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Volumetric combustion of torrefied biomass for large percentage biomass co-firing up to 100% fuel switch
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The co-firing of biomass and coal plays an important role in increasing the biomass power capacity and reducing greenhouse gas (GHG) emissions. The challenges of the large percentage biomass co-firing (over 20% on energy basis) in existing pulverized coal boilers are keeping the same steam parameters and having a high boiler efficiency and a stable operating.

The primary goal of this thesis is to develop a combustion concept for coal-fired boilers to enablea large percentage of biomass co-firing with up to a 100% fuel switch; these changes should increase the combustion efficiency, reduce CO2  and NOx emissions, improve the process efficiency, while maintaining the same steam parameters after switching fuels. To achieve these goals,  a  typical  biomass  pretreatment technology  called  torrefaction  has  been  employed to upgrade  the  biofuel  quality  in  terms  of  both  energy  density  and  chemical  properties. Consequently, a torrefaction based co-firing system has been proposed. In addition, a novel biomass combustion method called volumetric combustion has been designed; this process involves intense mixing and flue gas internal recirculation inside the combustion chamber, increasing the residence time of the biomass particles and making the temperature and gas species more uniform.

In this thesis, a series of studies based on experiments, CFD modelling, and process simulations have been performed. First, the raw material was palm kernel shells (PKS) that were torrefied over same residence time but at different temperatures in a laboratory-scale torrefaction reactor, producing three torrefied biomasses with different degrees of torrefaction. The devolatilization kinetics and char oxidation kinetics were determined based a series of high-temperature high-heating-rate tests in an isothermal plug flow reactor (IPFR), the obtained kinetic parameters were adopted for CFD modeling. Continually, the numerical investigations on the flame properties of the torrefied biomass and a 220 MWe coal-fired boiler performance were conducted, to understand the predicted results of the coal-fired boiler performance at varying biomass co-firing ratios. Afterward, analyses of the impacts of the degree of torrefaction and the biomass co-firing ratio on process operation, performance and electricity efficiency of a torrefaction based co-firing power plant were performed. Finally, the properties of the pollutants emitted from biomass volumetric combustions under various combustion modes and co-firing ratios were studied using Aspen Plus.

According to the results, the following conclusions can be reached: 1) a high heating rate enhances the yields of the volatiles for biomass devolatilization processes with the same final temperature; 2) the enhanced drag force on the biomass particles causes a late release of volatile matter and delays the ignition of the fuel-air mixture. Furthermore, oxidizers with lower oxygen concentrations normally generate larger flame volumes, lower peak flame temperatures and lower NO emission; 3) the co-firing simulation reveals that a boiler load reduction of less than 10% is observed when firing 100% torrefied biomass; 4) deep torrefaction is not recommended because the energy saved during biomass grinding is lower than that consumed by the additional torrefaction process; the electrical efficiency of power plant is reduced when increasing either the degree of torrefaction or the biomass substitution ratio; 5) the amount of flue gas that needs to be recycled for NOx reduction decreased when the percentage of co-fired biomass increased.

Overall, from the perspective of combustion, both the torrefaction process and volumetric combustion are promising steps toward realizing large percentage biomass co-firing in coal-fired boilers with high efficiency and reduced emissions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xii, 68 p.
Keyword [en]
Biomass, co-firing, torrefaction, torrefaction degree, kinetics, volumetric combustion, fuel switch
National Category
Chemical Sciences Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-140705ISBN: 978-91-7501-988-8 (print)OAI: oai:DiVA.org:kth-140705DiVA: diva2:692311
Public defence
2014-02-17, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140130

Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2014-01-31Bibliographically approved
List of papers
1. High-temperature rapid devolatilization of biomasses with varying degrees of torrefaction
Open this publication in new window or tab >>High-temperature rapid devolatilization of biomasses with varying degrees of torrefaction
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2014 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 122, 261-269 p.Article in journal (Refereed) Published
Abstract [en]

