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Emissions from realistic operation of residential wood pellets heating systems
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (MERO)ORCID iD: 0000-0002-4308-3980
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
Keyword [en]
Wood pellet, Heating, Combustion, boiler, stove, emissions, particulate matter
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-29582ISBN: 978-91-7485-246-2 (print)OAI: oai:DiVA.org:mdh-29582DiVA: diva2:872404
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
List of papers
1. Emissions Characteristics of residential pellet boilers during start-up and stop periods
Open this publication in new window or tab >>Emissions Characteristics of residential pellet boilers during start-up and stop periods
2010 (English)In: 3rd International Scientific Conference on Energy systems with IT / [ed] Erik Dahlquist, Jenny Palm, 2010, 120-130 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this study, gaseous emissions and particles are measured during start-up and stop periods for anover-fed boiler and an under-fed boiler. Both gaseous and particulate matter emissions arecontinuously measured in the laboratory. The measurement of gaseous emissions includes oxygen(O2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxide and (NO). The emissions rates arecalculated from measured emissions concentrations and flue gas flow. The behaviours of the boilersduring start-up and stop periods are analysed and the emissions are characterised in terms of CO, NO,TOC and particles (PM2.5 mass and number). The duration of the characterised periods vary betweentwo boilers due to the difference in type of ignition and combustion control. The under-fed boiler Bproduces higher emissions during start-up periods than the over-fed boiler A. More hydrocarbon andparticles are emitted by the under-fed boiler during stop periods. Accumulated mass of CO and TOCduring start-up and stop periods contribute a major portion of the total mass emitted during wholeoperation. However, accumulated mass of NO and PM during start-up and stop periods are notsignificant as the duration of emission peak is relatively short.

Keyword
Emissions, pellets, boilers, combustion, start-up, stop period, burn-out, ignition
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-29575 (URN)
Conference
3rd International Scientific Conference on Energy systems with IT
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2015-11-19Bibliographically approved
2. Particles and gaseous emissions from realistic operation of residential wood pellet heating systems
Open this publication in new window or tab >>Particles and gaseous emissions from realistic operation of residential wood pellet heating systems
2012 (English)In: Atmospheric Environment, ISSN 1352-2310, Vol. 59, 320-327 p.Article in journal (Refereed) Published
Abstract [en]

Gaseous and particulate emissions from six residential wood pellet heating systems are determined at a realistic six day operation sequence. The study aims to investigate the total emissions from a realistic operation of the heating systems including start-up and stop phases. Five combined solar and pellet heating systems and one reference boiler without solar system with an integrated DHW preparation was tested in a laboratory at realistic operation conditions. The investigated emissions comprised carbon monoxide (CO), nitrogen oxide (NO), total organic carbon (TOC) and particulate matter (PM2.5). In this study, the emissions are presented as accumulated total emissions from the whole six days period and the emissions from start-up and stop phases are also presented separately to evaluate the influence of the emissions from these phases on the total emissions. Emission factors of the measured systems from the six day period are between 192 and 547 mg MJ -1 for the CO emissions, between 61 and 95 mg MJ -1 for the NO, between 6 and 45 mg MJ -1 for the TOC, between 31 and 116 mg MJ -1 for the particulate matter and between 2.1 × 10 13 and 4 × 10 13 for the number of particles. The emissions from the start-up and stop phases are significantly high for the CO (63-95 %) and the TOC (48-93 %). NO and particles emissions are shown to dominate during stationary operation. However, 30-40 % of the particle emissions arise from the start and stop periods. It is also shown that the average emissions of CO, TOC and particles under the realistic annual conditions were higher than the limit values of two eco labels.

