A Novel Method Evaluating the Real-Life Performance of Firewood

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A Novel Method Evaluating the Real-Life Performance of Firewood Roomheaters in Europe Gabriel Reichert, Christoph Schmidl, Walter Haslinger, Harald Stressler, Rita Sturmlechner, Manuel Schwabl, and Christoph Hochenauer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03673 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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Energy & Fuels

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A Novel Method Evaluating the Real-Life

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Performance of Firewood Roomheaters in Europe

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Gabriel Reichert1,2, Christoph Schmidl*1, Walter Haslinger1, Harald Stressler1, Rita

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Sturmlechner1, Manuel Schwabl1, Christoph Hochenauer2 1

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BIOENERGY 2020+ GmbH, Inffeldgasse 21b, 8010 Graz, Austria

Graz University of Technology, Institute of Thermal Engineering – Thermal Energy Systems

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and Biomass, Inffeldgasse 25/B, 8010 Graz, Austria

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* Corresponding author: Christoph Schmidl; e-mail: [email protected];

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Address: BIOENERGY 2020+ GmbH, Gewerbepark Haag 3, 3250 Wieselburg - Land, Austria;

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Phone number: +43 (7416) 52238-24

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Keywords: Firewood combustion; Test protocol; Firewood roomheater; Emissions; Thermal

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efficiency; Real-life

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ACS Paragon Plus Environment

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ABSTRACT:

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In Europe, the official type test method (oTT) according to the standard EN 13240 evaluates the

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emissions and thermal efficiency of firewood roomheaters only under optimal conditions, i.e.

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heated-up and at nominal load. In this study a novel test method, called “beReal”, is presented

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which was developed to reflect real-life operation within the testing procedure. The “beReal” test

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concept consists of a heating cycle with eight consecutive batches and covers all typical phases

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of real-life operation. A specific procedure based on volume flow measurements evaluating the

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emission and the efficiency of the whole test cycle was defined. A comparative assessment of the

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”beReal“ and EN test protocol was conducted with a serial-production stove and compared with

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oTT results. The repeatability of EN and ”beReal“ test results regarding emissions and thermal

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efficiency was evaluated. The EN test results of this study were more than 200% higher

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compared to oTT results of the used stove model. Hence, it seems that the tested product during

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oTT differs from the serial-production products. The thermal efficiency of “beReal” tests was

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significantly lower compared to EN test results. However, regarding emissions no significant

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differences were observed by the comparative tests according to both test protocols with the

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serial-production stove. An implementation of the “beReal” test protocol as a quality label or

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standard combined with a market surveillance should be considered as an instrument to push

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innovation and technological development further and to enable better differentiation of good

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and poor products for the end customer.

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1. Introduction

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Harmonized European standard test protocols, which evaluate the combustion performance of

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new products before market introduction, have clearly driven the technological development of

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biomass small-scale heating appliances towards low emissions and high efficiency in the last

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decades. For example, carbon monoxide (CO) emissions of biomass boilers during standard type

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testing decreased significantly from about 15,000 mg/m³ (STP, dry, 13 vol.-% O2) to less than

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1,000 mg/m³ (STP, dry, 13 vol.-% O2) whereas thermal efficiency increased from around 50% to

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more than 90% 1.

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However, since a high share of harmful particulate and gaseous emissions of residential

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heating is still present 2 there is a need to improve real-life operation of biomass heating systems.

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In general, the testing of new products shall guarantee a minimum of product quality

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concerning operation performance and safety aspects. Testing conditions and procedures shall be

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well defined and transparent in order to offer equal opportunities for manufacturers. For

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manually operated wood stoves there are several European standards which are specifically

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defined for each type of technology:

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o EN 13240: Roomheaters fired by solid fuel – Requirements and test methods 3

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o EN13229: Inset appliances including open fires fired by solid fuels – Requirements and

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test methods 4

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o EN12815: Residential cookers fired by solid fuel - Requirements and test methods 5

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o EN15250: Slow heat release appliances fired by solid fuel – Requirements and test

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methods 6 o EN 15544: One off Kachelgrundöfen/ Putzgrundöfen (tiled/mortared stoves) – Dimensioning 7


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The EN standards cover definitions and requirements regarding used materials, product

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declarations, performance and safety testing. During the performance test CO emissions and

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thermal efficiency are assessed. In addition, this testing procedure is used to assess further

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emissions, like organic gaseous compounds (OGC) and particulate matter (PM) emissions in

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order to comply with national legal requirements.

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However, standard test methods for firewood stoves evaluate the appliance performance only

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at nominal load. The ignition of the first fuel batch and the heating-up of the stove is not

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considered, except for the testing procedure for slow-heat release appliances (EN 15250 60).

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Generally, ignition, different loads, load changes and the cooling down phase are not included in

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the EN test protocols. Consequently, testing according to EN test protocols in a quasi-stationary

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operation mode and a thermal equilibrium usually leads to best possible emission and efficiency

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results and should be highly repeatable. However, operating conditions referring to typical user

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behavior and intransient conditions, like ignition, heating-up and cooling down which occur in

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each heating operation in real-life, are not evaluated. This leads to official type test (oTT) results

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of low emissions and a high thermal efficiency. However, these results are never reached during

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real-life operation 8, 9. This effect is well-known for many different product classes. Recently,

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quite big differences between lab and field performance were found for the car industry. In case

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of stove manufacturers we want to stress that differences do not originate from illegal testing

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software, as has been reported for car manufacturers, but simply result from different conditions

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between lab testing and field operation.

