Tracking a Thermal Phenomen

NEW INSIGHTS IN THE BURNING AND COMBUSTION OF HEAVY –NON-VOLATILE ORGANIC LIQUIDS

 

Combustion processes are usually depicted by combinations of physical models of heating, melting, boiling, charring and combusting. Each of which can be described by invoking physical and chemical rules of the thumb.

Analyzing atomization at solid/liquid/vapor interfaces at carbonized substrates involves a complex of disciplines: application of the phase rule and electrochemical considerations to interfaces at the interior of the carbonized substrate and to those at its exposed exterior.

The description of atomization of “non-volatile liquids” at “3-phase” interfaces requires a step by step colloid chemical approach. Thus far no literature has been found on this subject matter.

 

When approaching the asphalt puzzle, I got help of prof. H.B. Levinsky [Department of Chemical Technology, University of Groningen]. He prompted one of his students to do a preliminary study on the combustibility of asphalt and suggested to start experiments by studying the asphalt candle. L. Faber covered quite a lot features of asphalt [www.rug.nl/wewi/_shared/publicaties/chemie/c102.pdf], but reported that asphalt candles do not support combustion.

A review [ www.nvon.nl/nvox/nvox2001/kaars.doc ] by H. De Gruijter and A. Van Rossum introduced me to thermophysical parameters of candle flames. Their data proved to be very constructive. I was staggered! The aerosol originating from the dark zone of the candle flame does have the same chemical composition as the wax of the candle. Popular websites on the pecularities of candle flames refer to this aerosol as being "vaporous", neglecting the fact that the vapour pressure of heavy organics is indeed very small.

The data of De Gruijter and Van Rossum on the oxygen level inside the dark zone provided my breakthrough! Some deducting of the thermal and electrochemical conditions showed that the oxygen activity at the very surface of the wick could indeed only be very low.

          These findings have been summarized in Slide 3.

 

I started to scrutinize the features of the combustyin process by taking place at candles.

By putting the domains of the candle into categories - making use of the phase rule I started to scrutinize the features of the combustion process taking place at candles.-and estimating the heat balance of these domains their jigsaw pieces fell into place.

The wick and flame can be differentiated into eight domains and contains 2 separate combustion zones: the lower blue envelope and the combination of the upper blue envelope and the yellow combustion zone, see Slide 4

A summary of these observations are presented in this slide. The features of a model of the atomization process taking place at the surface of the wick of a candle should meet and explain each of these observations!

 

The observations presented in Slide 5 indicate the activity of colloidchemical and electrochemical processes. From the very start I assumed that the surface of the wick did become charged electrically by gaseous electrochemical active species and also that this charging energized the atomization process. But the driving force that directs the aerosol into the dark zone of the flame did remain unclear for some time.

The notion that the "burner" contains two separate combustion zones, see Slide 4, that - could - operate independently, provided the step forward. "In vitro" experiments showed that a submerged wick started to atomize when heated to a temperature inside the temperature range of the wick.

Pyrolysis at the liquid/solid interface of the interior of the wick! At temperatures still well below the range where the liquid phase starts to pyrolyze homogeneously.

Heterogeneous pyrolytic processes became highlighted! This elucidation provided the final stepping stone to the model of the atomization process. I could develop a finite concept of "pyrolytic carburation".

My concept of pyrolytic carburation is based on manifest elements. 

  1. The notion that pyrolysis of organic liquid generates gas, radicals and electrochemical active species is an adage of organic chemistry.
  2. Inside the dark zone the aerosol appears at the surface of the wick; this is an empirical observation.
  3. When exposed to electrochemical active species, carbonized substrates - being composed of carbon particles -, become charged by oxidation-reduction processes; this is an electrochemical axiom.
  4. The liquid phase becomes pressured by gas - generated by pyrolysis throughout the carbonized substrate; a logical deduction.
  5. The inner hydrodynamic pressure squeezes liquid to the surface where protuberances are sheared off by electrical forces; a common colloidchemical phenomenon.
  6. Their hydrodynamic momentum drives droplets into the gaseous thermal lift; a logical deduction.

The combination of these elements into the model of "pyrolytic carburation" yields views in many directions!

It took some time to discern the parameters that control the kinetics of pyrolytic carburation. Early experiments with nightlight burners fueled by paraffin and vegetable oils (sunflower up to cod liver oil) showed that already 1 wt % of asphalt could douse the candle flame. Curiously it showed also that such burners when heated to 200-250 C inclined to douse. A few switched to a regime supporting a colourless microflame.

To tackle my colloidchemical assumptions I measured at this temperature range the electric resistivity of these mixtures and of "pure" asphalt. The polarization of the wick inside the dark zone of the flame turned out to be evidently negative (when measured between its base and top).

These results prompted me to study the parameters of the model and to develop specific experimental methodology.

Unexpected effects showed, e.g. the carbonization of cotton - when operated as a wick or when submerged in some heated paraffin bath - carbonized cotton seems to be imprinted by the specific molecular structure of the liquid with which it has been pyrolyzed. Lateron this imprint shows when atomizing different fuels.

 Similar ways to imprint wetting phenomena and capillary forces  have become hot items to nanotechnology [ www.mf.mpg.de/en/abteilungen/dietrich/research

To become familiar with the pecularities of pyrolysis a set of rather "heavy organics" was collected, viz. C14-C20 alkanes and alkenes, inclusive some alkane carboxilic acids and alcohols. According to the model a major point of interest should be the electrochemical activity of the pyrolytic gas [H2, H2O, CO2, radicals]. The collection was complemented with some petrol additives and organic redox couples.