Keyword
Combustion; Emissions; Pellet boiler; Pellet stove; Start-up phase; Stop phase; Wood pellet
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-29574 (URN)10.1016/j.atmosenv.2012.05.016 (DOI)2-s2.0-84863482378 (Scopus ID)
Note

Export Date: 18 November 2015

Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2015-11-19Bibliographically approved
3. Emissions characteristics of a residential pellet boiler and a stove
Open this publication in new window or tab >>Emissions characteristics of a residential pellet boiler and a stove
2010 (English)In: / [ed] Swedish Bioenergy Association, 2010, 85-88 p.Conference paper, Published paper (Refereed)
Abstract [en]

Gaseous and particulate emissions from a residential pellet boiler and a stove are measured at a realistic 6-day operation sequence and during steady state operation. The aim is to characterize the emissions during each phase inorder to identify when the major part of the emissions occur to enable actions for emission reduction where the savingscan be highest. The characterized emissions comprised carbon monoxide (CO), nitrogen oxide (NO), total organic carbon(TOC) and particulate matter (PM 2.5). In this study, emissions were characterised by mass concentration and emissionsduring start-up and stop phases were also presented in accumulated mass. The influence of start-up and stop phases onthe emissions, average emission factors for the boiler and stove were analysed using the measured data from a six-daystest. The share of start-up and stop emissions are significant for CO and TOC contributing 95% and 89% respectively atthe 20kW boiler and 82% and 89% respectively at the 12 kW stove. NO and particles emissions are shown to dominateduring stationary operation.

Keyword
emissions, pellet boiler, stove, combustion, start-up, stop
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-29576 (URN)978-91-977624-1-0 (ISBN)
Conference
World Bioenergy Conference 2010
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2015-11-19Bibliographically approved
4. Emissions factors fromresidential wood pellet boilers characterized into start-up, steady operationand stop emissions
Open this publication in new window or tab >>Emissions factors fromresidential wood pellet boilers characterized into start-up, steady operationand stop emissions
2014 (English)In: Energy and fuels, ISSN 0887-0624, Vol. 28, no 4, 2496-2505 p.Article 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.

Keyword
Emissions factors, wood pellet, boilers, start-up, steady operation, stop
National Category
Energy Engineering
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-27236 (URN)
Available from: 2014-12-30 Created: 2014-12-30 Last updated: 2015-12-21
5. Emissions and deposit properties from combustion of wood pellet with magnesium additives
Open this publication in new window or tab >>Emissions and deposit properties from combustion of wood pellet with magnesium additives
Show others...
2013 (English)In: Ranliao Huaxue Xuebao/Journal of Fuel Chemistry and Technology, ISSN 0253-2409, Vol. 41, no 5, 530-539 p.Article in journal (Refereed) Published
Abstract [en]

This work studies the amount of gaseous and particle emissions and deposits on heat exchanger surfaces in a boiler fired with commercially available pellets and with pellets primed with magnesium oxide and magnesium hydroxide. The combustion experiments were performed on a residential boiler of 20 kW. Substrates placed in the heat exchanger was analysed with SEM-EDX-mapping to evaluate the chemical composition of the deposits. The results show that particle emissions (PM 2.5) using the additives increased by about 50% and the mass of the deposits in the flue gas heat exchanger (excluding loose fly ash) increased by about 25% compared to the combustion of pellets without additives. The amount of additives was found to be eight times higher than the amount of the main alkali metals potassium (K) and sodium (Na) which leads to the assumption that the additives were overdosed and therefore caused the problems reported. The SEM analysis of the substrates placed in the flue gas heat exchanger indicate that the deposits of sodium (Na), potassium (K), chlorine (Cl) and sulphur (S) decrease using the additives. If this was due to the expected chemical reactions or due to the loose fly ash covering the substrates after the test, could not be determined in this study.

Keyword
softwood pellets; magnesium; additives; particle emissions; deposit formation
National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-29578 (URN)10.1016/S1872-5813(13)60029-8 (DOI)2-s2.0-84879517759 (Scopus ID)
Note

Export Date: 18 November 2015

Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2016-05-16Bibliographically approved

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