AU2006202587A1

AU2006202587A1 - Soil remediation system

Info

Publication number: AU2006202587A1
Application number: AU2006202587A
Authority: AU
Inventors: John Anthony Lucas; Louis James Wibberley
Original assignee: Innova Soil Technology Pty Ltd
Current assignee: Innova Soil Technology Pty Ltd
Priority date: 1998-09-28
Filing date: 2006-06-16
Publication date: 2006-07-06
Anticipated expiration: 2019-09-16
Also published as: AU2009238280B2; AU2006202587B2; AU2009238280A1

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Soil remediation system The following statement is a full description of this invention, including the best method of performing it known to us:

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1A SSOIL REMEDIATION SYSTEM Field of the Invention The present invention relates to the remediation of soil contaminated with hydrocarbons, utilising thermal desorption followed by thermal oxidation.

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5 Background Art There are numerous other types of processes for remediating soils, including soil washing, in-situ air stripping, in-situ vitrification, stabilisation, vacuum extraction and solvent extraction. However, the most universally proven and efficient method for removing organics from soil is thermal desorption, which together with treatment or destruction of the desorbed organics is termed thermal remediation. Hydrocarbon contaminants which are treatable with thermal remediation include: Volatile organic compounds (VOC) eg petrol, diesel, Aromatic hydrocarbons eg benzene, tars, Dioxins and furans, Semi-volatile organic compounds (SVOCs), Polynuclear aromatic hydrocarbons (PAHs or PNAs), Polychlorinated biphenyls (PCBs), and Pesticides (eg organochlorins such as dieldrin and aldrin).

Thermal remediation of contaminated soil uses heat to physically separate hydrocarbon based contaminants from feed material which may be, for example, directly recovered soils, sediments, sludges or filter cakes. The separated hydrocarbons are then combusted or thermally oxidised to produce essentially carbon dioxide and water vapour.

The most common process configuration involves a counter-current direct N fired desorber, but there are numerous variants. The most common alternative is the co-current desorber, which produces a hotter contaminated offgas stream. To INO avoid cooling these gases to enable fabric filtration, a cyclone is used to remove some of the dust prior to thermal oxidation, followed by gas cooling then fabric filtration. In another variant the functions of the thermal desorber and oxidiser are In combined by arranging to combust the contaminant gases within a metal jacketed combustion chamber within a rotary desorber.

SUnited States patent 5658094 discloses an arrangement in which heat exchangers are used for preheating combustion air for a thermal desorber. In that arrangement, there is described a combined (all metal) rotary device, a type of rotary kiln with internal indirect heating of both soil and combustion air, which is claimed to carry out combined thermal desorption and thermal oxidation.

German patent application 3447079 describes a process in which the contaminated soil is thermally treated in a rotary kiln by the direct addition of hot combustion gases and/or air. The decomposition products are partially combusted in the rotary kiln, with the remaining production gas fed to a waste gas combustion chamber where it is afterburnt at high temperatures. In general, the postcombustion waste gases are cooled and released into the atmosphere.

Various other methods of thermal remediation of soil are described in United States patents 5,455,005, 5,393,501, 4,715,965, 4,974,528, and 5,378,083.

The main difference between different technologies is the equipment used for thermal desorption, which may be one of four main types, the advantages and disadvantages of which are summarised in Table 1 (obtained from various sources, including W.L. Troxler et al, "Treatment of non-hazardous petroleumcontaminated soils by thermal desorption technologies", Jnl of Air and Waste, Vol. 43, Nov. 1993, and W.C. Anderson, "Innovative site remediation technology", Thermal Desorption, WASTECH, 1993).

Table 1 Main Types of Thermal Remediation A~ r- 00

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Auvantages Disadvantages Direct fired rotary kilns High rates of heat transfer. Larger thermal oxidiser than for indirect Smaller desorber than indirect fired.

fired. Dilution strategies are usually required Simplest, most robust. for hydrocarbon contamination levels of to avoid exceeding the LEL of Most flexible to variation in feed desorber ofigases.

material and type and level of contamination.