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Consequently, the differentiation of product qualities concerning emissions and thermal

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efficiency which refer mainly to intransient operating conditions and user related aspects is poor

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and customers have insufficient information for their selling decision. Furthermore, legal


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authorities cannot achieve the desired effect of reduced emissions and increased efficiency in the

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field by tightening of ELVs. The same criticism was also mentioned for biomass boilers in

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previous publications 10, 11.

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Consequently, there is an interest to develop and implement new test methods that are capable

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to evaluate the performance of the appliances closer to real-life operation to push technological

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innovation and technological development further and to enable better differentiation of good

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and poor products for the end customer

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In a European R&D project, called “beReal” 12, new test protocols for firewood roomheaters

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(EN 13240 3) and pellet stoves (EN 14785 13) were developed under collaboration of R&D

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institutes, stove manufacturers and industry associations. The new test protocols for these types

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of appliances aim at an evaluation of the combustion performance regarding emissions and

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thermal efficiency under testing conditions close to real-life 15. The new methods should

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generally be capable to be used as standard test protocols. For economic reasons the possibility

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of testing the appliance in one day was a basic requirement of the industry partners. The test

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procedures of both methods are based on the findings of different user surveys 15, 16 and long

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term field tests 17 in order to consider typical aspects of user behavior in the test concept.

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Specific operational aspects, e.g. the ignition mode, the effect of flue gas draught 18, air valve

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settings and fuel characteristics 19, were assessed by laboratory tests. For firewood roomheaters a

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one-page user manual, called Quick-User-Guide (QUG), was developed, which defines the

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specific operation mode for each appliance 17. The QUG is the basis for heating operation during

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testing. Moreover, it should act as a user manual in real-life. The reproducibility and real-life

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relevance was demonstrated by a Round-Robin-Test 20 and during a field test campaign 21.


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This study aimed at a comparison of the testing procedure and the emission and efficiency

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results of the novel test protocol “beReal” and the existing standard type test protocol

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exemplarily with one firewood roomheater (EN 13240). Therefore, comparative combustion

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experiments according to both test protocols were conducted. The results of the own EN test

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results were compared with oTT results. The results of CO, OGC and PM emissions were

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compared with future Ecodesign requirements which will come into force in 2022 in whole

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Europe 22. Fundamental differences of test procedures and data evaluation and their effect on the

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test results were identified. The repeatability of both methods was assessed and compared by

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calculating the coefficient of variation (CV). Finally, potentials and limitations of the “beReal”

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test protocol towards the effect on real-life evaluation are presented.

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2 Materials and Methods

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2.1 Fuel

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Beech (“Fagus sylvatica”) firewood with an average length of 0.33 m was used for all

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combustion tests (Table 1). The beech firewood of the ignition batch was split in small pieces

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for producing the kindling material. The firewood pieces derived from trees grown in the

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Austrian Province “Lower Austria”. It was bought as ready-to-use products from a local

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firewood producer 23 and was stored covered outside until the respective combustion tests were

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conducted.

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Table 1. Chemical properties of used fuel (chemical analysis included also bark) Moisture content* W

Net calorific value* Hu

Ash content a

Carbon C

Hydrogen H

Nitrogen N

(MJ/kg)

(g/kg, d.b.)

(kg/kg, d.b.)

(kg/kg, d.b.)

(mg/kg, d.b.)

EN 147741:2009 24

EN 14775:2009 25

EN 14775:2009 26

0.16

14.88

8.3

(kg/kg) Analysis standard Beech firewood (“Fagus sylvatica”)

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EN 15104:2011 27

0.485

0.0607

Sulfur S (mg/kg, d.b.)

Chlorine Cl (mg/kg, d.b.)

EN 15289:2011 28

1110

95

9

*as received/ d.b. = dry base/

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2.1 Combustion appliance

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A commercial firewood roomheater, classified according to EN 13240 3, was used for the

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combustion tests 29. The most relevant oTT results are summarized in Table 2.

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Table 2: Performance characteristics of the used stove model assessed during oTT according to

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EN 13240 Parameter

Unit

Official type test (oTT) results

Nominal thermal heat output

kW

5.5

%

80

vol.-%

0.09

*mg/m³

1125

Nitrogen oxides (NOx)

*mg/m³

94

Organic gaseous compounds (OGC)

*mg/m³

73

Particulate matter (PM)

*mg/m³

12

°C

299

Thermal efficiency

Carbon monoxide (CO)

Temperature at flue outlet

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* based on standard temperature (273.15 K) and pressure (101,325 Pa) (STP), dry, referred to 13 vol.-% O2

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The combustion air supply is conducted to the stove by a central pipe and is delivered into the

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combustion chamber as primary, secondary and tertiary air. Primary air is supplied via two

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vertical cleavages at the left and right corner of the combustion chamber. Secondary air is

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supplied via holes at the back wall of the combustion chamber and tertiary air enters the

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combustion chamber as window flushing air. An adaption of combustion air supply is possible

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manually with one leaver. Therefore, the proportion of primary, secondary and tertiary air cannot

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be influenced by the user separately.

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2.3 Experimental test setup and measurements

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For the combustion experiments the firewood stove was placed on a balance (Mettler Toledo

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PTA 455 – 600, accuracy ± 50 g) at the laboratory test stand (Figure 1).


