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Thermally homogenous gasification of biomass/coal/waste for medium or high calorific value syngas production
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

 Today’s problems with emissions of green house gases, land filling of waste and depletion of the oil reserves calls for new energy systems based on alternative fuels like biomass and waste. Gasification is an attractive technology for the use of such solid fuels. Conventional gasification, in the vast majority of cases, uses in-reactor heat release from combustion of part of the feedstock, possibly coupled with a limited preheating of the agent, to obtain the necessary temperatures in the gasifier bed. During recent years, a new gasification technology, using highly preheated gasification agents (> 1273 K), has been developed. The extra heat brought into the process by the high temperature agent reduces the amount of feedstock that has to be oxidized to supply the necessary heat and the use of highly preheated agents has previously proven to have several positive effects on the fuel gas quality.In difference to the previous work on gasification with highly preheated agents, this thesis primarily focuses on the fundamental aspects namely, mass conversion, heating and ignition. It starts by considering single fuel particles or thin beds of fuel particles inserted into highly preheated agents. Mass conversion, heating and ignition are reported in function of the temperature and oxygen concentration of the agent and formulas for the prediction of ignition time and ignition mechanism are developed. The perspective is then widened to include the whole gasifier bed. Simulations of fixed bed batch gasification using highly preheated agents are performed with a mathematical model and used to study how the high agent temperature influences the mass conversion, devolatilisation front rate and the temperature distribution in the fixed fuel bed. Further, the gas quality and gasification efficiency are studied by means of large scale experiment. Ultimately, a thermodynamic analysis of the whole autothermal gasification system, including both a regenerative preheating system and the gasifier, is made.The particle study reports results from experiments with wood and coal and agents consisting of mixtures of nitrogen and oxygen in various proportions. It is shown that an increase in agent temperature from 873 K to 1273 K make the conversion process faster, mostly due to an early onset of the devolatilisation (fast drying) but also due to an increased devolatilisation rate (at least in the case of wood). The time to ignition also decreases significantly, particularly so between 873 and 1073 K. Further, it is shown that the higher the agent temperature, the more pronounced was also the tendency of the coal particles to heat significantly faster in oxygen diluted conditions (5,10 and 21% oxygen) than in inert (0% oxygen) or oxygen rich conditions (30, 50, 80 and 100% oxygen). An increase in agent temperature is also shown to reduce the dependency of the process on the oxygen concentration, at least in diluted conditions (5-21% oxygen). The results also indicate that for coal an increase in the oxygen concentration, specifically in the region above the atmospheric concentration, leads to a decreased dependency on the agent temperature. It is finally shown in the experiments with agent temperatures of 1073 and 1273 K that a flame is promptly formed even in very low concentrations of oxygen.The gasifier study reports results from simulation of batch air gasification and experiments in both batch and continuous up-draft fixed bed gasifier with wood and waste derived fuel and air and mixtures of air and steam. It is shown that the conversion process is faster the higher the air temperature. In particular somewhere between air temperatures of 623 K and 803 K the process behaviour changes. In fact, the devolatilisation rate is significantly increased in this region while it increases less sharply with air temperature below and above this temperature window. The temperature distribution in the bed shows less sharp gradients at high temperature (> 803 K) than at low temperatures (< 623 K). It is also showed experimentally and in fairly large scale that the use of highly preheated air for the gasification of biomass and waste derived fuels can produce - in continuous mode – relatively high yields of product syngas with relatively high fractions of combustible gases and probably also low content of tar. The efficiency of the gasification under these conditions, even when the extra heat input in the preheated agent is considered in the computation of the gasification efficiency, is shown to be comparable to that of conventional gasification techniques. The results also shows that with the use of steam in the agent, the content of hydrogen can be further increased with respect to gasification with only preheated air.In base of the results of the particle study and the gasifier study it is shown that a there exists two regimes of operation in function of the agent temperature, separated by the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature. The value of this temperature depend on material properties and the kinetics of the reaction, thus also on the oxygen concentration. When agent temperatures below the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature are used, the drying and devolatilisation are mainly controlled by the heat released by reactions. The heating of the fuel particles and their devolatilisation are relatively slow and the devolatilisation rate is highly oxygen dependent. In a fixed bed, the devolatilisation front rate is low and the bed is characterised by significant temperature gradients.When the agent temperature is higher than the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature, the drying and devolatilisation are mainly controlled by the convective heat transfer from the preheated agent and the released volatiles ignite very fast even in diluted conditions. This results in very efficient heat transfer to the fuel particles. In the fixed fuel bed the process is characterized by a high devolatilisation front rate. Thus, the temperature gradients in the bed are significantly reduced and the gasification can be said to be thermally homogeneous. Thanks to high rates of heat transfer and mass conversion, the heating value of the dry produced syngas is high with high concentrations of combustible species. The ignition of the volatiles and the high temperatures all along the bed presumably contributes to the reduction of the tar content even in up-draft configurations. The high temperatures also allows for operation with reduced air – to – fuel ratios which further increased the value of the produced gas (thanks to less dilution by nitrogen).The system study presents a concept for an autothermal system including both preheating and gasification. Results from a thermodynamic analysis of such a system are reported. Autothermal operation of a thermally homogeneous gasifier is possible only in a twin component system in which the gasifier is coupled to a preheating system able to reach preheating temperatures well above the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature. It is shown that to reach certain temperature levels of the gasification air, heat exchange between product gas and air is not enough and the preheating system has to improve the temperatures involved, for example by burning part of the produced gas in a regenerative preheater. Further, it is shown that in comparison to gasifier without such a system for additional preheating, the autothermal Thermally Homogeneous Gasification system has the ability to significantly improve the gas quality (in terms of heating value of the dry gas) without losing energy- or exergy efficiency to an appreciable extent.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , v, 60 p.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4902ISBN: 978-91-7415-105-3 (print)OAI: oai:DiVA.org:kth-4902DiVA: diva2:2104
Public defence
2008-09-29, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 14:00 (English)
Opponent
Supervisors
Note
QC 20100903Available from: 2008-09-19 Created: 2008-09-18 Last updated: 2010-09-03Bibliographically approved
List of papers
1. Ignition of single coal particles in high-temperature oxidizers with various oxygen concentrations
Open this publication in new window or tab >>Ignition of single coal particles in high-temperature oxidizers with various oxygen concentrations
Show others...
2008 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 87, no 6, 974-987 p.Article in journal (Refereed) Published
Abstract [en]

