http://www.itebe.org/telechargement/textesconferences/be2000/lasselsbergeranglaisR

QUALITY MARKING AND ENVIRONMENTAL TESTING OF SMALL-SCALE BIOMASS BOILERS IN AUSTRIA Leopold LASSELSBERGER Federal Institute of Agricultural Engineering (BLT), A-3250 Wieselburg Tel +43-7416-52175, Fax +43-7416-52175-45, email: direktion@blt.bmlf.gv.at

Abstract The article surveys the development of hand and automatically stoked biomass furnaces. It includes boiler technology, (international) standards, testing and its results.

Heating private homes with wood has a very long tradition in Austria. Over the past decade the combustion quality of Austrian heating systems could be considerably improved. Emission values were reduced and boiler efficiency could be increased. The test reports published by BLT Wieselburg underline this development. The publication of the European standard EN 303-5 "Heating boilers for soild fuels" guarantees standardised testing and assessment of biomass furnaces on a European level.

1

Introduction

In Austria, a country which is densely wooded (46 %), 25 % of the overall energy requirement are covered by water power and biomass. 140 PJ out of a total of 1290 PJ (i.e. about 11 %) are supplied by the latter. This contrasts with 5.4 % on a European level, a share which should be raised to 12.5 % by 2010.

Even though there are slightly decreasing figures for wood furnaces at the moment (approx. 570000 of the Austrian households are heated with wood, i. e. about 18.5 %), wood is still of increasing importance, a fact which is due to the steady CO2 increase in the atmosphere [I]. Biomass is mostly used in residential heating systems and in district heating plants with varying heat output. In 1998 about 2300 chipped wood furnaces were installed, 2000 of which had an heat output of up to 100 kW. Moreover more than 1300 pellet boilers were sold. Approx. 12000 to 15000 new log wood boilers were installed [II].

In Austria small-scale heating systems (furnaces with a nominal heat output of 4 kW to 400 kW) are used for heating rooms as well as for hot water production. Furnaces are distinguished according to their field of application (for central heating systems, for heating rooms or hot water production) and according to the type of fuel used. There are several types of biomass boilers: hand stoked furnaces (log wood boilers) and automatically stoked furnaces (chipped wood and pellet boilers).

Boiler testing and the publication of the test results help to improve boiler technology, as exact measurements and advanced test methods allow insights into the combustion process which might be of use to manufacturers and customers. Thus, the publication of the test results also stimulates the competition amongst the manufacturers of biomass boilers.

As an officially authorised institute BLT Wieselburg conducts type tests of heating boilers for biofuels.

About 40 small-scale furnaces are tested a year, of which more than 50 % achieve positive test results [III]. When BLT Wieselburg started those tests 20 years ago, appropriate test methods still had to be developed.

Lasselsberger

Landtechnische Forschung/1999

Small-scale biomass boilers

2

2

Technology of small-scale boilers

Hot-water central heating systems consist of a boiler with an integrated burner, the hot water distribution system with pumps and tubes, the radiators in the rooms, a hot water storage tank, a control unit and the flue gas chimney. The boilers are operated with natural gas, liquid petrol gas, fuel oil, coal and coke, with log wood, chipped wood or pellets. The following types of wood-fired boilers are available in the power range up to 100 kW.

2.1

Hand stoked boilers

Log wood boilers are stoked manually with wood logs or coarse chipped wood. In modern boilers with underburning systems the combustion air is mostly supplied as primary or secondary air by means of air blowers. controlling unit oxygene sensor forced draft filling chamber heat exchanger combustion air combustion chamber

Figure 1: Hand stoked log wood boiler

2.2

Automatically stoked boilers

Small chipped wood furnaces for rooms are mostly stoked automatically with chipped wood up to 30 mm in size. Most of the time boilers are equipped with heat exchangers integrated into the burning chamber or with preheating systems. controlling unit combustion chamber discharge auger

combustion air

Figure 2: Automatically stoked chipped wood furnace Pellet furnaces are used as boilers for central heating or as chimney stoves for rooms. Figure 4 shows a typical chimney stove with automatic stoking.