Indirect fired rotary kilns May allow economic recovery Unsuitable for heavy contamination, of hydrocarbons. especially of long chain or aromatic Lower dust losses from hydrocarbons (tars).

desorber. Larger desorber.

Higher moisture soils severely impair capacity.

Combination direct/indirect Process simplification by using Inability to process large gas volumes.

fired desorber, with a single process step.

integral thermal oxidiser Lower peak soil temperatures will prevent practical decontamination of heavily contaminated soils, especially with PAHs or PCBs.

Less suitable for high moisture soils.

Direct fired conveyors, As for indirect fired rotary kilns. As for indirect fired rotary kilns.

including metal belts and Improved control over solids screws residence time.

Direct fired fluidised beds Highest process intensity. Increased complexity.

Increased dust losses/recycling of dusts.

Requires fine and uniform sized material (normally less than Increased maintenance (abrasion).

SRemediation plants may be either stationary or mobile, with the larger, Sstationary plants being restricted to remediation of large heavily contaminated nsites (eg large integrated steelworks sites), regional clusters of contaminated sites, or under circumstances where transport of contaminated materials is economic and not hazardous.

r- 00oO Key technical factors in thermal remediation include: Solids temperature and contact time.

Soil moisture when treated.

Actual soil hydrocarbon contaminants present.

Other contamination, eg chlorine compounds and heavy metals.

S Extraneous rubble.

It is an object of the present invention to provide an improved method and apparatus for remediating soil contaminated with hydrocarbons that is capable in preferred embodiment of optimising energy usage and operating costs for a given soil throughput, and that is preferably adaptable to treat short chain, long chain, aromatic, and polychlorinated hydrocarbons. In particular embodiments, it is further desired to minimise environmental impacts, especially greenhouse gases, NOx and dioxin/furan emissions.

Summary of Invention The invention accordingly provides, in a first aspect, a process for remediating soil contaminated with hydrocarbons, including: desorbing the hydrocarbon contaminants from a bed of the soil by thermal desorption in a treated desorption chamber and thereafter combusting the contaminants in a thermal oxidiser; -wherein combustion air for said desorption chamber and said thermal oxidiser, and said desorbed contaminants prior to admission to said thermal 004778030 O oxidiser, are preheated by heat exchange with offgases from the thermal oxidiser; ;and wherein, after said heat exchange with said combustion air and said desorbed IND contaminants, said offgases from the thermal oxidiser are rapidly quenched to below 200°C.

oo 5 The invention further provides, in its first aspect, apparatus for remediating soil S contaminated with hydrocarbons, including: a first fumrnace defining a desorption chamber in which a bed of said soil may be treated to separate the hydrocarbon contaminants from the soil by thermal desorption; a second furnace for combusting hydrocarbon contaminants by thermal oxidation; respective ducts for conveying combustion air to said desorption chamber and to said second furnace, and for conveying the desorbed contaminants from the desorption chamber to the second furnace; and a heat exchange configuration arranged for preheating said combustion air and said desorbed contaminants by heat exchange with offgases from the second furnace; and a quenching module for rapid quenching of said offgases to below 200'C after said heat exchange with said combustion air and said desorbed contaminants.

Preferably, the heat exchange configuration is further arranged in a series configuration so that said offgases preheat the combustion air first and then the desorbed contaminants.

Advantageously, the heat exchange configuration is directly installed in the hot gas duct at the offgas outlet end of the second furnace for thermal oxidation, and is preferably arranged for co-current flow. The leading tube bank of the heat exchange configuration preferably incorporates variable tube spacing to facilitate the .004778030 6

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O aforementioned direct installation (preferably without radiation shields or excess metal temperatures).

IDThere may be an energy dump valve from the heat exchange configuration venting of excess preheated air as will occur during treatment of higher contaminated soil. Preferably, the process and energy dump valve are controlled to maintain metal oo temperatures above 5000C, to minimise dioxin formation from PCB or salt contaminated soils, but below 7000C to minimise metal oxidisation, corrosion and expansion damage.

