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Emissions factors fromresidential wood pellet boilers characterized into start-up, steady operationand stop emissions
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (MERO)ORCID iD: 0000-0002-4308-3980
Högskolan i Dalarna.
2014 (English)In: Energy and fuels, ISSN 0887-0624, Vol. 28, no 4, p. 2496-2505Article in journal (Refereed) Published
Abstract [en]

Gaseous and particulate emissions from three residential wood pellet boilers and a stove were characterized and quantified at start-up, steady (full, medium and low combustion power), and stop phases. The aim was to characterize the emissions during the different phases of boiler operation and to identify when the major part of the emissions occur to enable actions for emission reduction where the savings can be highest. The investigated emissions comprised carbon monoxide (CO), nitrogen oxide (NO), total organic carbon (TOC), and particulate matter (PM 2.5). In this study, particle emissions were characterized by both number and mass concentration. The emission characteristics at high combustion power were relatively similar for all tested devices while significant differences in CO and TOC were observed at lower combustion power. Highest CO and TOC emissions are produced by the bottom fed boiler at low combustion power. The accumulated start-up emissions of the tested devices varied in the ranges of 0.5–12 g CO, 0.1–0.7 g NO, 0.1–2 g TOC, 0.12–2.9 g PM2.5, and 2.4 × 1013 to 3.1 × 1014 particles PM2.5. The accumulated stop emissions varied in the ranges 4–15.5 g CO, 0.01–0.11 g NO, 0.02–1.6 g TOC, 0.1–1.3 g PM2.5, and 3.3 × 1013 to 1.4 × 1014 particles PM2.5. The bottom fed boiler B1 had higher start-up and stop emissions than the tested top fed boilers and more particle emissions were accumulated in start-up phase than in stop phases of boiler B1, B3, and stove S1. Number of particles emitted from residential wood pellet combustion is dominated by fine particles smaller than 1 μm and similar particle distribution both in number and mass were observed for the tested devices. The start-up phase generated higher accumulated particle mass than the stop phase.

Place, publisher, year, edition, pages
2014. Vol. 28, no 4, p. 2496-2505
Keywords [en]
Emissions factors, wood pellet, boilers, start-up, steady operation, stop
National Category
Energy Engineering
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-27236OAI: oai:DiVA.org:mdh-27236DiVA, id: diva2:775021
Available from: 2014-12-30 Created: 2014-12-30 Last updated: 2015-12-21
In thesis
1. Emissions from realistic operation of residential wood pellets heating systems
Open this publication in new window or tab >>Emissions from realistic operation of residential wood pellets heating systems
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Emissions from residential combustion appliances vary significantly depending on the firing behaviours and combustion conditions, in addition to combustion technologies and fuel quality. Although wood pellet combustion in residential heating boilers is efficient, the combustion conditions during start-up and stop phases are not optimal and produce significantly high emissions such as carbon monoxide and hydrocarbon from incomplete combustion. The emissions from the start-up and stop phases of the pellet boilers are not fully taken into account in test methods for ecolabels which primarily focus on emissions during operation on full load and part load.

 The objective of the thesis is to investigate the emission characteristics during realistic operation of residential wood pellet boilers in order to identify when the major part of the annual emissions occur. Emissions from four residential wood pellet boilers were measured and characterized for three operating phases (start-up, steady and stop). Emissions from realistic operation of combined solar and wood pellet heating systems was continuously measured to investigate the influence of start-up and stop phases on total annual emissions. Measured emission data from the pellet devices were used to build an emission model to predict the annual emission factors from the dynamic operation of the heating system using the simulation software TRNSYS.

 Start-up emissions are found to vary with ignition type, supply of air and fuel, and time to complete the phase. Stop emissions are influenced by fan operation characteristics and the cleaning routine. Start-up and stop phases under realistic operation conditions contribute 80 – 95% of annual carbon monoxide (CO) emission, 60 – 90% total hydrocarbon (TOC), 10 – 20% of nitrogen oxides (NO), and 30 – 40% particles emissions. Annual emission factors from realistic operation of tested residential heating system with a top fed wood pelt boiler can be between 190 and 400 mg/MJ for the CO emissions, between 60 and 95 mg/MJ for the NO, between 6 and 25 mg/MJ for the TOC, between 30 and 116 mg/MJ for the particulate matter and between 2x1013 and 4x1013 /MJ for the number of particles. If the boiler has the cleaning sequence with compressed air such as in boiler B2, annual CO emission factor can be up to 550 mg/MJ. Average CO, TOC and particles emissions under realistic annual condition were greater than the limits values of two eco labels. These results highlight the importance of start-up and stop phases in annual emission factors (especially CO and TOC). Since a large or dominating part of the annual emissions in real operation arise from the start-up and stop sequences, test methods required by the ecolabels should take these emissions into account. In this way it will encourage the boiler manufacturers to minimize annual emissions.

 The annual emissions of residential pellet heating system can be reduced by optimizing the number of start-ups of the pellet boiler. It is possible to reduce up to 85% of the number of start-ups by optimizing the system design and its controller such as switching of the boiler pump after it stops, using two temperature sensors for boiler ON/OFF control, optimizing of the positions of the connections to the storage tank, increasing the mixing valve temperature in the boiler circuit and decreasing the pump flow rate. For 85 % reduction of start-ups, 75 % of CO and TOC emission factors were reduced while 13% increase in NO and 15 % increase in particle emissions was observed.

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2015
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 195
Keywords
Wood pellet, Heating, Combustion, boiler, stove, emissions, particulate matter
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-29582 (URN)978-91-7485-246-2 (ISBN)
Public defence
2015-12-14, rum Clas Ohlson, Högskolan Dalarna, Borlänge, 10:15 (English)
Opponent
Supervisors
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2015-11-25Bibliographically approved

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Win, Kaung Myat

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