past and more recently companies are entering the U.S.
market and forming manufacturing partnerships with U.S. boiler makers.
Figure 1. European efficiency requirements based on higher heating value
In addition to very tight thermal efficiency standards, the Europeans also have adopted a parallel
supporting set of regulations on the emissions for all biomass combustion devices. There is a strong
inverse relationship between energy efficiency and emissions. That is, as efficiency increases, emissions
decrease, often in a nonlinear fashion. Higher-efficiency units are typically capable of meeting tight
emissions requirements without any post-combustion control technologies. To maximize efficiency and
minimize emissions, boilers should be operated at a relatively high output, also known as the “Nominal” or
“Rated” output. To facilitate this type of operation, European regulations often require a matched, thermal
storage heat reservoir for cord-wood boilers that is external to the boiler itself (as opposed to the large
water jacket surrounding the combustion chamber on conventional OWBs). This storage vessel, which is
also called an “accumulator tank” allows the entire charge of wood in the firebox to be burned cleanly at
high efficiency without periods of oxygen starvation and smoldering or idle operation as a means to
modulate heat output. Oxygen starvation, used as a control parameter in conventional OWBs to modulate
heat output, leads to the smolder combustion and extremely high emissions. Thermal storage is highly
recommended even for pellet-fired boilers with automated combustion control. Figure 2 is a schematic of a
pellet boiler, pellet-storage bin, accumulator (thermal storage) tank and solar-thermal hot water system.
Figure 2. Schematic of a residential installation: (1) pellet boiler, (2) accumulator tank, (3) pellet
storage bin, and (4) solar-thermal panels (Courtesy of Maine Energy Systems).
European success in the area of high efficiency, low emissions, and growth of high-tech manufacturing jobs
in wood combustion technology, can be thought of as a “three-legged stool” that rests on 1) high
performance requirements for thermal efficiency, 2) tight standards for emissions, and 3) repeatable and
easily-conducted test methods for (1) and (2) that guide and encourage the best technologies.
1.1 DESIGN AND OPERATIONAL DIFFERENCES: TWO-STAGE COMBUSTION BOILERS
All combustion processes require a vaporized, or “gas” fuel source in order for combustion to proceed.
This is true of a dinner candle as well as a jet aircraft engine: vaporized fuel is what burns. The word
“gasification” as it relates to fuel preparation prior to combustion is used by virtually any and all
manufacturers of any biomass burning device. First a combustible vapor must be produced, and then the
evolved fuel vapor is fed to the chemical reaction process identified as “flame”. The distinguishing
characteristics of two-stage combustors is that the generation of combustible fuel vapors are physically and
process separated from the dominant heat-release of the vapor combustion step. In conventional
combustion (i.e. fireplaces, OWBs, traditional wood stoves, campfires, etc.), the fuel vaporization
generation process is not separated spatially from the dominate heat-release flame front. In two-stage
combustor systems, there is a small flame that is needed to force the evolution of combustible vapor from
the solid fuel charge, followed by a separate fuel-vapor combustion process step. This separation of fuel
vapor preparation from combustion heat release is the basis for the term “Two-Stage Combustion”.
Compared to open combustion where fuel (cordwood) is vaporized and burned in one continual step, twostage
combustion principles provide the combustion engineer designer the opportunity to control flame
properties leading to clean emissions and high thermal efficiency.
Figure 3. Two-stage, split-wood, down-draft gasification boiler (Froeling, 2005).
Figure 3 is an example of a typical down-draft, two-stage wood boiler. The top chamber contains the
charge of cord wood with a small flame bed at the base which serves to force the generation of combustible
vapor fuel. This fuel jet is then pre-mixed with secondary pre-heated air and burns in a separated chamber
at the bottom. Heat transfer then follows at the back. Sophisticated controls regulate the flow of air to both
the upper chamber to generate combustible vapors from the supply of fuel wood, and also to the vapor
combustion step (producing the main heat release) in the lower combustion chamber lined with refractory.
A process schematic of two-stage combustion is shown below:
Figure 4. Process schematic of two stage combustion (NYSERDA, 2010).