The heat exchange configuration may have a hot gas by-pass duct and damper system Sin either or both the offgas duct or by-pass duct to control hot gas flow through both the combustion air and contaminants heat exchanges.

The heat exchanger for the contaminants may have either co-current or counter current flow, and may be adapted to be made reversible depending on operating conditions.

In a second aspect, the invention provides a process for remediating soil contaminated with hydrocarbons, including: desorbing the hydrocarbon contaminants from a bed of the soil by thermal desorption in a treated desorption chamber and thereafter combusting the contaminants in a thermal oxidiser, combusting the desorbed contaminants at least in part within said desorption chamber by controlled admission of air into said chamber above said bed to effect such combustion; wherein the separated contaminants are treated in said thermal oxidiser in at least two stages, including a combustion stage in which the contaminants are combusted with a first supply of combustion air at a substantially adiabatic temperature in the range 900 12000C, and a second stage in which a second supply of combustion air is admitted for combustion of residual compounds and for controlling the offgas outflow temperature.

.004778030 6a 0 In its second aspect, the invention further provides apparatus for remediating soil contaminated with hydrocarbons, including; Q first furnace means defining a desorption chamber in which a bed of said

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00 Ssoil may be treated to separate the hydrocarbon contaminants from the soil Sby thermal desorption;

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second furnace means for combusting hydrocarbon contaminants by thermal oxidation; 00oo means for controlled admission of air into said desorption chamber above 0 said bed to effect in the said chamber at least partial combustion of said IND desorbed contaminants in gaseous form; means for conveying the products of said at least partial combustion to said second furnace means for further combustion therein; and wherein said second furnace for thermal oxidation includes at least two stages including a combustion stage in which the contaminants are combusted with a first supply of combustion air at a substantially adiabatic temperature in the range 900 1200 0 C, and a second stage in which a second supply of combustion air is admitted for combustion of residual compounds and for controlling the offgas outflow temperature.

Preferably, the desorption chamber is provided in a rotary kiln that thereby constitutes the first furnace means and is preferably inclined. The contaminated soil, which is advantageously optimally sized and prepared, is preferably admitted to an upper, cooler end of the rotary kiln at a controlled rate, and the rotation of the kiln then causes the soil to move down the inside of the kiln towards the hotter end containing a burner. The heat from the burner and other exothermic reactions in the kiln heats the soil, causing it to dry and "desorb" (a term which includes without limitation evaporation, decomposition and gasification) contained hydrocarbon contaminants.

Preferably, the at least partial combustion of the contaminants in the desorption chamber occurs both in close proximity to the soil bed and in the hot gas stream passing along the desorber. The air admitted to effect such combustion may be injected at the burner end of the desorption chamber. The first ID 8 O furnace means is preferably a high velocity desorber burner which provides a Shighly collimated stream of high temperature gases along the centre of the desorber.

In the preferred operation of the first stage of the thermal oxidiser, preheated near stoichiometric amounts of combustion air, preheated dedusted 00 tt desorber offgases, ie desorbed contaminants, and auxiliary fuel are injected, Spreferably via a nozzle mix burner. The fuel rate and preheat to this burner is ND arranged to give said adiabatic flame temperature of the mixture of 900-1200°C, and thus avoids localised high temperatures and high NO, from the use of preheated combustion air. However, the temperature is sufficient -to destroy any gaseous contaminants in the desorber gases. These hot gases then pass into the second zone of the thermal oxidiser where cold or preheated combustion air is injected into the hot gas stream to provide additional mixing and oxygen for combustion of residual compounds, and to control the gas inlet temperature to the heat exchangers.

The invention also extends to methods or apparatus incorporating both of the aspects of the invention.

The offgas from thermal oxidation may be further treated (eg after said heat exchanges in the first aspect of the invention) by one or more modular off-gas treatments according to the nature of the original contaminants, and the requirements of the soils being remediated. For low chlorine containing soils, such an off-gas treatment system may be omitted, and replaced with a short stack. For higher chlorine containing soils, where the risk of dioxin or hydrochloride containing gases is evident, a scrubber section may be used. A suitable scrubber can treat most of the offgases. A small bleed of hot off-gas or preheated combustion air is allowed to by-pass the scrubber to provide reheating of the scrubbed gas stream in the stack thereby preventing drooping or visible plumes.