Lasselsberger

Landtechnische Forschung/1999

Small-scale biomass boilers

3

filling chamber auger

combustion chamber

ash chamber

Figure 3: Pellet stove

3

Type tests

As there had been no international standards even experts found it difficult to compare the results of the individual European test institutes. Yet, in 1999 the European standard EN 303-5 ("Heating boilers for solid fuels"), which was set up under Austrian chairmanship, was published [IV]. This standard allows standardised testing and assessment of biomass furnaces in Europe.

At the moment the federal governments of Austria are converting the respective agreements following Art. 15a of the Austrian Federal Constitutional Act (on safety measures for small furnaces and on saving energy) into federal laws [V]. This guarantees standardised testing, assessment and type approval of furnaces in the Austrian federal states.

3.1

Boiler tests and the Austrian law

If tests are conducted according to EN 303-5 and the Austrian law, the furnace is operated within the heat output range stated by the manufacturer. Heat output, boiler efficiency and combustion period are determined. Moreover the composition of the combustion gas is analysed and the exit flue temperature, the draught and the emission properties are determined. During the test period the furnace is to be operated in such a way that continuous running is possible. The minimum heat output can be regulated manually or automatically by means a control device.

Emission limits and boiler efficiency have to be determined at nominal and minimum heat output. For hand stoked furnaces the minimum partial load has to be up to 50 % of the nominal heat output, for automatically stoked furnaces up to 30 %. If small hand stoked furnaces do not meet the requirements at minimum load, the furnace has to be equipped with an accumulator tank, a fact which has to be indicated on the type plate as well as in the technical records.

Table 1: Emission limits for biofuels (following Art. 15a BV-G) emission limits [mg/MJ] NOx OGC dust *) 1100 150 80 60 **) *) 500 150 40 60 CO

hand stoked automatically stoked *) **)

only applies for wood furnaces can be exceeded by 50 % at 30 % of the nominal heat output

Lasselsberger

Table 2: Boiler efficiency furnace hand stoked up to 10 kW >10 to 200 kW >200 kW automatically stoked up to 10 kW >10 to 200 kW >200 kW

boiler efficiency 73 % (65.3 + 7.7 log Pn) % 83 % 76 % (68.3 + 7.7 log Pn) % 86 %

Landtechnische Forschung/1999

Small-scale biomass boilers

3.2

4

The test rig at BLT Wieselburg

In autumn 1997 two new test rigs were opened (with a maximum heat output of 300 kW). The power range of Test rig I is up to 100 kW, that of Test rig II 100-300 kW. Both test rigs are equipped with 3 heat circuits. The heat is dissipated by means of two central heat exchangers with a radial ventilator and can be used for heating the test rigs and the adjacent rooms.

Test rig I 100 KW

Test rig II 300 KW

Figure 4: Test rigs BLT Wieselburg The construction of the new test rig was funded by the Ministery of Agriculture and Forestry and the Ministery of Economic Affairs.

3.2.1

Technical equipment

The test rigs are equipped with heat exchangers. The heat output is determined by measuring the amount of water in the heat circuit and its decrease in temperature (following DIN 4702-2).[VI] Flue gas outlet by means of a vertical exhaust gas section, generation of draught by means of insulated chimneys (200 mm, 250 mm, 300 m and 400 mm in diameter), 9 m above ground level, draught control.

Determination of the heat output: The volume flow is determined by means of a magneto-inductive flowmeter (Promag 33 A by Endress & Hauser); the outlet and the return temperature are measured by two resistance thermometres (Pt 100, 1/3 DIN), calibrated in pairs.

The exhaust gas temperature is determined by means of multiple point measurement with 5 thermocouples in the exhaust gas section.

The draught is determined by a ring balance.

For the determination of the CO and CO2 (in dry exhaust gas) content a non-dispersive IR gas analyser (NGA 2000 by Fisher-Rosemount), with a measuring range from 0-5 % CO, 0-20000 ppm CO, 0-24 % CO2 is used. Further IR gas analysers: BINOS I and BINOS 100 (by Leybold-Heraeus), measuring range 0-20 % CO2 and 0-50000 ppm CO as well as 0-500 ppm and 0-5000 ppm CO. The O2 content (of the dry exhaust gas) is determined by means of a paramagnetic gas analyser (NGA 2000 by Firma Fisher-Rosemount), with a measuring range of 0-20 %.

Dust content: A dust analyser by StrĂśhlein (with a nominal exhaust gas volume of 4 m3/h) is applied.

