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Thermodynamic aspects and heat transfer characteristics of HiTAC furnaces with regenerators
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Oxygen-diluted Combustion (OdC) technology has evolved from the concept of Excess Enthalpy Combustion and is characterized by reactants of low oxygen concentration and high temperature. Recent advances in this technology have demonstrated significant energy savings, high and uniform thermal field, low pollution, and the possibility for downsizing the equipment for a range of furnace applications. Moreover, the technology has shown promise for wider applications in various processes and power industries.

The objectives of this thesis are to analyze the thermodynamic aspects of this novel combustion technology and to quantify the enhancement in efficiency and heat transfer inside a furnace in order to explore the potentials for reduced thermodynamic irreversibility of a combustion process and reduced energy consumption in an industrial furnace. Therefore, theoretical and experimental investigations were carried out.

The 2nd law of thermodynamics analyses of OdC systems have been carried out for cases in which the oxidizer is either oxygen (Flameless-oxy-fuel) or air (High Temperature Air Combustion, HiTAC). The analyses demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-diluted combustion process that utilizes both gas- and/or heat-recirculation. Furthermore, the results showed that an oxygen-diluted combustion system that utilizes oxygen as an oxidizer, in place of air, results in higher 1st and 2nd law efficiencies.

Mathematical models for heat regenerators were developed to be designing tools for maximized heat recovery. These models were verified by heat performance experiments carried out on various heat regenerators.

Furthermore, experiments were performed in a semi-industrial test furnace. It was equipped with various regenerative burning systems to establish combustion and heat transfer conditions prevailing in an industrial furnace operating based on HiTAC. The tests were carried out at seven firing configurations, two conventional and five HiTAC configurations, for direct and indirect heating systems.

Measurements of energy balance were performed on the test furnace at various configurations in order to obtain the 1st law efficiency. Moreover, local measurements of temperature, gas composition, and heat fluxes in the semi-industrial test furnace were performed to find out the main characteristics of HiTAC flame and the effects of these characteristics on the heating potential, i.e., useful heating in the furnace. In the case of HiTAC, these measurements showed uniformities of chemistry, temperature, temperature fluctuation, and heat fluxes profiles. The values of fluctuations in temperature were small. The high speed jets of the fuel and air penetrated deep into the furnace. The fuel gradually disappeared while intermediate species gradually appeared in relatively high concentrations and at broader regions inside the furnace. These findings indicate: a large reaction zone, low specific combustion intensity in the flame, low specific fuel energy release, and high heat release from this large flame. In addition to the thermodynamic limitations to the maximum temperature of the Oxygen-diluted Combustion, the low specific energy release of the fuel and the high heat release from the flame to its surroundings cause this uniform and relatively moderate temperature profile in a HiTAC flame, consequently suppressing thermal-NO formation.

Heat flux and energy balance measurements showed that heating potential is significantly increased in the case of HiTAC compared to that in the conventional case, implying much more energy savings than the apparent heat recovery from the heat regenerators, and consequently much less pollutants emissions. Therefore, it is certain that this large HiTAC flame emits more thermal radiation to its surroundings than the conventional flame does, in spite of the moderate-uniform temperature profile of the flame. This intense heat flux was more uniform in all HiTAC configurations, including the indirect heating configuration, than that of the conventional-air combustion configuration.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , xiv, 68 p.
Keyword [en]
Combustion, flameless, oxy-fuel, heat-recirculation, gas-recirculation, heat transfer, furnace, radiant-tube, regenerative burner, honeycomb, fixed-bed
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-541ISBN: 91-7178-223-0 (print)OAI: oai:DiVA.org:kth-541DiVA: diva2:14359
Public defence
2005-12-15, F4/Sem and F3, Lindstedtsvägen 26, KTH Main Campus, 10:00
Opponent
Supervisors
Note
QC 20101011Available from: 2005-12-06 Created: 2005-12-06 Last updated: 2010-10-11Bibliographically approved
List of papers
1. Thermodynamic aspects of oxygen-deficient
Open this publication in new window or tab >>Thermodynamic aspects of oxygen-deficient
2005 (English)In: Archives of Thermodynamics, ISSN 1231-0956, Vol. 26, no 2, 29-44 p.Article in journal (Refereed) Published
Abstract [en]