For gases of intermediate chlorine compound content, a module comprising an ambient air quenching module may be used, wherein a large volume of ambient air is injected into the offgases to rapidly quench them to less than 2000C.

O 9 0 Brief Description of the Drawings

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nFigure 1 is a block flow diagram of an apparatus incorporating

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_embodiments of the principal aspects of the invention; and Figure 2 is a diagram depicting combustion of desorbed contaminants in oO n 5 the desorber kiln.

O Description of Preferred Embodiments The illustrated system includes a pair of furnaces 20, 30, being a slightly inclined countercurrent rotary kiln 20 for effecting thermal desorption and a 2stage thermal oxidiser 30. The off-gases 32 from thermal oxidiser 30 pass directly through a 2-stage heat exchanger 40. In the first stage 42 of the series arrangement, itself consisting of a pair of sub-stage tube banks M, L, cold combustion air admitted along supply duct 41 is pre-heated for delivery to the lower, burner ends of desorber kiln 20 and oxidiser 30 by respective combustion IN O air ducts 43a, 43b. In the second stage 44 of heat exchanger 40, again consisting of respective sub-stage tube banks J, K, off-gases (including desorbed 1n contaminants) recovered from the upper end of desorber kiln 20 via line 21, and cleaned and dedusted by cyclone 22 and bagfilter 23, are pre-heated for delivery to the burner end of thermal oxidiser 30 via contaminant vapours supply line Preheating may be to a temperature in the range 350-500°C. 00 Sized and otherwise prepared soils requiring remediation are transferred at Sa controlled rate into the upper or cooler end of desorber kiln 20 at delivery port 24. The desorber kiln is inclined so that its rotation causes the soil to move down inside the kiln towards the burner end 20a. The heat from the burner 27 and from other exothermic reactions in the kiln, heats the soil, causing it to dry and desorb contained hydrocarbon contaminants.

The pre-heated combustion air in delivery duct 43a for desorber kiln 20 is divided into a first stream 25 for burner 27, and a second stream 26 of overbed combustion air for effecting at least partial combustion of the desorbed hydrocarbon contaminants within the kiln. This combustion takes place both in close proximity to the soil bed in the kiln and to the soil particles cascading through the hot gas stream, and in the hot gas stream passing along its interior. A suitable kiln for the desorber 20 is a high velocity burner such as the North American Hi Ram kiln burner, which provides a highly collimated stream of high temperature gases along the centre of the kiln. Application of this burner type with the abovementioned admission of overbed air 26 ensures efficient and reliable ignition of hydrocarbons as they evolve from the soil as it progresses along the kiln, as depicted in Figure 2.

In the case of soils with high hydrocarbon contamination levels, energy conservation will be secondary to controlling the level to be below the Lower Explosive Limited (LEL) (typically 11/2 of the desorber off-gas. For this situation, the temperature of the desorber off-gases in duct 21 may be increased by controlling both the energy input to the desorber burner 27 and the amount of insitu combustion, to allow dilution of the desorber off-gases prior to gas cleaning.

Controlled amounts of water may be injected via sprays 29 located in duct 21 immediately downstream of the desorber kiln. Thus as hydrocarbon contamination Sincreases to a value above the LEI, combustion is controlled in the kiln both to I\N mimimise energy consumption and to keep the hydrocarbon level in the off-gas below the LEL: energy efficiency and safety issues must both be managed.

r- 0 0 5 Remediated soil is recovered from desorber kiln at 28 at burner end 21a.