In this investigation, coal pellets were combusted using a high temperature oxidizer with varying oxygen concentration, using a small scale batch reactor able to preheat the oxidizer to 1273 K. In base of the experimental results, the influence of oxygen concentration on the ignition mechanism, the solid temperature inside the particle at the moment of ignition, the mass lost at the moment of the ignition and ignition time is analyzed and discussed. A theoretical basis for the division of the conditions tested into three ignition regimes is developed and a formula for the prediction of the ignition time directly from the material and oxidizer temperature and oxygen concentration is proposed.

Keyword
flameless oxidation, high-temperature air combustion, coal, ignition, pulverized coal, mild combustion, air, gasification, mechanisms, behavior
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-17368 (URN)10.1016/j.fuel.2007.06.027 (DOI)000253894800041 ()2-s2.0-38849104321 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Development of a thermally homogeneous gasifier system using high-temperature agents
Open this publication in new window or tab >>Development of a thermally homogeneous gasifier system using high-temperature agents
2006 (English)In: Clean Air, ISSN 1561-4417, Vol. 7, no 4, 363-379 p.Article in journal (Refereed) Published
Abstract [en]

An advanced twin component gasification system, named Thermally Homogenous Gasification (THG), is developed. Development, testing and numerical simulations of the THG have shown that increased temperature of the gasification agent, results in a higher gasification rate, higher ignition front rate, higher molar fraction of combustible species in the product gas (CO, H2 and CmHn), and consequently a higher LHV. Moreover, there exists a critical gasification agent temperature above which preheating is no longer efficient if the purpose is to maximise the yield of gaseous products.

