2010: NYSERDA on Differences Between European & U.S. OWBs and Between 2 stage biomass gasifiers and OWBs (one-stage)

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

Regulations (CFR).


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.



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

and 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


complete combustion.




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.

EPA, 2008).


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.


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?

This entry was posted in Biomass Particulates Questioned (Industrial Biomass), Industrial Wood Gasification with fluidized bed reactors, OWB regulations. Bookmark the permalink.

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