The vapours exiting the desorber in duct 21 typically at around 275°C, typically Scomprise 50% steam, 5% carbon dioxide, 44% nitrogen, and approximately 1 volatile hydrocarbons contaminants desorbed from the soil bed. As previously mentioned these vapours are cleaned of solid matter entrained from the kiln by cyclone 22 and/or bagfilter 23 before being pre-heated in heat exchanger stage 44 and injected into the thermal oxidiser via line The thermal oxidiser 30 is a 2-stage refractory-lined chamber comprising one or more burners to assist complete combustion of the hydrocarbon contaminated vapours from the thermal desorber. Typically the gases are heated and combusted at 1000-1200°C for approximately 1000ms. To minimise NO, formation, and to decrease radiation to the front of the heat exchanger, the thermal oxidiser has two sequential combustion zones; i) the primary combustion zone and, ii) the post-combustion zone Preheated combustion air, preheated contaminant vapours and auxiliary fuel are injected into the primary combustion zone using, preferably, but not restricted to, a nozzle mixing burner or burners 36. The air in the gas mixture is controlled to give an overall stoichiometric or slightly sub-stoichiometric combustion. Additional unheated combustion air is injected via ports around the periphery and at the entry to an afterburner 55 to give an overall excess oxygen in the hot gases of approximately 3% to ensure complete destruction of contaminant hydrocarbons, to provide additional turbulence, and to control the temperature of the gases entering the heat exchanger to typically between 950 and 1100°C. Gas temperatures above 1100°C will lead to decreased heat exchanger life.

Features of heat exchanger 40 include a wider tube spacing for the leading rows of tubes (typically three rows, to decrease convective heat transfer to these rows subject to high radiant heat fluxes), in bank M, and an energy dump valve N 50. The combination of these features allows direct installation of heat exchanger in the hot gas duct at the outlet of thermal oxidiser 30, without the need for _n radiation shields and without incurring excess metal temperatures. This saves weight and cost. Dump valve 50 allows venting of excess pre-heated air from the 5 leading tube bank M during operation. This dumping allows accurate control of r-- 00 the process energy balance with varying moisture and hydrocarbon contamination N levels. In addition, this facility decreases manufacturing costs for the heat i exchanger by allowing the use of lower alloy steels, and increases heat exchanger life.

c-i An optional feature to cope with even more extreme and variable operating conditions is to equip heat exchanger 40 with a bypass duct 55 and associated damper (either in one or both of the heat exchanger stages), to further increase the flexibility of the process to treat higher contaminated soils, and to improve the operational safety of the heat exchanger stages.

The heat exchanger features, together with controlled combustion of hydrocarbon contaminants in desorber kiln 20, the use of nozzle mixing burners, and the 2-stage combustion in thermal oxidiser 30, combine to minimise overall energy consumption and therefore operating costs, greenhouse gas and NOx emissions, and to increase throughput by minimising the gas volumes processed.

These features also allow maintenance of metal temperatures above 5000C to minimise dioxin formation from PCB or salt-contaminated soils, but below 7000C to minimise exchanger metal oxidisation and corrosion. In addition, the system design allows control such that the heat exchanger exit gas temperature is maintained above 6000C to further minimise dioxin formation.

It is believed that, relative to no pre-heating, a total 55% reduction in energy consumption is achieved with the illustrated system by pre-heating all combustion air and the contaminant hydrocarbon vapours, at a level where combustion of hydrocarbon vapours in desorber kiln 20 is at about 20%. The reduction in energy consumption is complemented by reduced C02 and NO, levels.

A further advantage of preheating is that the size of the thermal oxidiser in particular, and to a lesser extent the kiln and the baghouse, can be reduced.

C The drawing also illustrates several modules for further off-gas treatment downstream of heat exchanger 40. These modules may be variously provided according to the characteristics of the contamination. For low chlorine containing 00 5 soils, there is no further off-gas treatment and a short refractory line stack 60 is

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N1 utilised. This approach minimises water and electrical energy consumption.

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For high chlorine or PCB containing soils, where the risk of dioxin or C hydrochloride containing gases is high, a scrubber section 62 is used to quench the off-gases and remove the chlorides. A preferred embodiment under these conditions is to allow a small bleed of hot off-gas (about 10%, depending on contamination levels) to bypass the scrubber on line 63 to provide sufficient reheating of the scrubbed gas stream in the stack to prevent drooping or visible fumes. A proportion of the pre-heated combustion air may also be delivered to this bypass 63 by a delivery duct 43c.