Torrefied biomass is a coal-like fuel that can be burned in biomass boilers or co-fired with coal in co-firing furnaces. To make quantitative predictions regarding combustion behavior, devolatilization should be accurately described. In this work, the devolatilization of three torrefied biomasses and their parent material were tested in an isothermal plug flow reactor, which is able to rapidly heat the biomass particles to a maximum temperature of 1400 degrees C at a rate of 10(4) degrees C/s, similar to the conditions in actual power plant furnaces. During every devolatilization test, the devolatilized biomass particles were collected and analyzed to determine the weight loss based on the ash tracer method. According to the experimental results, it can be concluded that biomass decreases its reactivity after torrefaction, and the deeper of torrefaction conducted, the lower the biomass reactivity. Furthermore, based on a two-competing-step model, the kinetic parameters were determined by minimizing the difference between the modeled and experimental results based on the least-squares objective function, and the predicted weight losses exhibited a good agreement with experimental data from biomass devolatilization, especially at high temperatures. It was also detected that CO and H-2 are the primary components of the released volatile matters from the devolatilization of the three torrefied biomasses, in which CO accounts for approximately 45-60%, and H-2 accounts for 20-30% of the total volatile species.

Keyword
Torrefaction, Pulverized solid biomass, CFD, Flame, High temperature
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-140690 (URN)10.1016/j.fuel.2014.01.012 (DOI)000331544100030 ()2-s2.0-84893546247 (Scopus ID)
Note

QC 20140305

Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-12-06Bibliographically approved
2. Volumetric combustion of biomass for CO2 and NOx reduction in coal-fired boilers
Open this publication in new window or tab >>Volumetric combustion of biomass for CO2 and NOx reduction in coal-fired boilers
2012 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 102, 624-633 p.Article in journal (Refereed) Published
Abstract [en]

To meet the urgent environmental targets, substituting coal with biomass has been considered to be an effective and promising method over the last decades. In this paper, a new concept of volumetric combustion is proposed and further developed to achieve 100% fuel switching to biomass in large scale coal-fired boilers. Volumetric combustion not only changes the in-furnace flow but also affects the combustion reactions by the intensive mixing and internal recirculation of the flue gases. Firstly, the volumetric combustion properties of the wood pellets were investigated experimentally. An Aspen model was then used to thermodynamically describe and study the volumetric combustion with three different types of fuel, and the emission properties of CO2 and NOx were compared. Finally, two applications of volumetric combustion were discussed. It is concluded that the wood pellets ignited and combusted much faster than the coal pellets and had a larger combustion volume when combusted under lower oxygen concentration conditions, and the ignition time was almost independent of the oxygen concentration when the oxidizer was preheated to 1000 degrees C. In addition, the NOx emissions decreased as the recirculation ratio of the flue gas increased, and as the percentage of biomass used in co-firing increased, the amount of flue gas that needs to be recycled for reduction of NOx decreased. Thus, the volumetric combustion is beneficial as it reduces the operation cost of NOx reduction. The volumetric combustion would be an attractive technology for co-firing a large proportion of biomass in coal-fired boilers with high boiler efficiency and effective emissions reduction.

Keyword
Volumetric combustion, Biomass, Co-firing, NOx, CO2
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-104108 (URN)10.1016/j.fuel.2012.06.083 (DOI)000308804500075 ()2-s2.0-84866614182 (Scopus ID)
Note

QC 20121029

Available from: 2012-10-29 Created: 2012-10-29 Last updated: 2017-12-07Bibliographically approved
3. Flame characteristics of pulverized torrefied-biomass combusted with high-temperature air
Open this publication in new window or tab >>Flame characteristics of pulverized torrefied-biomass combusted with high-temperature air
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2013 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 160, no 11, 2585-2594 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the flame characteristics of torrefied biomass were studied numerically under high-temperature air conditions to further understand the combustion performances of biomass. Three torrefied biomasses were prepared with different torrefaction degrees after by releasing 10%, 20%, and 30% of volatile matter on a dry basis and characterized in laboratory with standard and high heating rate analyses. The effects of the torrefaction degree, oxygen concentration, transport air velocity, and particle size on the flame position, flame shape, and peak temperature are discussed based on both direct measurements in a laboratory-scale furnace and CFD simulations. The results primarily showed that the enhanced drag force on the biomass particles caused a late release of volatile matter and resulted in a delay in the ignition of the fuel-air mixture, and the maximum flame diameter was mainly affected by the volatile content of the biomass materials. Furthermore, oxidizers with lower oxygen concentrations always resulted in a larger flame volume, a lower peak flame temperature and a lower NO emission. Finally, a longer flame was found when the transport air velocity was lower, and the flame front gradually moved to the furnace exit as the particle size increased. The results could be used as references for designing a new biomass combustion chamber or switching an existing coal-fired boiler to the combustion of biomass.