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Energy & Fuels

Figure 1. Scheme and picture of experimental test setup (all dimensions in m)

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0.33 m of the flue gas pipe were not insulated. The following part of the measuring section

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(0.15 m diameter) was insulated with a layer of 0.05 m glass wool. The combustion tests were

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carried out under controlled draught conditions of 12 ± 2 Pa. The flue gas draught (∆p) was

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monitored 50 mm downstream the gas analysis using a differential pressure manometer

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(Thermokon DPT 2500-R8). Gaseous composition of the flue gas (FGC) was measured

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continuously over the entire test duration. CO2, O2, and CO were measured with a multi-gas-

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analyzer (NGA 2000-MLT4). OGC was measured with an FID (M&A Thermo-FID PT63LT) as

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total hydrocarbons (THC) based on methane (CH4) equivalents. A thermocouple (type K)


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centrally placed in the flue gas pipe was used for measuring the flue gas temperature T1. The

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volume flow conditions were assessed by measuring the flue gas velocity (v) with a vane wheel

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anemometer (Höntzsch ZS25: accuracy ± 0.1 m/s) and the flue gas temperature T2

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(thermocouple, type K) after reducing the inner diameter of the flue gas pipe to 99 mm. Ambient

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air temperature was measured with a thermocouple (type K) at the distance of around 2 m next to

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the firewood stove on the level of the flue outlet (1.5 m above the floor).

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PM emissions (PM) were measured gravimetrically by out-stack measurement according to

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VDI 2066-1 guideline 30. Isokinetic sampling was adjusted based on the flue gas velocity

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measurements. The retention of the particles was carried out with a stuffed quartz wool cartridge

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and a downstream plane filter which both were heated up to 160°C during the measurements.

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Rinsing of the sampling probe was carried out with acetone after each combustion experiment.

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The total mass of rinsing was distributed mass-weighted to all PM measurements of the

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experiment. For pre- and post-conditioning the stuffed filter cartridges, the plane filters and the

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rinsing pots were dried at 180°C for 1 hour and subsequently cooled down in a desiccator for at

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least 8 hours until the thermal equilibrium was reached. The collected mass of particles was

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determined by weighing the filters before and after the measurements on a precision balance

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(Type: Sartorius ME 235P, accuracy ± 0.01 mg).

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2.4 Combustion Experiments

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2.4.1 Real-life test protocol – “beReal”

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Three combustion experiments according to the “beReal” test protocol were carried out 31.

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Each combustion experiment consisted of a heating cycle with eight consecutive batches and a

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subsequent cooling down phase (= 9 test phases). The heating cycle started with the ignition


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batch followed by four batches at nominal load (batch 2-5) and three batches at partial load

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(batch 6-8). The cooling down phase is defined as the duration when the refilling criteria of the

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eighth batch is reached until the flue gas temperature T1 decreases to 50°C (Figure 2).

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Figure 2. Test and PM sampling phases according to the “beReal” test protocol

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Nominal load represents 100% of the fuel mass (1.5 kg) per batch whereas partial load is

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defined as 50% of the nominal load batch mass (0.75 kg). For nominal and partial load the total

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batch mass was equally divided on two firewood pieces without bark. The length of the firewood

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pieces was 0.33 m. The firewood pieces were placed crosswise in the combustion chamber as

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defined in the manual and the QUG. The lighting of the ignition batch was carried out by the top-

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down ignition technique as defined by the manufacturer in the QUG. Therefore, two pieces of

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firewood, each 0.5 kg, were placed on the grate. On top of these two firewood pieces eight pieces

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of kindling material (0.5 kg) were stocked crosswise in three layers (1. Layer: 3 Pieces, 2. Layer:

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2 pieces, 3. Layer: 3 pieces). The starting aids, (FLAMAX: ECOLOGICAL WOOD WOOL

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based of wood and wax, rolled and soaked to small balls 32) were placed between the two

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kindling pieces used for the second layer (Figure 3).


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Figure 3. Lighting procedure of the ignition batch according to the top-down ignition technique.

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Recharging criteria of “beReal” is defined as the measured CO2 concentration of the flue gas

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reached 25% of the maximum CO2 concentration of the respective batch. In the case that the

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maximum CO2 concentration of the respective fuel batch was lower than 12 vol.-% the

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recharging was done at a CO2 value of 3 vol.-% absolutely. The air lever for adjustment of

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combustion air supply was set to fully open during the ignition batch and the nominal load

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batches (batch 2-5). When changing to part load operation the setting of combustion air supply

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was changed to decrease the combustion air supply. Therefore, the lever was adjusted 40% in the

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direction of “totally closed” settings. After finishing heating operation the lever remained at the

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settings of part load operation. This behavior results in increased thermal heat losses after

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finishing heating operation, but this is a common practice of users of firewood stoves 15. After

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cooling down the remaining residues in the combustion chamber were collected. Subsequently,

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the share of unburnt carbon was determined according to standard EN 14775 33.

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2.4.2 Standard type test protocol – EN 13240

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Three combustion experiments according to the EN 13240 test procedure were conducted. Each

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combustion experiment consisted of a heating cycle with seven consecutive batches (Figure 4).


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Energy & Fuels

Figure 4. Test and PM sampling phases according to the EN 13240 test protocol

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For each EN test five test batches were conducted. Deviating from EN 13240 test protocol the

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flue gas temperature was measured with the thermocouple (T1) instead of a suction pyrometer.

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This was defined due to the impossibility to enable the required flue gas velocity of 20-25 m/s in

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the suction pyrometer with the used gas sampling system. Previous tests showed that a sampling

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velocity below this range results in too low measured flue gas temperatures which effects

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efficiency determination significantly 19. PM sampling was carried out in each test batch (Figure

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4). According CEN/TS 15883 34 PM sampling started 3 minutes after recharging a new fuel

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batch and closing the door and lasted for 30 minutes. A fuel batch was terminated according to

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the signal of the balance when the mass of the test fuel batch was combusted (± 0.05 kg acc. to

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EN 13240). The ignition of the first fuel batch was performed as described for the beReal test

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procedure. The second fuel batch, consisting of two firewood pieces with a total mass of 1.5 kg,

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was recharged when the flames of the ignition batch extinguished. Basic firebed was specified

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according to the balance signal when the flames of the preheating batch extinguished. The air

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valve setting during ignition and preheating was set at “fully open”.