The dust is extracted by means of a quartz wool filter fixed directly behind the sampling probe. The partial exhaust gas volume is determined by means of a dry gas counter and a drying column connected in line. The sampling point for the determination of the dust content is right behind the exhaust gas section.

OGC content (hydrocarbons): FID (by JUM, Type VE 5); sampling by means of filters and pipes (heated to 180°C); determination in moist exhaust gas.

Lasselsberger

Landtechnische Forschung/1999

Small-scale biomass boilers

5

NOx content: A gas analyser by ECO Physics (Type CLD 700 El-ht; chemiluminiscence) is used; sampling by means of heated filters and pipes; determination in moist exhaust gas.

Further gas analyser (in dry exhaust gas): CLD 502 by TECAN AG including an exhaust gas cooling system.

The data is recorded by data acquisition systems (Schlumberger Scorpio SI 3535 G) in 10 sec. intervals; the mean value of 6 measurements is calculated and then stored.

4

Test results

The development of Austrian heating systems since the mid nineteen eighties has brought about considerable improvements in combustion quality. The emission values of organic gaseous carbons and of carbon monoxide have been reduced to 1/10-1/100 of the original value, a development which can be observed for hand as well as automatically stoked furnaces. Over the last 10 years the boiler efficiency has been increased from an average of 60 % to 80-90 %. Moreover the quality of design was improved, boilers became ready for multiple production and are now easier to operate (so for instance, the combustion period of hand stoked log wood boilers is now 4 to 8 hours at nominal heat output).

4.1

Hand stoked boilers

The emission values of hand stoked furnaces used to be clearly load-dependent. New developments in boiler technology allow striking results for log wood boilers at partial load. Low emission values can now also be achieved at minimum heat output. If the operating instructions specified by the manufacturers are followed, the required emission limits should be kept even at partial load.

4.2

Automatically stoked boilers

By adapting the energy supply to the amount of heat required, the operation of chipped wood furnaces can be considerably improved at partial load. The fuel and air supply is controlled by sensors (e. g. lambda probe, OGC probe, O2 probe) so that the combustion can be optimised and the required limits can be met even at 30 % of the nominal heat output.

As the fuel of pellet boilers is very homogeneous, it is easier to control the combustion process. The test results of central heating boilers show low emission values and a high efficiency. Chimney stoves have a higher O2 content in the exhaust gas which is due to the air purging. So there are higher losses in the exhaust gas.

100

Efficiency [%]

90 80 70 60 50 40 80

82

84

86

88

90

92

94

96

98

Year

Figure 5: Efficiency of biomass boilers (log wood, chipped wood and pellets)

Lasselsberger

Landtechnische Forschung/1999

Small-scale biomass boilers

6

30000

CO [mg/Nm3]

25000 20000 15000 10000 5000 0 80

82

84

86

88

90

92

94

96

98

Year

Figure 6: CO emissions of (log wood, chipped wood and pellet) boilers

5

Summary and conclusion

BLT Wieselburg has been testing biomass furnaces since 1979. Test methods which are now standardised on an international level allow the comparison of test results obtained by different institutes. The publication of those test results stimulates the competition amongst manufacturers of biomass boilers and leads to substantial improvements in combustion quality. New boiler technology now allows for an easy operation of hand and automatically stoked boilers, thereby offering high boiler efficiency and low emission values.

6 [I] [II] [III] [IV] [V] [VI]

References NÖ ENERGIEBERICHT (1996), Bericht über die Lage der Energieversorgung in Niederösterreich 1995. Wien.

HACKSCHNITZELERHEBUNG (1998). NÖ Landes-Landwirtschaftskammer. Wien.

Bundesanstalt für Landtechnik (1979-1998). Prüfberichte über Heizkessel ÖNORM EN 303-5 (1 .Juli 1999). Heizkessel für feste Brennstoffe, mit handbeschickten und automatisch beschickten Feuerungen, mit einer Nennwärmeleistung bis 300 kW. Vereinbarungen des Bundes und der Bundesländer gemäß Art. 15a BV-G über die "Einsparung von Energie" und über Schutzmaßnahmen betreffend Kleinfeuerungen" (1997) DIN 4702-2 (1990). Heizkessel, Regeln für die heiztechnische Prüfung.

Lasselsberger

Landtechnische Forschung/1999