Keyword
Torrefaction, Pulverized solid biomass, CFD, Flame, High temperature
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-129606 (URN)10.1016/j.combustflame.2013.05.010 (DOI)000324006400026 ()2-s2.0-84882864740 (Scopus ID)
Note

QC 20131004

Available from: 2013-10-04 Created: 2013-10-03 Last updated: 2017-12-06Bibliographically approved
4. CFD Approach for Unburned Carbon Reduction in Pulverized Coal Boilers
Open this publication in new window or tab >>CFD Approach for Unburned Carbon Reduction in Pulverized Coal Boilers
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2012 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 26, no 2, 926-937 p.Article in journal (Refereed) Published
Abstract [en]

Low-NO, technologies are widely used in pulverized coal boilers, but they usually produce high levels of carbon in the fly ash. High levels of unburned carbon represent fuel loss, so the overall boiler efficiency is reduced. Additionally, the higher carbon content affects the suitability of fly ash for cement applications. The purpose of this paper is to provide a CFD approach for unburned carbon reduction by optimizing operating conditions. In this paper, three different boiler loads were simulated: 200 MW, 170 MW, and 140 MW. The air supply System was simulated previously for preparing as precise as possible boundary conditions. At last, the unburned carbon level of every burner was investigated, and the effects of residue residence time and the local fuel air momentum ratio are discussed in detail. According to the predicted results, operating conditions and the residence time of the coal particles affects the unburned carbon level in fly ash. Operating conditions play a more significant role during the combustion process, while the residence time affects char burnout only when the burner's location is low. Therefore, it is concluded that a cost-effective method could be developed for reducing the unburned carbon level in ash and correspondingly, the loss on ignition level. First, it is necessary to determine which burners are operating under poor conditions through CFD analysis. Then, the fuel air momentum ratios of those burners should be modified by changing the operating conditions, meanwhile increasing the residence time of coal particles to ensure complete combustion.

National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-92356 (URN)10.1021/ef201700a (DOI)000300275100017 ()2-s2.0-84863156208 (Scopus ID)
Note
QC 20120417Available from: 2012-04-17 Created: 2012-04-02 Last updated: 2017-12-07Bibliographically approved
5. Co-firing based on biomass torrefaction in a pulverized coal boiler with aim of 100% fuel switching
Open this publication in new window or tab >>Co-firing based on biomass torrefaction in a pulverized coal boiler with aim of 100% fuel switching
2012 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 99, 344-354 p.Article in journal (Refereed) Published
Abstract [en]

Torrefied biomass has several benefits, such as higher energy density, good grindability, higher flowability and uniformity. The process of torrefaction moves the chemical and physical properties of raw biomass close to that of bituminous coal, which allows co-utilization with high substitution ratios of biomass in the existing coal-fired boilers without major modifications. In this study, a torrefaction based co-firing system was proposed and studied. Devolatilization and char oxidize kinetics of the torrefied biomass have been investigated experimentally. CFD modeling of co-firing with varying substitutions of torrefied biomass in a pulverized coal boiler have been carried out. To figure out the boiler performance when co-firing torrefied biomass, five different cases were involved and simulated, coal only, 25% biomass, 50% biomass, 75% biomass, and 100% biomass on thermal basis, respectively. The results showed torrefaction is able to provide a technical option for high substitution ratios of biomass in the co-firing system. The case-study pulverized coal boiler could be fired 100% torrefied biomass without obvious decreasing of the boiler efficiency and fluctuation of boiler load. More positively, the net CO 2 and the NO x emissions significantly reduced with increasing of biomass substitutions in the co-firing system.