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For each test batch (batch 3-7) a mass of beech firewood of 1.5 kg equally proportioned over

228

two firewood pieces without bark was used. The firewood pieces had a length of 0.33 m and

229

were placed crosswise in the combustion chamber as defined in the manual. The air valve

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settings for the test batches were reduced as specified in the manual. The lever of combustion air


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supply was adjusted 20% in the direction of “totally closed” settings. At this setting the results of

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thermal heat output were close to the values as specified in the declaration of performance based

233

on oTT results.

234 235

2.5 Data evaluation and statistical analysis

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Emission concentrations were calculated for both test protocols for CO, OGC and PM

237

emissions. They were calculated based on dry flue gas at standard temperature and pressure

238

conditions (STP: 0°C, 101,325 Pa) and subsequently transferred to 13 vol.-% O2. For the

239

standard EN 13240 the average CO, OGC and PM emission concentrations of each test batch

240

were calculated (see “Supporting Information”: Equation S1- Equation S4). Out of the five test

241

batches the average of the best two batches regarding CO emissions was used as EN 13240 test

242

result.

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For “beReal” the evaluation of emissions was widely following the standard EN 13240. In

244

contrast to the evaluation of single batch results (EN 13240) the whole heating cycle consisting

245

of nine test phases was considered (for details see “Supporting Information”).

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For both test protocols the indirect approach of thermal efficiency determination was used.

247

Accordingly, the thermal (‫ݍ‬௔ ) and chemical (‫ݍ‬௕ ) flue gas losses as well as the losses due to

248

unburnt constituents in the residues (‫ݍ‬௥ ) were considered (Equation 1).

249

Equation 1. Evaluation of the thermal efficiency according to the indirect determination

250

approach

251 252

ߟ% = 100 − ሺ‫ݍ‬௔ + ‫ݍ‬௕ + ‫ݍ‬௥ ሻ

(1)

η%…thermal efficiency; in %


14

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Energy & Fuels

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‫ݍ‬௔ …Proportion of thermal flue gas losses; in %

254

‫ݍ‬௕ …Proportion of chemical flue gas losses; in %

255

‫ݍ‬௥ …Proportion of losses through combustible constituents in the residues; in %

256 257

Details about the evaluation procedure of emissions and thermal efficiency for both test protocols can be found in the “Supporting Information”.

258

For evaluation of the repeatability the coefficient of variation (CV) was calculated. For

259

statistical evaluation of the test results of “beReal” and EN 13240 a Student´s t-test was applied

260

using “MS-Excel” software. Significant differences were defined by p-values ≤ 0.05 and highly

261

significant differences by p-values ≤ 0.01.

262 263

3. RESULTS & DISCUSSION

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3.1 Fundamental differences of test protocols

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The “beReal” test protocol is based on the specifications given in the QUG and reflects a more

266

realistic heating operation than the actual standards. The test procedure covers typical elements

267

of heating operation in real-life, i.e. ignition, heating operation in different load settings and the

268

cooling down process (Figure 5). The QUG defines relevant aspects of user behavior, like the

269

ignition technique, the number and mass of used firewood pieces and kindling material for

270

ignition, nominal load and part load as well as the lever settings of combustion air supply for the

271

different test phases. Only firewood pieces of beech are permitted. The use of firewood with bark

272

is not obligatory for testing since comparative combustion experiments showed no significant

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influence of bark on “beReal” test results regarding CO, OGC and PM emissions 20. As kindling

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material beech or spruce may be used. The use of bio-based fire starters is mandatory. At least


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two firewood pieces have to be used for each batch. The distribution of total batch mass over the

276

used firewood pieces shall be similar (± 10%). The mass of the ignition batch has to be at least

277

80% of the batch mass used for nominal load heating operation. Heating operation at partial load

278

is obligatorily defined by using 50% of the mass of nominal load. Refilling of batch loads is

279

carried out according to a CO2 flue gas measurement which represents the instant of time when

280

flames are extinguished. This was observed as a typical instant of time for refilling according to

281

user surveys 15, 16.

282 283

Figure 5. Aspects of real-life heating operation that are considered by the test protocols

284

EN13240 and beReal

285

The EN test protocol considers predominantly appliance specific design aspects, like the

286

combustion chamber and air staging design, used materials and the leakage rate of the appliance

287

(Figure 5). As test fuel firewood of beech, birch or hornbeam is obligatory. There are fuel

288

requirements regarding chemical properties, e.g. moisture content (16 ± 4 %; m/m) and ash

289

content (< 1%; m/m), but the length of firewood pieces and the distribution of batch mass over

290

the number of firewood pieces are not defined. EN testing is only conducted at nominal load

291

which represents a performance evaluation of emissions and efficiency under optimal and quasi-

292

stable heating operation. In contrast to the “beReal” test protocol, transient conditions, e.g.


16

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ignition and pre-heating (batch 1 and 2) of the stove, load changes or cooling down phases are

294

not considered in the EN test procedure (Figure 5).

295

But also the “beReal” test protocol covers not all situations or operational conditions which

296

can occur during real-life operation. User behavior that deviates from the QUG is not considered.