The oxygen deficient combustion (ODC) is characterized by reactants of low oxygen concentration and high temperature. This work is devoted to analysis of such combustion process from the thermodynamic point of view. It demonstrated the possibilities for reducing thermodynamic irreversibility of combustion by considering the oxygen-deficient combustion process that utilizes both gas- and heat-recirculation. Furthermore, an ODC system utilizes oxygen as oxidizer has higher 1st and 2nd low efficiencies compared to an ODC system using air as oxidizer. This study is a technical guidance for further efficiency-improvement in combustion process especially because the temperature increase due to the reaction in an ODC system is mild.

Keyword
Combustion, Gas recirculation, Heat recirculation, Irreversibility
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-8878 (URN)
Note
QC 20101011Available from: 2005-12-06 Created: 2005-12-06 Last updated: 2010-10-11Bibliographically approved
2. Thermal performance analysis on a two composite material honeycomb heat regenerators used for HiTAC burners
Open this publication in new window or tab >>Thermal performance analysis on a two composite material honeycomb heat regenerators used for HiTAC burners
2005 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 25, no 17-18, 2966-2982 p.Article in journal (Refereed) Published
Abstract [en]

Honeycomb heat regenerators do not only reduce the fuel consumption in a high temperature air combustion (HiTAC) burning system but also provide the necessary high temperature of combustion air. A two-dimensional simulation model was developed to numerically determine the dynamic temperature and velocity profiles of gases and solid heat-storing materials in a composite material honeycomb regenerator. Consequently, the energy storage and the pressure drop are calculated and the thermal performance of honeycomb heat regenerator is evaluated at different switching times and loading. The model takes into account the thermal conductivity parallel and perpendicular to flow direction of solid and flowing gases. It considers the variation of all thermal properties of solid material and gases with temperature, Moreover, the radiation from combustion flue gases to the storage materials was considered in the analysis, The results are presented in a non-dimensional form in order to be a design tool as well, These analyses were applied on a regenerator made of two layers of ceramic materials, one is pure alumina and other is cordierite. This regenerator is contained in a. 100 kW twin-type regenerative-burning system used for HiTAC. The effectiveness and the energy recovery rate were 88% and 72% respectively at nominal operating range of the regenerator and the pressure drop across the twin regenerator system wits 1.16 kPa. The periodic steady state condition is reached after about 11 min and it takes only 2 min of operation until the temperature of combustion air remains above the self-ignition temperature that is required for HiTAC. Furthermore. these mathematical analyses show good agreement with experiments made on the same regenerator. In the experiments, the dynamic behavior of the heat regenerator operation was considered in order to compensate measurement readings for this effect.

Keyword
regenerator, heat exchanger, honeycomb, HiTAC, burner, mathematical simulation
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-8879 (URN)10.1016/j.applthermaleng.2005.03.004 (DOI)000231682900023 ()2-s2.0-23844490300 (Scopus ID)
Note
QC 20101011Available from: 2005-12-06 Created: 2005-12-06 Last updated: 2010-10-11Bibliographically approved
3. Physical properties of a LPG flame with high-temperature air on a regenerative burner
Open this publication in new window or tab >>Physical properties of a LPG flame with high-temperature air on a regenerative burner
2004 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 136, no 4, 567-569 p.Article in journal (Refereed) Published
Keyword
air temperature, article, combustion, cooling, exhaust gas, fire, furnace, high temperature, priority journal, technique, temperature dependence, thermal analysis
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-8880 (URN)10.1016/j.combustflame.2003.12.006 (DOI)000220338400013 ()2-s2.0-1342289625 (Scopus ID)
Note
QC 20100927Available from: 2005-12-06 Created: 2005-12-06 Last updated: 2012-03-22Bibliographically approved
4. Heat transfer characteristics of HiTAC heating furnace using regenerative burners
Open this publication in new window or tab >>Heat transfer characteristics of HiTAC heating furnace using regenerative burners
2006 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 26, no 16, 2027-2034 p.Article in journal (Refereed) Published
Abstract [en]