In contrast to two-stage wood combustion, OWBs characteristically have the fuel vapor generation and
combustion of these vapors taking place up through the stack of fuel wood charged to the boiler. This is
called “through combustion”.
Control of the ratio of air (oxygen) for combustion to the fuel vapor available for mixing and burning
(Air/Fuel ratio) is critical to emissions control. The A/F control in modern automobiles is accomplished by
an oxygen sensor located in the exhaust providing feedback many times per second, allowing computer
controls to modulate air and fuel to the engine for the wide range of operation conditions which keeps CO
combustion emissions in very narrow limits, regardless of load or vehicle operation mode. Like
the automobile, two-stage wood appliances are moving in the direction of ever tighter control and
modulation of A/F and some are using oxygen sensors (lambda control) in the downstream to provide even
tighter real-time continuous control. The left-hand side of Figure 5 shows the CO
(left Y axis) and CO
(right Y axis) emissions for typical through combustion process where uncontrolled combustion leads to
large swings in both emissions products as the fuel charge is depleted during a test burn. The upper righthand
side of Figure 5 is from a comparable two-stage combustion device where the relative constant levels
of both CO and CO
2 are seen between times when the unit is re-charged with wood. NOTE
: the scale
range markings for CO emissions for the OWB graph are twice as large as for the two-stage data.
The lower right-hand figure in Figure 5 is the same data from a modern tightly controlled pellet boiler,
demonstrating even tighter windows of CO
2 and CO control. The combination of CO2
and CO emissions at
any given moment in the burn cycle is a key indicator of low emission [low CO], high efficiency and
Figure 5. Comparisons of real-time CO/CO2 emissions for through combustion, two-stage split
wood, and 2-stage pellet boiler combustion performance (Hartmann, 2003).
The track record of huge gains in both efficiency and low emissions in Europe is shown in Figures 6 and 7
below where many test data points are shown from a wide range of devices. Over a 25-year period, thermal
efficiency has improved from about 55% to over 90% based on the lower heating value. CO emissions
have in the same time frame reduced from nearly 15,000 mg/m
3 to 50 mg/m3
Figure 6. Efficiency improved from 55% to > 90% (based on LHV) over 25 years (Voglaur, 2005).
Figure 7. Carbon monoxide emissions drop from 15,000 to < 50 mg/m3 over 25 years (Voglaur, 2005).
1.2 U.S. EPA Technology Forcing Activities
In an effort to encourage improved emission performance of new OWBs sold in the U.S. the EPA in 2007
implemented a voluntary program for Outdoor Wood-fired Hydronic Heaters “OWHH”. This EPA program
utilizes a test method known as “
Method 28 OWHH
” adapted from the historic Wood Stove Method 28 of
1988 for application to OWBs with operation test modes selected to capture the range of OWB operations.
A schematic of how the OWB is set up for operation in Method 28-OWHH is below:
Figure 8. Schematic of experimental set up for boiler evaluation according to Method 28 OWHH (U.S.
This first step by EPA to reduce emissions from OWBs included a voluntary partnership agreement, testing
by EPA-accredited laboratories, reporting of test results, and EPA issuance of labels and hangtags denoting
qualification of Phase 1 (Orange Tag) and Phase 2 (White Tag) models. In conjunction with EPA’s effort,
the Northeast States for Coordinated Air Use Management (NESCAUM) developed a model rule for OWB
emissions to leverage implementation of the test reviews completed under EPA’s voluntary program
(NESCAUM, 2007). In the absence of federal regulations, eleven states have adopted or proposed OWB
regulations (not voluntary) based on the NESCAUM Model Rule. In March 2010, the EPA Voluntary
Program Phase 1 “Orange Tag” (0.6 lb/MMBTU input) was retired and only the Phase 2 “White Tag” level
(0.32 lb/MMBtu output) is now recognized in the program. This change in the emissions performance
requirement, in conjunction with linking the emissions standard to thermal output, rewards higher
efficiency systems and is a positive supporting link between efficiency and low emissions.