My equipment allowed digital recording of weight, voltage and dc resistivity. Maxi nightlight cups were used, equiped with cotton and glassfiber wicks, to burn in an atmosphere with controlled oxygen level and heat conductivity. Fuel consumption was related to the oxygen level and the heat conductivity of the gaseous ambient.

The study of pyrolysis in test tubes quickly got routines allowing visual observation of "pyrolytic" gas developed by submerged carbonized substrates. The electric resistivity was determined by using the nickel sheats of parallel oriented thermocouples as electrodes. Inserting a small test tube into the larger one and connecting adjacent liquids by glassfiber cord the classical "half-cell" construction came within reach.

Organic collection:

C14 - C16 - C18 - C20 n-alkane, 1-n-alkene, 1-n-alkane carboxilic acid/alcohol

Ferroce, MMT, benzoquinone, tetrabromoquinone, tetramethylquinone

Equipment:

Toploading balance (Snug 150/0.005g);  recording digitally weight (Excel graphs)

Electrometer (Keithley 614); recording digitally voltage and resistivity

Maxi nightlight cup, equiped with standardized cotton/glassfiber wick

Pyrex test tube, equiped with 2 nickel sheated thermocouples & temperature monitor

Air thermostat (Bosch GHG 660 LCD)

Cotton/glassfiber wick, equiped with Pt/Pt-Ir thermocouple/electrode (Ø50micron)

Glass bell, equiped with gas distribution (Air/oxygen/nitrogen/helium/carbondioxide)

Oxygen monitor (Greisinger GOX 100), recording digitally

This slide presents various sorts of data:

# At a submerged wick C18 alkane & alkene atomize at a temperature slightly lower than the temperature of the wick in the dark zone of a NightLight. (red numbers)

# At the "atomizing" temperature their resistivity of C18 alkane & alkene is higher than that of paraffin. (blue numbers)

#  Ranking fuel consumption: C18 dekane > paraffin > C18 dekene (black numbers)

# Asphalt lowers the resistivity of alkane, alkenes (and paraffin)

# Already 1 wt% of asphalt tends to douse the combustion of C18 alkane & alkene

# The resistivity of asphalt at the temperature of the wick is very low (blue number)

The equipment available allows the determination of relations between molecular structure of organics, their pyrolytic carburation and various kinds of electrical and thermal properties. The data obtained by making use of carefully dimensioned wicks and standardized Maxi nightlights are reproducible to a satisfactory degree

By comparing the data of resistivity and fuel consumption of alkanes and alkenes of equal molecular length a rather general picture emerged. Unsaturateness caused a lowering of resistivity and rate of pyrolytic carburation; the more so in the presence of asphalt.

Pyrolytic carburation may be visualized as being an engine made up from the six processes mentioned in Slide 7. Each process controls in some manner the overall rate of atomization. This engine dissipates energy. It can be shown that a large part of this energy has an electrochemical origin. The electrical part of the engine can be presented by a so called "3-D equivalent electrical network", consisting of elemental loops tailored to the volume elements of the carbonized substrate and its gaseous ambient.

 

For this discussion it will be sufficient to mention that it can be shown that in each of those elemental loops the polarization of solid/gas interface and the resistivity of the liquid controls the electric turnover of the loop to the overall rate of carburation. To some degree these turnovers are proportional to the magnitude of the solid/gas polarization and inversely proportional to the resistivity of the liquid.

The model of pyrolytic carburation predicts the reality of ways to manipulate the rate of atomization taking place during the combustion of droplets containing some solid phase, viz. soot. By facilitating atomization the amount of liquid will diminish more rapidly, leaving the solid phase less time to grow. According to this principle disseminating the liquid with nanoparticles will promote atomization and reduce the rate of growth of soot particles.

In the same manner the model predicts that a lowering of the resistivity of the liquid will lower the rate of atomization. To retard in this manner combustion new leads will develop to the class of flame retardant materials.

Analogous to the methods of nanotechnology, wetting phenomena and capillary forces of carbonized substrates could become controllable by imprinting their interfaces.

To summarize:

 

The atomizing of combustible liquids by carbonized substrate is a complex colloidal phenomenon. It is also a very common natural one that occurs everywhere, any moment of the day. This privilege seems to have been hidden from scientific scrutiny.

Its remarkable features include that the rate of atomizing is controlled at the interior of the substrate, whereas the size aerosol droplets tends to become controlled at its exterior. Both processes can be analyzed and manipulated by various means.

This presentation has been put forward to Shell Global Solutions, Chester, U.K.

 

 

 

J.J.C. Oomen PhD

 

Appeltern, 10-07-2005

 

Slide 15

 

Track Record

 

 

 

•       1983 De Kaars, review by H. de Gruijter and A. van Rossum

•       1997 Aware of Indonesian asphalt-firewood problem

•       2002 Small scale use of asphalt as a fuel (flyer by J.J.C. Oomen)

•       2002 Towards small scale use of asphalt as a fuel, RuG report by J. Faber (prof. H.B. Levinsky, University of Groningen, Netherlands)

•       2002 RuG: cooperation concluded.

•       2003 Pre-ignition of less volatile organics, notes by J.J.C. Oomen

•       2004 Pyrolysis related to molecular structure fuel

•       2004 Concept of “pyrolytic carburation

•       2005 Development of test tube experiments

•       2005 Put forward to Shell Global Solutions, Netherlands and UK.