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Combustion of gasified biomass:: Experimental investigation on laminar flame speed, lean blowoff limit and emission levels
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is among the primary alternative energy sources that supplements the fossil fuels to meet today’s energy demand. Gasification is an efficient and environmental friendly technology for converting the energy content in the biomass into a combustible gas mixture, which can be used in various applications. The composition of this gas mixture varies greatly depending on the gasification agent, gasifier design and its operation parameters and can be classified as low and medium LHV gasified biomass. The wide range of possible gas composition between each of these classes and even within each class itself can be a challenge in the combustion for heat and/or power production. The difficulty is primarily associated with the range in the combustion properties that may affect the stability and the emission levels. Therefore, this thesis is intended to provide data of combustion properties for improving the operation or design of atmospheric combustion devices operated with such gas mixtures.

The first part of this thesis presents a series of experimental work on combustion of low LHV gasified biomass (a simulated gas mixture of CO/H2/CH4/CO2/N2) with variation in the content of H2O and tar compound (simulated by C6H6). The laminar flame speed, lean blowoff limit and emission levels of low LHV gasified biomass based on the premixed combustion concept are reported in paper I and III. The results show that the presence of H2O and C6H6 in gasified biomass can give positive effects on these combustion parameters (laminar flame speed, lean blowoff limit and emission levels), but also that there are limits for these effects. Addition of a low percentage of H2O in the gasified biomass resulted in almost constant laminar flame speed and combustion temperature of the gas mixture, while its NOx emission and blowoff temperature were decreased. The opposite condition was found when H2O content was further increased. The blowoff limit was shifted to richer fuel equivalence ratio as H2O increased. A temperature limit was observed where CO emission could be maintained at low concentration. With C6H6 addition, the laminar flame speed first decreased, achieved a minimum value, and then increased with further addition of C6H6. The combustion temperature and NOx emission were increased, CO emission was reduced, and blowoff occurs at slightly higher equivalence ratio and temperature when C6H6 content is increased. The comparison with natural gas (simulated by CH4) is also made as can be found in paper I and II. Lower laminar flame speed, combustion temperature, slightly higher CO emission, lower NOx emission and leaner blowoff limit were obtained for low LHV gas mixture in comparison to natural gas.

In the second part of the thesis, the focus is put on the combustion of a wide range of gasified biomass types, ranging from low to medium LHV gas mixture (paper IV). The correlation between laminar flame speed or lean blowoff limit and the composition of various gas mixtures was investigated (paper IV). It was found that H2 and content of diluents have higher influence on the laminar flame speed of the gas mixture compared to its CO and hydrocarbon contents. For lean blowoff limit, the diluents have the greatest impact followed by H2 and CO. The mathematical correlations derived from the study can be used to for models of these two combustion parameters for a wide range of gasified biomass fuel compositions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , 81 p.
Series
TRITA-KRV, ISSN 1100-7990 ; 13:03
Keyword [en]
biomass gasification; gasified biomass; laminar flame speed; blowoff; emissions
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-120570ISBN: 978-91-7501-710-5 (print)OAI: oai:DiVA.org:kth-120570DiVA: diva2:615594
Public defence
2013-04-22, M3, Brinellvägen 64, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130411

Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2014-01-16Bibliographically approved
List of papers
1. Influence of water vapour and tar compound on laminar flame speed of gasified biomass gas
Open this publication in new window or tab >>Influence of water vapour and tar compound on laminar flame speed of gasified biomass gas
2012 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 98, 114-121 p.Article in journal (Refereed) Published
Abstract [en]

Biomass can be converted to a gaseous fuel through gasification in order to be used in higher efficiency conversion equipment. Combustion of a gaseous fuel generally allows for higher combustion temperatures than that of a solid fuel leading to the higher efficiency. However, the gasified biomass gas (GBG) contains condensable compounds, such as water vapour and tars, which both will affect the subsequent combustion process with respect to emission levels and flame stability. Cleaning of the GBG prior to combustion is very costly and therefore further research is needed on direct combustion of GBG containing these condensable compounds, in order to develop stable combustion techniques for GBG. The laminar flame speed is a main parameter that relates to other important flame properties such as stability, extinction limit and flashback. Each of GBG components have different chemical and transport properties, which then influences the laminar flame speed of GBG. In this study, the individual effect of water vapour (H2O) and tar compound addition in simulated GBG on laminar flame speed is investigated at atmospheric pressure. The tar compound used is benzene (C6H6) and simulated GBG used is CO/H-2/CH4/CO2/N-2 mixture. Experiments were carried out with conical burner stabilized flame and a Schlieren photography system. The volume fraction of additives in the fuel mixture was varied: for H2O from 0% to 5% and for C6H6 from 0% to 10%. The unburned fuel air mixture was preheated and the temperature was maintained at T-i = 398 K to avoid condensation of the liquid. It was found that measured laminar flame speed of GBG-air mixture decreases with addition of H2O in the fuel mixture. While, non-monotonic behaviour is shown with addition of C6H6. Initially, as the volume fraction of C6H6 incremented, the laminar flame speed decreases, reaching a minimum value, and then increase.