EPA is currently in the process of updating the emissions performance requirements (regulations, not
voluntary) of all residential wood heating technologies including wood stoves, pellet stoves, wood boilers
and others. These New Source Performance Standards (NSPS) revisions will require that all newly
manufactured wood-fired heating systems meet emission levels that represent the current best demonstrated
technology; i.e., all new units sold in the United States will have to meet the level of performance of today’s
best designs. The EPA must complete an evaluation of “best demonstrated technology (BDT)” that will
include emissions data gathered under the voluntary program as the basis of the certification test for NSPS
for wood boilers. EPA is examining emissions and cost data from wood-fired heating technologies from
both within and outside of the United States to determine appropriate BDT emission standards as applicable
to the forthcoming NSPS EPA rule.
Two-stage wood-fired units do not operate in the same fashion as conventional OWBs that Method 28-
OWHH was designed to evaluate. The two-stage combustion boilers have a primary combustion chamber
where volatile components of the wood fuel are gasified under oxygen-starved conditions at relatively low
temperatures. In the secondary chamber the volatiles are combusted with preheated air under oxygen-rich
conditions. Such a two-stage setup promotes complete combustion and high efficiencies. These units also
tend to have a higher level of sophistication including operational controls that utilize a microprocessor,
temperature and gas sensors, and variable speed blowers that lead the boiler to operate at high-loads and
avoid idling modes. It is because of the staged combustion and controls that resulting thermal efficiencies
and emissions performance is so improved relative to single combustion chamber appliances.
Conventional OWBs typically have a large water volume surrounding a large volume firebox, and are
operated in alternating burn-smolder modes as the call for heat varies as seen in Figure 5. In order to
capture this high emission mode in testing for approval, EPA required that the Method 28-OWHH include a
certain amount of operation be conducted in a very low output mode identified as “Category I”, defined in
Method 28-OWHH as less than 15% of the full output capacity (measured rated output) for the unit.
Two-stage wood appliances operating at high-efficiency were not designed to operate in a smolder or idle
mode. Neither do these commonly have a large water volume in the combustion vessel itself like most
conventional OWBs (typically several hundred gallons). They are more commonly operated with small
water jacket volumes (typically about 40 gallons) and run in high output modes, and/or with thermal
storage systems. Most two stage boilers can cycle on and off to meet low load situations, although most
were designed for heat storage that is separated from the combustion unit.
1.3 PURPOSE OF WORKSHOP
The New York State Energy Research and Development Authority (NYSERDA) convened a workshop on
April 6, 2010, with firms manufacturing or importing high-efficiency wood-fired boilers in the Northeast.
The purpose of the workshop was to discuss operational details of two-stage wood boilers and modern
pellet- or chip-fired boilers with respect to energy efficiency and the duty-cycles in the European-based EN
303-5 and Method 28-OWHH for Voluntary OWB certification tests.
Participants were given a summary of the workshop purpose and a link to Method 28-OWHH. On the day
of the workshop, after an introductory session and charge to the participants, the attendees were split into
two groups; one to address cordwood-fired boilers and the other to address pellet- and chip-fired boilers.
Questions given to the groups to address were:
1) Are the test burn cycle modes in Method 28-OWHH (Cat I-II-III-and IV) representative of how a
two-stage combustion boiler operates?
2) Are there fundamental differences in the way your boiler operates that prevents it from functioning
properly in the Method 28-OWHH test modes?
3) For each procedure in the test, is your boiler able/ready to operate for the duration of the mode?
4) What design elements, tools, or techniques does your boiler employ to avoid idling or smoldering?
5) What sizing parameters would you recommend for the use of hot water storage or accumulator
tanks in the test and for installation in a residence?
6) In the context of the existing Method 28-OWHH procedure, what would you recommend to
improve the duty cycle (modes) in the test to more appropriately reflect recommended boiler
7) In the context of an Energy Star or Annual Fuel Utilization Efficiency, what would you
recommend the duty cycles (modes) in the test be to more appropriately reflect application of the
technology for a central New York State climate?
8) What boiler sizing recommendations do you have? Is this compatible with sizing of oil-fired
systems using Manual J, Residential Load Calculations, published by the Air Conditioner
Contractors of America?