297

Therefore, increased emissions which can occur due to maloperation, e.g. due to the use of

298

inappropriate fuels like wet firewood 35, 36, a too high (overload) or too low (low load) batch

299

size 37-39, inappropriate ignition technique 18 and air settings 40, or the use of litter 41 are not

300

considered by the “beReal” test procedure (Figure 5). Furthermore, the chimney system which

301

induces the flue gas draught according to dimensions, construction materials, weather and

302

temperature conditions is not considered in detail within the “beReal” test procedure. The

303

constant flue gas draught of 12 ± 2 Pa does not respect the effect of different draught conditions

304

on emissions and thermal efficiency for different types of appliances 18. In long-term field

305

measurements an average flue gas draught of 18 Pa was observed 17. Experimental combustion

306

tests at different firewood roomheaters revealed no statistically significant effect of draught

307

conditions on gaseous and particulate emissions in the range of 12 to 48 Pa. However, for all

308

stoves used in that study thermal efficiency decreased significantly at higher draught

309

conditions 18. Based on these results the “beReal” flue gas draught was defined equally as the EN

310

test protocol at 12 ± 2 Pa. A minimum flue gas draught of 12 Pa is required by manufacturers

311

manual that normal heating operation is guaranteed. Furthermore, potential technologies that can

312

be used as retrofit applications in order to control the flue gas draught, combustion air supply or

313

to reduce emissions, e.g. filters or catalysts, are not part of “beReal” testing. Differences of

314

“beReal” and EN test protocol regarding measuring methods and data evaluation are summarized

315

in Table 3.


17

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Page 18 of 34

316

Table 3. Differences of the EN 13240 and “beReal” test protocol regarding measuring methods

317

and data evaluation EN 13240 test protocol & CEN/TS 15883 for PM

“beReal” test protocol

All components obligatory

(O2, CO2, CO, OGC)

Obligatory only CO and O2 or CO2; OGC typically tested according to CEN/TS 15883 due to national ELVs

Measuring interval

≤ 1 minute

≤ 10 seconds

Particulate (PM)

Not part of the standard, but typically tested according to CEN/TS 15883 due to national ELVs

Part of the test method, basically according to VDI2066-1, in future new EN-PME test method should be applied 42

Fixed volume flow

Isokinetic; at least proportional of volume flow

3 minutes after refilling

Before opening the combustion chamber door for refilling

30 minutes (first 3 minutes of start-up phase not respected)

Entire batch duration respected: start-up, intermediate and burn-out phase

Flue gas temperature

Suction pyrometer*

Thermocouple centrally placed in the flue gas pipe

Velocity of flue gas

Not required assumed)

Balance

Required for determination duration and refilling

Parameter Measurements Gaseous components:

emissions

Sampling Sampling start

Sampling duration

(constant

volume

of

flow

batch

Obligatory measuring parameter, calibration for calculation of the average flue gas velocity (ܿ௠௘௔௡ ) required (see Figure S2 of „Supporting Information“). Not required, refilling according CO2 flue gas measurement

Data evaluation Emission concentrations: OGC, PM

Average of two batches arbitrarily chosen

Volume weighted average of total test duration; PM emissions measured in batch 1, 3, 5 and 7

Thermal efficiency

Indirect determination

Indirect determination

Thermal flue gas losses

Specific calculation based on fuel composition, CO, CO2 and flue gas as well as ambient air temperature

Absolute calculation based on ܿ௠௘௔௡ , ܿ௣ of dry air and the flue gas as well as ambient air temperature (see “Supporting Information”)

Chemical flue gas losses

Specific calculation based on fuel composition, flue gas composition (CO, CO2) as well as ambient air temperature

Absolute calculation based on CO mass flow

CO,

Losses through combustibles in the residues

318

Fixed value: 0.5% of thermal efficiency

Calculated based on the remaining mass in the combustion chamber and ash box (see Equation S11 of “Supporting Information”)

*in this study: thermocouple centrally located in the flue gas pipe (see Figure 1)


18

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The main differences of measuring methods between EN 13240 and “beReal” testing is the use

320

of the suction pyrometer for flue gas temperature measurement, the balance signal for

321

specification of batch duration and the flue gas velocity measurement as essential basis for the

322

“beReal” data evaluation. The PM measurement procedure is basically not part of the standard

323

EN 13240, but is typically performed according to CEN/TS 15883: “German and Austrian

324

method” 34 within EN testing. For “beReal” testing PM measurement is basically performed

325

according to VDI 2066-1 guideline 30 with an “out-stack” measuring approach. The main

326

difference between both measuring protocols is the isokinetic sampling according VDI 2066-1

327

compared to a fixed sampling volume flow according to CEN/TS 15883. Furthermore, rinsing of

328

the sampling probe is obligatory according to the guideline VDI2066-1 and the heating

329

temperature of filters is higher (at least 160°C for VDI 2066-1 guideline compared to at least

330

70°C for CEN/TS 2066-1). In this study PM sampling for both test protocols were carried out

331

according VDI2066-1 guideline in order to increase the comparability of test results.

332 333

3.2 Comparison of test results of emissions and thermal efficiency

334

For evaluation of the accuracy of flue gas velocity measurements the calculated and measured

335

total flue gas volume of each “beReal” test were compared (Table 4). The mean deviation was

336

below 20% which is required according to the beReal test procedure 31. t-test results showed no

337

statistical significant difference (p = 0.10).

338 339 340


19

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Page 20 of 34

341

Table 4. Comparison of calculated and measured total flue gas volume of beReal tests as quality

342

assurance for velocity measurement Unit beReal Test

Test 1 Test 2 Test 3

343

m³, STP, wet

*beReal:

*beReal:

Deviation

ࢂ࢝, ࡿࢀࡼ, ࢉࢇ࢒ࢉ࢛࢒ࢇ࢚ࢋࢊ

ࢂ࢝, ࡿࢀࡼ, ࢓ࢋࢇ࢙࢛࢘ࢋࢊ

calculated vs. measured

103.6

109.5

5.4%

106.1

118.4

10.4%

110.1

122.9

10.4%

Average of deviation

8.7%

*… see Equation S13 and Equation S14 of “Supporting Information”

344 345

No significant differences between EN 13240 und “beReal” combustion experiments for

346

gaseous and particulate emissions (CO: p = 0.517; OGC: p = 0.998; PM: p = 0.177) were

347

observed. The absolute differences for average CO and OGC results were marginal (< 5%). PM

348

emissions of “beReal” test results were around 17% higher compared to EN 13240 results

349

(Figure 6). This could be explained by the immediate start of PM sampling at the beginning of

350

the batch and the consideration of all characteristic combustion phases, defined as start-up,

351

intermediate and burn-out phases 43.