The aim is to experimentally study the various modes of heat transfer and to investigate the effect of the HiTAC flame characteristics on the heat transfer intensity and uniformity inside a setni-industrial test furnace using various industrial regenerative burners and various flame configurations namely; single-flame, twin-flame counter. twin-flame parallel and twin-flame stagger. Measurements of local instantaneous and average temperatures, heat fluxes and gas composition at several locations inside the furnace were carried out. It was observed that the HiTAC flame with highly reduced temperature fluctuations. turbulent intensity and combustion intensity have a larger reaction zone than a conventional flame. This large flame emits more thermal radiation in spite of its uniform and reduced temperature. Furthermore, the convective heat transfer was found to be uniform and as high as 30% of the total heat transfer to an object surface in the furnace. On the other hand, the very high reduction of NOx emission is a consequence of the low temperature and temperature fluctuation levels of the HiTAC flames. The above findings are valid to a similar extent in all burners and configurations but to less extent in the twin-flame counter configuration.

Keyword
heat transfer, combustion, furnace, regenerative burner
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-8881 (URN)10.1016/j.applthermaleng.2005.12.016 (DOI)000238798200031 ()2-s2.0-33744970679 (Scopus ID)
Note
Uppdaterad från "In press" till published: 20101011. QC 20101011Available from: 2005-12-06 Created: 2005-12-06 Last updated: 2010-10-11Bibliographically approved
5. Increase of the Effective Energy from the Radiant Tube Equipped with Regenerative System in Comparison with Conventional Recuperative System
Open this publication in new window or tab >>Increase of the Effective Energy from the Radiant Tube Equipped with Regenerative System in Comparison with Conventional Recuperative System
2005 (English)In: IFRF Combustion Journal, ISSN 1562-479X, no 03, 1-17 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents the experimental results of High Temperature Air Combustion (HiTAC)investigations with the use of a radiant tube, in order to compare the Regenerative System (RS)with a conventional recuperative system.For this work a semi-industrial HiTAC test furnace was equipped with the W-shape RadiantTube. The working length of this tube was around 7,0 m and the diameter was 0,195 m. Theradiant tube was operated in sequence with a conventional recuperative system and aRegenerative System. The recuperative burner was mounted in the upper end of the tube. TheRS consisted of two burners, equipped with honeycomb ceramic regenerators, mounted to bothends of the radiant tube. The temperature profile of the tube wall was monitored by 74thermocouples located along the tube. Additional temperatures, flow rates and pressures weremeasured to assess and compare the energy balance of both systems. Pollutant emissions,including NOx and CO, as well as the exhaust gas composition were measured. The tests werecarried out over a wide range of parameters: firing power from 75 kW to 155 kW, furnacetemperature from 670°C to 950°C and an oxygen molar fraction in the exhaust gases set at 3%.LPG was used as a fuel in all tests.Test results show that the temperature profiles along the tube were more uniform when theregenerative system was used. The cross-sectional temperature distribution for the tube wasalso more uniform. Because of the relatively flat temperature distribution along the tube, moreenergy from the radiant tube can be emitted using RS in comparison with the conventionalrecuperative burner, for the same maximum temperatures of the tube. In certain conditions, theincrease of energy release can be up to 100%.Energy balance calculations show that the efficiency of the Regenerative System can be up to25% higher than that of the recuperative system, mainly due to very low temperature of fluegases for RS operation. Although, the preheated air temperature used for combustion wasmuch higher in the case of the regenerative system (in some tests as high as 960°C), the NOxemission was found to be almost the same in both cases.

Keyword
High Temperature Air Combustion, Radiant Tube, Regenerative Burners
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-8882 (URN)
Note
QC 20101011Available from: 2005-12-06 Created: 2005-12-06 Last updated: 2010-10-11Bibliographically approved

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