Keyword
Biomass, CFD, Co-firing, Pulverized coal boiler, Torrefaction
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-101326 (URN)10.1016/j.apenergy.2012.05.046 (DOI)000308049400037 ()2-s2.0-84864806915 (Scopus ID)
Note

QC 20120829

Available from: 2012-08-29 Created: 2012-08-27 Last updated: 2017-12-07Bibliographically approved
6. Process simulation of co-firing torrefied biomass in a 220 MWe coal-fired power plant
Open this publication in new window or tab >>Process simulation of co-firing torrefied biomass in a 220 MWe coal-fired power plant
Show others...
2014 (English)In: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227, Vol. 84, 503-511 p.Article in journal (Refereed) Published
Abstract [en]

Torrefaction based co-firing in a pulverized coal boiler has been proposed for large percentage of biomass co-firing. A 220 MWe pulverized coal-power plant is simulated using Aspen Plus for full understanding the impacts of an additional torrefaction unit on the efficiency of the whole power plant, the studied process includes biomass drying, biomass torrefaction, mill systems, biomass/coal devolatilization and combustion, heat exchanges and power generation. Palm kernel shells (PKS) were torrefied at same residence time but 4 different temperatures, to prepare 4 torrefied biomasses with different degrees of torrefaction. During biomass torrefaction processes, the mass loss properties and released gaseous components have been studied. In addition, process simulations at varying torrefaction degrees and biomass co-firing ratios have been carried out to understand the properties of CO2 emission and electricity efficiency in the studied torrefaction based co-firing power plant. According to the experimental results, the mole fractions of CO2 and CO account for 69-91% and 4-27% in torrefied gases. The predicted results also showed that the electrical efficiency reduced when increasing either torrefaction temperature or substitution ratio of biomass. A deep torrefaction may not be recommended, because the power saved from biomass grinding is less than the heat consumed by the extra torrefaction process, depending on the heat sources.

Keyword
Torrefaction, Biomass, Co-firing, CO2 reduction
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-140691 (URN)10.1016/j.enconman.2014.04.075 (DOI)000338601100051 ()2-s2.0-84901016938 (Scopus ID)
Note

QC 20140812. Updated from manuscript to article in journal.

Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-12-06Bibliographically approved
7. Effects of Flue Gas Internal Recirculation on NOx and SOx Emissions in a Co-Firing Boiler
Open this publication in new window or tab >>Effects of Flue Gas Internal Recirculation on NOx and SOx Emissions in a Co-Firing Boiler
2013 (English)In: International Journal of Clean Coal and Energy, ISSN 2168-1538, Vol. 2, no 2, 13-21 p.Article in journal (Refereed) Published
Abstract [en]

Volumetric combustion has been developed to realize a high substitution ratio of biomass in co-firing boilers, which features an intensive flue gas internal recirculation inside furnace. However, the characteristics of NOx and SOx emissions in large-scale boilers with volumetric combustion were not fully clear. In this paper, an Aspen Plus model of volumetric combustion system was built up based on a co-firing boiler. In order to characterize the reductions of NOx and SOx, three biomass substitution ratios were involved, namely, 100% biomass, 45% biomass with 55% coal, and 100% coal. The effects of flue gas recirculation ratio, air preheating temperature, oxygen concentration, and fuel types on pollutants emission in the volumetric combustion system were investigated. According to the results, it was concluded the higher substitution ratio of biomass in a co-firing boiler, the lower emissions of NOx and SOx. Moreover, flue gas internal recirculation is an effective pathway for NOx reduction and an increased recirculation ratio resulted in a significant decreasing of NOx emission; however, the SOx increased slightly. The influences of air preheating temperature and O2 concentration on NOx emission were getting weak with increasing of recirculation ratio. When 10% or even higher of flue gas was recycled, it was observed that almost no NOx formed thermodynamically under all studied conditions. Finally, to reach a low emission level of NOx, less energy would be consumed during biomass combustion than coal combustion process for internal recirculation of flue gas.

Keyword
Flue Gas Internal Recirculation, Co-Firing, NOx; SOx
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-140693 (URN)10.4236/ijcce.2013.22002 (DOI)
Note

QC 20140130

Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-03-30Bibliographically approved

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Citation style
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  • Other style
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Output format
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