352

In contrast, comparing the oTT with “beReal” test results big differences were observed for all

353

measured emission concentrations. Compared to the oTT results the CO emissions of “beReal”

354

testing were around 150% higher, OGC around 80% and PM emissions around 241%. When

355

comparing the oTT test results with the EN 13240 test results of this study also clear deviations

356

for CO, OGC and PM emissions can be seen. The highest deviation of about 192% was observed

357

for PM emission test results (Figure 6). The future Ecodesign-ELV for CO and OGC emissions

358

were not met by the average results of “beReal” and EN 13240 test results. For OGC emissions

359

only one, for PM emissions two of the three conducted “beReal” combustion tests met the


20

Page 21 of 34

360

Ecodesign-ELVs. However, the average beReal test results were higher than Ecodesign-ELVs.

361

All three EN 13240 test results of PM emissions met the Ecodesign-ELVs. (Figure 6). 180 160 140 120 100 80 60 40 20 0

mg/m³

3500 STP, dry, 13 vol.-% O2

3000

2808

2695

2500 2000 1500

1125

1000 500 0

60 132

50

132

41

40 73

35

30 20

12

10 0 PM

OGC

CO

362

Figure 6. Emission results of combustion experiments according to the EN 13240 and “beReal”

363

test protocol in comparison to the official type test results (oTT) for the respective stove model.

364

The black line represents the Ecodesign threshold value of the respective parameter.

85

80.0

80 75 %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

70

68.7

70.2

acc. to beReal

acc. to EN 13240

65 oTT (EN 13240)

60 55 0 Thermal efficiency

365

Figure 7. Thermal efficiency results of combustion experiments according to the EN 13240 and

366

“beReal” test protocol in comparison to the official type test results (oTT).

367

The comparison of thermal efficiency results showed similar results compared to the gaseous

368

emissions (Figure 7).The absolute difference between the “beReal” and EN 13240 tests were

369

1.5% which was statistically highly significant (p ≤ 0.01). The difference between EN 13240 test

370

results and oTT results were 9.2% absolutely. Comparing the share of losses which were


21

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Page 22 of 34

371

calculated according to the indirect efficiency determination approach it is obvious that the

372

highest deviation is mainly caused by the losses due to unburnt material (Table 5).

373

Table 5. Share of thermal and chemical flue gas losses as well as losses due to combustibles in

374

the residues of EN 13240 and “beReal” test protocol Average losses of three tests

Unit

EN 13240

beReal

Thermal losses (ࢗࢇ )

kJ losses/kJ fuel input

0.275

0.238

Chemical losses (ࢗ࢈ )


0.018

0.024

Losses due to unburnt material (ࢗ࢘ )


0.005*

0.051**/ 0.064***

375

*fixed value according to standard EN 13240 for wood fuel/

376

** ‫ݍ‬௥, ௕௘ோ௘௔௟ …see Equation S11 of “Supporting Information”

377

***‫ݍ‬௥, ௘௫௣௘௥௜௘௠௡௧௔௟ …see Equation S12 of “Supporting Information”

378

The proposed 0.5% is clearly lower compared to the calculated (5.1%) and measured (6.4%)

379

value of this study. The EN 13240 respects only the residues passing through the grate due to the

380

argumentation that the ash which remains on the grate is used at the next heating operation for

381

ignition. However, the ignition batch is never respected by the EN 13240 testing procedure and

382

an ash and charcoal layer on the grate could also negatively affect the combustion performance

383

of the ignition batch. Consequently, the results indicated that the general use of 0.5% losses due

384

to unburnt material is too low. This is also confirmed by Mack et al. 2017 and Schüssler 2017

385

which investigated qr losses between 0.4% - 1.8% and 3.6% - 4.7% respectively for two

386

stoves 44, 45. Furthermore, the results showed that Equation S11 could be used as an

387

approximation, but an uncertainty of 25% can be estimated.

388

Comparing the EN 13240 test results with the oTT results, it is obvious that it was not possible

389

to reproduce the oTT results. Potential reasons for the high deviations are manifold and are


22

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Energy & Fuels

390

summarized in Table 6. The main reasons refer most probably to specifics of the tested

391

appliance and the used fuel. Additionally, differences in operation due to room for interpretation

392

of the standard EN 13240 and measuring errors could have been responsible for lower emissions

393

and higher thermal efficiency of oTT test results.

394

Table 6. Potential reasons for high deviations of EN 13240 test results of this study and oTT

395

results Parameter

Reason

Explanation of potential effects on the test results

Test appliance

No serial-production appliance is used

In this study the tested appliance was a serial product. This might have resulted in a different operating performance

Fuel

Test fuel is manufacturer

The single firewood pieces can be optimal designed and placed in the combustion chamber.

Testing procedure

Number of test batches not limited

In this study the number of test batches was limited. For each test 5 test batches were performed.

Flue gas temperature measurement

Suction pyrometer is used

Measurement method is error-prone. A deviation from the required suction velocity (20-25m/s) results in too low flue gas temperature 19. Consequently, thermal flue gas losses are underestimated and thermal efficiency is overestimated in the oTT.

Settings combustion supply

Room for interpretation: No constant air valve settings and “delayed” closing of combustion chamber door after refilling.