Keyword
Gasified biomass gas, Laminar flame speed, Water vapour, Benzene vapour
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-102606 (URN)10.1016/j.apenergy.2012.03.010 (DOI)000306889200013 ()2-s2.0-84862204229 (Scopus ID)
Funder
StandUp
Note

QC 20120921

Available from: 2012-09-21 Created: 2012-09-21 Last updated: 2017-12-07Bibliographically approved
2. Temperature, emission and lean blowoff limit of simulated gasified biomass in a premixed combustor
Open this publication in new window or tab >>Temperature, emission and lean blowoff limit of simulated gasified biomass in a premixed combustor
(English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118Article in journal (Other academic) Submitted
Abstract [en]

Biomass can be converted to a gaseous fuel through gasification in order to be used in higher efficiency conversion. Combustion of gasified biomass gas (GBG) in gas turbines, for example, potentially reduces the CO2 emission compared to natural gas and diminishes the dependence of fossil fuels. However, the wide variety in the gas composition and its lower heating value will affect the subsequent combustion process with respect to emission levels and flame stability. In this study, premixed combustion of simulated GBG is investigated experimentally at atmospheric pressure and compared with pure CH4 (simulated natural gas). Combustion performance in terms of emission levels and blowoff is observed. The GBG fuel with noncombustible to combustible components ratio of 1.5 is tested in comparison with pure CH4 at fixed input thermal load. The GBG fuel consists of a mixture of CO/H2/CH4/CO2/N2and its proportion reassembles the mixture from air‐blown gasification. The high diluent content decreases the lower heating value (LHV) and increases the volumetric flow compared to CH4. As a result, lower combustion temperature and different flame region than CH4were found in the combustor. However, the GBG combustion still can be stabilized at lower temperature and leaner condition compared to CH4 while maintaining low CO and NOx emissions. As low as ~15 ppm and ~5ppm of CO and NOxemissions, respectively, could be achieved at an equivalence ratio equal to 0.5. It was found that at a combustion temperature below ~800oC, both CO and UHC start to rise from their stable and low concentration. At different input thermal loads, a shift in the optimum operating condition for the GBG combustion was found. No auto‐ignition or flashback events were found during the combustion of GBG in all experiment conditions tested. The results show the possibility to use both GBG and natural gas in one and the same combustor without compromising low emission levels.

Keyword
gasified biomass gas; premixed, atmospheric combustion; CO, UHC and NOx emissions
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-120572 (URN)
Note

QS 2013

Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2017-12-06Bibliographically approved
3. Influence of water vapour and tar compound on combustion of simulated gasified biomass
Open this publication in new window or tab >>Influence of water vapour and tar compound on combustion of simulated gasified biomass
(English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153Article in journal (Other academic) Submitted
Abstract [en]

Gasification is a thermo‐chemical process which converts biomass fuel into a gaseous mixture, gasified biomass, which can be used in various prime movers. For heat and power generation, using gasified biomass in a combustion device, for example, can give lower undesired emission compared to direct combustion of solid biomass. However, with regards to its variety in composition and lower heating value, the combustion behaviour of gasified biomass may differ from natural gas. The main objective of this study is to investigate the influence of water and tar compound on the combustion of simulated gasified biomass, which mainly contains CO, H2, CH4, CO2, N2. The combustion tests are conducted at atmospheric pressure in a premixed combustor. At a fixed input thermal load, CO and NOx emission levels, combustion temperature, and blowoff characteristics of gasified biomass are observed while varying the volume fraction of water (H2O) or benzene (C6H6) vapours in the fuel mixture. With low H2O level in the fuel mixture, the combustion temperature is almost constant, while NOx emission is decreased compared to dry gasified biomass. On the contrary, the combustion temperature decreases and NOx emission is almost constant at higher H2O content. A temperature limit was observed where CO emission could be maintained at low concentration. The blowoff limit was shifted to higher equivalence ratio. The blowoff temperature was first slightly decreased at lower H2O level and raised when H2O level is further increased. With the content of C6H6, the combustion temperature and NOx emission enhanced, while CO emission was reduced. The blowoff occurs at slightly higher equivalence ratio and temperature compared to gasified biomass without C6H6. The study shows that the presence of H2O and C6H6 in gasified biomass may give positive effects on the emission characteristics during combustion, but also that there are limits for these effects.

Keyword
Gasified biomass; water vapour; benzene vapour; CO, UHC, NOx emissions; blowoff
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-120573 (URN)
Note

QS 2013

Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2017-12-06Bibliographically approved
4. Correlation of laminar flame speed and lean blowoff limit with the fuel composition of gasified biomass
Open this publication in new window or tab >>Correlation of laminar flame speed and lean blowoff limit with the fuel composition of gasified biomass
(English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153Article in journal (Other academic) Submitted
Abstract [en]

The composition of the product gas produced from a biomass gasification process varies largely depending on several operational factors. The present study gathers the combustion information of different fuel mixtures that resemble the wide range of product gases from biomass gasification process. Two combustion parameters that are laminar flame speed, SL and lean blowoff limit, ERblowoff have been studied as functions of the content of H2 in the fuel mixture as well as the ratios of CO/H2, hydrocarbons/H2 and diluents/H2. From the plotted graphs, mathematical correlations between the parameter studied and the component of the gas mixture have been derived. The equations developed can be used to calculate the laminar flame speed and blowoff equivalent ratio for a wide range of gasified biomass. The graphs show that the H2 content and diluents/H2 ratio have the greatest influence on the laminar flame speed of the gas mixture and higher effect compared to the influence by the ratio of CO/H2 and hydrocarbons/H2. For the lean blowoff limit, the descending order of influence is the ratio of diluents/H2, H2 content and the ratio of CO/H2. While no importance on the lean blowoff limit is observed for the ratio of hydrocarbons/H2.

Keyword
gasified biomass, laminar flame speed, blowoff
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-120574 (URN)
Note

QS 2013

Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2017-12-06Bibliographically approved

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