Keyword
Biomass, Computer simulation, Ignition, Mathematical models, Biomass fuels, Fixed beds, High-temperature air, Pebble bed gasifier, Gasification, Biomass, Computer simulation, Gasification, Ignition, Mathematical models, biofuel, biomass, homogeneity, numerical model, temperature
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24360 (URN)10.1615/InterJEnerCleanEnv.v7.i4.40 (DOI)2-s2.0-34147223561 (Scopus ID)
Note
QC 20100903Available from: 2010-09-03 Created: 2010-09-03 Last updated: 2017-12-12Bibliographically approved
3. Performance analysis of a fixed-bed biomass gasifier using high-temperature air
Open this publication in new window or tab >>Performance analysis of a fixed-bed biomass gasifier using high-temperature air
2006 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 87, no 3, 235-245 p.Article in journal (Refereed) Published
Abstract [en]

Gasification of biomass using high-temperature agents (air/steam) has been proven to have many features superior to those of conventional gasification using low-temperature agents. In this work, an experimental fixed-bed gasifier is utilized to investigate the gasification of biomass using high-temperature air up to 1473 K. A mathematical model has been formulated for the prediction of the main chemical and physical processes and is used to study the influence of temperature, oxygen concentration and flow rate of the feed gas. Predicted gas species concentration profiles and their maximum values are in reasonable agreements with the measurements. The results show that: (a) When the temperature of feed gas is increased, a higher gasification rate, a higher ignition front rate, and higher molar fractions of fuel gases (CO, H-2 and CmHn), thus a higher LHV are obtained. (b) Increased oxygen concentration leads to higher peak values of the fuel gas concentrations, a higher gasification rate, and a larger ignition front rate.

Keyword
mathematical modeling, gasification, biomass fuel, fixed bed, high-temperature air, municipal solid-wastes, packed-beds, gasification, incineration, combustion, model, coal
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-15394 (URN)10.1016/j.fuproc.2005.08.004 (DOI)000234937700008 ()2-s2.0-30344434068 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. Effect of operating conditions on tar and gas composition in high temperature air/steam gasification (HTAG) of plastic containing waste
Open this publication in new window or tab >>Effect of operating conditions on tar and gas composition in high temperature air/steam gasification (HTAG) of plastic containing waste
2006 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 87, no 3, 223-233 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the high temperature air/steam gasification (HTAG) technique has been tested for a fuel in pellet form made from waste material of woody and plastic origin. The feedstock was gasified in an updraft fixed bed reactor by mixtures of air and steam (102 Nm(3)/h, 4% to 82% steam) preheated to 1400 degrees C, a temperature well above the fluid temperature of the feedstock. The produced gas was analyzed with respect to composition, including a detailed characterization of the tar. Lower heating values up to 9.5 MJ/Nm(3) and gas yields as high as 3.4 Nm(3)/kg were reported, indicating the process to be highly efficient for waste-to-energy applications. The composition of the tars, suggested extensive cracking as a result of the high temperatures of the outgoing gas.

Keyword
gasification, tar, waste, biomass
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24361 (URN)10.1016/j.fuproc.2005.08.002 (DOI)000234937700007 ()2-s2.0-30344447717 (Scopus ID)
Note
QC 20100903Available from: 2010-09-03 Created: 2010-09-03 Last updated: 2017-12-12Bibliographically approved
5. A thermodynamic analysis of high temperature agent gasification (HTAG) using biomass and air
Open this publication in new window or tab >>A thermodynamic analysis of high temperature agent gasification (HTAG) using biomass and air
(English)In: Clean Air, ISSN 1561-4417Article in journal (Other academic) Submitted
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24362 (URN)
Note
QC 20100903. Ingår även i Proceedings of the 9th Conference on Energy for a Clean Environment, July 2007, Povoa de Varzim, Portugal.Available from: 2010-09-03 Created: 2010-09-03 Last updated: 2017-12-12Bibliographically approved

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