Combustion chamber door is not closed immediately after recharging or air settings are fully open during the first three minutes after refilling. Consequently, a faster ignition process and lower emissions during the start-up phase is achievable.

Room for interpretation: Emission evaluation is often made until CO2 reached 4 vol.-%

Emission data from the beginning of the batch load until CO2 is 4% is respected. High CO emissions during burn-out phase are neglected.

Larger measuring interval

In this study measuring intervals of 1 second were used for both test protocols. A measuring interval of 1 minute, which is permissible according to EN 13240 standard, might disregard short emission peaks

Sensitivity of measuring parameter

PM measurement is error-prone due to leakage rates in the sampling train which couldn´t be recognized during measurement. A leakage results in lower PM emission measurements.

for air

Data evaluation

Measuring interval

Measuring for PM

error

supplied

by

the

396 397


23

Energy & Fuels

398

3.3 Repeatability 16

CV (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

14 12 10 8 6 4 2 0

2.1

acc. to EN 13240

9.4

8.6 5.8

acc. to beReal

12.5

12.0

5.2

4.8 3.3

3.0

2.8 1.5

O2

2.4 0.9

CO2

CO

OGC

PM

T flue gas

0.6

0.4

Thermal efficiency

Thermal heat output

399

Figure 8. Coefficient of variation (CV) for selected test results according to the “beReal” and

400

EN13240 test protocol

401

For most parameters the EN 13240 test protocol revealed a lower CV and consequently a

402

better repeatability compared to “beReal” test protocol (Figure 8). The CV of CO emissions was

403

the only parameter which revealed a lower variation of results for “beReal” tests compared to EN

404

13240 test results (Figure 6).The CV of all selected parameters was below 14% which is similar

405

or even lower comparing CV results from test protocols of advanced biomass cookers ranging

406

from 10% to about 30% 46. The higher variability of “beReal” test results is most probably due to

407

considering the different combustion phases of the heating cycle, especially phases with transient

408

conditions (Figure 2). In these phases, i.e. ignition, load changes, cooling down, the variations

409

for single test cycles might be higher than the variability of selecting two batches at nominal load

410

from a test cycle (Figure 4, Figure 5).

411

Comparing the results of the CV it is evident that the EN 13240 test protocol has a higher

412

reliability compared to the “beReal” test protocol and is therefore appropriate for setting a

413

benchmark between different products. However, considering the higher variability of operating

414

phases respected by the “beReal” test protocol the reliability of the new test protocol is quite


24

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Energy & Fuels

415

good and suitable for setting a benchmark between different products under more realistic

416

conditions.

417 418

4. CONCLUSIONS

419

For realization of an emission reduction in real-life operation and for pushing technological

420

development further new test protocols shall focus on real-life heating operation. The testing

421

procedures have to reflect real-life heating operation as closely as possible, but also ensure

422

repeatability and reproducibility of the tested parameters. Transient phases, like cooling down,

423

load changes and ignition processes shall be included in the testing cycle since they typically

424

occur in each heating cycle in real-life. Suitable measuring methods for evaluating the different

425

heating phases have to be applied and a quality assurance concept must guarantee the correct

426

application and evaluation of measurements. If necessary, emission limit values and efficiency

427

requirements have to be adapted to new test methods. In this study such a real-life oriented test

428

concept for firewood roomheaters called “beReal” was presented.

429

The official type test results for CO, OGC and PM emissions were not reachable with the used

430

serial-production stove. For example, the deviation was up to 192% for PM emissions. In

431

general, it seems that future Ecodesign requirements for CO and OGC emissions are hardly

432

reachable with serial-production products, although they have low official type test results. As a

433

consequence legal initiatives should establish an efficient market surveillance in order to

434

guarantee that testing results are valid for serial-production products that are sold on the market.

435

Compared to the oTT results the “beReal” test method resulted in 150% higher CO, 80% higher

436

OGC and 241% higher PM emissions. Although the “beReal” test protocol is not capable to

437

evaluate maloperation and the total heating system consisting of the appliance, the chimney and


25

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

438

the user behavior, the test protocol evaluates the appliance performance more realistically. This

439

was also confirmed during a comprehensive testing campaign in the field 21. In order to evaluate

440

the potential applicability of the “beReal” test concept for assessing emission factors further field

441

tests are necessary. Based on that results the “beReal” test procedure might be adapted.

442 443

Acknowledgements

444

This study was done in the project “ClearSt” that was financially supported by the Austrian

445

Research Promotion Agency (under Grant Agreement no. 848940) in the frame of the program

446

“Forschungspartnerschaften” and in the frame of the COMET program managed by the Austrian

447

Research Promotion Agency (FFG) under Grant Agreement no. 844605 (“CleanStoves”). The

448

COMET program is co-financed by the Republic of Austria and the Federal Provinces of

449

Burgenland, Lower Austria and Styria.

450

Notes

451

The authors declare no competing financial interest.

452


26

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Energy & Fuels

453

5. REFERENCES

454

(1) SCHWARZ M., CARLON E.: Load cycle test for biomass boilers, , IEA Bioenergy Task

455

32 workshop: Practical test methods for small-scale furnaces, 5th Central European Biomass

456

Conference, Graz, Austria, 18-20 January 2017, 19.01.2017, Graz, Austria. Online available:

457

http://www.cebc.at/en/service/publications/5th-central-european-biomass-conference/workshop-

458

iea/ (accessed at 08.03.2017)

459

(2) IEA, World Energy Outlook, Special report, Energy and air pollution, 2016.

460

Online available: http://www.iea.org/publications/freepublications/publication/WorldEnergyOutl

461

ookSpecialReport2016EnergyandAirPollution.pdf (accessed at 04.10.2016)

462 463 464 465 466 467 468 469 470 471

(3) ÖNORM EN 13240:2001 + AC:2003: Roomheaters fired by solid fuel – Requirements and test methods, Austrian institute for standardization, Vienna, 2003 (4) ÖNORM EN 13229:2007: Inset appliances including open fires fired by solid fuels – Requirements and test methods, Austrian institute for standardization, Vienna, 2007 (5) ÖNORM EN 12815:2007: Residential cookers fired by solid fuel - Requirements and test methods, Austrian institute for standardization, Vienna, 2007 (6) ÖNORM EN 15250:2007: Slow heat release appliances fired by solid fuel - Requirements and test methods, Austrian institute for standardization, Vienna, 2007 (7) ÖNORM EN 15544:2009: One off Kachelgrundöfen/ Putzgrundöfen (tiled/mortared stoves) – Dimensioning, Austrian institute for standardization, Vienna, 2009

472

(8) HARTMANN H.: Status on emissions, regulation and technical improvements and future

473

developments for residential wood burning appliances in Germany, Seminar on wood


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combustion and air quality in Aarhus/Denmark, March 15th, 2012. Online available:

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http://www.skorstensfejerfredensborg.dk/upload/5.%20hans%20hartmann,%20tfz.pdf (accessed

476

at 08.03.17)

477

(9) REICHERT G., SCHMIDL C., HASLINGER W., MOSER W., AIGENBAUER S., FIGL

478

F., WÖHLER M.: Investigation of user behavior and operating conditions of residential wood

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combustion (RWC) appliances and their impact on emissions and efficiency, Central European

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Biomass

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http://www.biomasseverband.at/veranstaltungen/tagungen-und-vortraege/4-mitteleuropaeische-

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biomassekonferenz/biowaerme-simulation-und-effizienzoptimierung/ (accessed at 08.03.2017)

Conference,

Graz,

15-18

January

2014,

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p.

Online

available:

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(10) CARLON E., SCHWARZ M., GOLICZA L., KUMAR V., PRADA A., BARATIERI M.,

484

HASLINGER W., SCHMIDL C.: Efficiency and operational behaviour of small-scale pellet

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boilers installed in residential buildings, Applied Energy 155 (2015) 854 – 865.

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(11) SCHWARZ M., HECKMANN M., LASSELSBERGER L., HASLINGER W.:

487

Determination of annual efficiency and emission factors of small-scale biomass boiler, Central

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European Biomass Conference 2011, 26-29 January 2011, 28.01.2011. Online available:

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https://www.bioenergy2020.eu/files/publications/pdf/Schwarz_etal_Annual_Efficiency.pdf

490

(accessed at 08.03.2017)

491

(12) http://www.bereal-project.eu/ (accessed at 08.03.2017)

492

(13) ÖNORM EN 14785:2006: Residential space heating appliances fired by wood pellets -

493

Requirements and test methods, Austrian institute for standardization, Vienna, 2006


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(14) Rita Sturmlechner, Gabriel Reichert, Harald Stressler, Christoph Schmidl, Manuel

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Schwabl, Walter Haslinger, Heike Öhler, Johannes Bachmaier, Robert Mack, Hans Hartmann,

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Marius Wöhler.: beReal – Development of a new testing method close to real-life for domestic

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biomass room heaters, ProScience 3 (2016), 123-128. DOI:10.14644/dust.2016.020

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(15) REICHERT G., SCHMIDL C., HASLINGER W., SCHWABL M., AIGENBAUER S.,

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WÖHLER M:, HOCHENAUER C.: Investigation of user behavior and assessment of typical

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operation mode for different types of firewood room heating appliances in Austria, Renewable

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Energy 93, 2016, 245 – 245. DOI: 10.1016/j.renene.2016.01.092

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(16) WÖHLER M., ANDERSEN J. S., BECKER G., PERSSON H., REICHERT G., SCHÖN

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C:, SCHMIDL C., JAEGER D., PELZ S. K.: Investigation of real life operation of biomass room

504

heating appliances – Results of a European survey, Applied Energy 169, 2016, 240 – 249. DOI:

505

10.1016/j.apenergy.2016.01.119

506

(17) REICHERT G., HARTMANN H., HASLINGER W., OEHLER H., PELZ S., SCHMIDL

507

C., SCHWABL M., STRESSLER H., STURMLECHNER R., WÖHLER M., HOCHENAUER

508

C.: „beReal“ - Development of a new test method for firewood roomheaters reflecting real life

509

operation, Proceedings of 24th European Biomass Conference and Exhibition, 6-9 June , 2016,

510

Amsterdam, Netherlands, 382 – 387.

511

(18) REICHERT G., HARTMANN H., HASLINGER W., OEHLER H., MACK R.,

512

SCHMIDL C., SCHÖN C., SCHWABL M., STRESSLER H.,STURMLECHNER R.,

513

HOCHENAUER C.: Effect of draught conditions and ignition technique on combustion

514

performance of firewood roomheaters, Renewable Energy 105, 2017, 547 – 560. DOI:

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10.1016/j.renene.2016.12.017


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516

(19)

REICHERT

G.,

SCHMIDL

C.,

Page 30 of 34

STRESSLER

H.,

STURMLECHNER

R.,

517

HOCHENAUER C, HARTMANN H., SCHÖN C., MACK R., OEHLER H.: The „beReal“

518

Project – Scientific Highlights, IEA Bioenergy Task 32 workshop: Practical test methods for

519

small-scale furnaces, 5. Central European Biomass Conference, 18-20.01. 2017, 19.01.2017,

520

Graz, Austria

521

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A Novel Method Evaluating the Real-Life Performance of Firewood

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