For gases of intermediate chlorine compound content, an ambient air quenching module is used, wherein a large volume of ambient air is injected at into stack 60 to rapidly quench (within less than 750ms) the off-gases to below 2000C. Such a module might comprise, for example, a fan sucking in ambient air or an ejector powered by the hot offgases.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A process for remediating soil contaminated with hydrocarbons, including: Idesorbing the hydrocarbon contaminants from a bed of the soil by thermal desorption in a treated desorption chamber and thereafter combusting the contaminants r- 5 in a thermal oxidiser; o00 combusting the desorbed contaminants at least in part within said desorption ID chamber by controlled admission of air into said chamber above said bed to effect such combustion; wherein the separated contaminants are treated in said thermal oxidiser in at 0 least two stages, including a combustion stage in which the contaminants are combusted with a first supply of combustion air at a substantially adiabatic temperature in the range 900 12000C, and a second stage in which a second supply of combustion air is admitted for combustion of residual compounds and for controlling the off-gas outflow temperature.

2. A process according to claim 1, wherein the desorption chamber is provided in a rotary kiln.

3. A process according to claim 2, wherein the rotary kiln is inclined.

4. A process according to claim 2 or 3, including admitting the contaminated soil to an upper, cooler end of the rotary kiln at a controlled rate and wherein rotation of the kiln causes the soil to move down the inside of the kiln towards the hotter end containing a burner.

A process according to any one of claims 1 to 4, wherein the at least partial combustion of the contaminants in the desorption chamber occurs both in close proximity to the soil bed and in the hot gas stream passing along the desorption chamber. 004829825" IND S

6. A process according to any one of claims 1 to 5, including injecting air at the burner end of the desorption chamber.

7. A process according to any one of claims 1 to 6, wherein in the first stage of the thermal oxidiser, preheated near stoichiometric amounts of combustion air, preheated dedusted desorber off-gases, and auxiliary fuel are injected. 00oO S

8. A process according to claim 7, wherein the fuel rate and preheat to this burner is 0 arranged to give said adiabatic flame temperature of the mixture of 900 1200 0 C.

9. A process according to any one of claims 1 to 8, wherein in the second zone of the thermal oxidiser, cold or preheated combustion air is injected into the hot gas 0 stream to provide additional mixing and oxygen for combustion of residual compounds, and to control the gas inlet temperature to the heat exchangers.

A process according to any one of claims 1 to 9, wherein the off-gas from thermal oxidation is further treated by one or more modular off-gas treatments.

11. Apparatus for remediating soil contaminated with hydrocarbons, including: first furnace means defining a desorption chamber in which a bed of said soil may be treated to separate the hydrocarbon contaminants from the soil by thermal desorption; second furnace means for combusting hydrocarbon contaminants by thermal oxidation; means for controlled admission of air into said desorption chamber above said bed to effect in the said chamber at least partial combustion of said desorbed contaminants in gaseous form; means for conveying the products of said at least partial combustion to said second furnace means for further combustion therein; 004829825' 16 O wherein said second furnace for thermal oxidation includes at least two stages including a combustion stage in which the contaminants are combusted with a first supply of combustion air at a substantially adiabatic temperature in the range 900 1200 0 C, and a second stage in which a second supply of combustion air is admitted for INO combustion of residual compounds and for controlling the off-gas outflow temperature. oO

12. Apparatus according to claim 11, wherein the desorption chamber is provided in a rotary kiln that thereby constitutes the first furnace means.

13. Apparatus according to claim 12, wherein the rotary kiln is inclined.

14. Apparatus according to any one of the claims 11 to 13, wherein the first furnace 0 means is a high velocity desorber burner which provides a highly collimated stream of high temperature gases along the centre of the desorber.

Apparatus according to any one of claims 11 to 14, further including, in the first stage of the thermal oxidiser, means to inject preheated near stoichiometric amounts of combustion air, preheated dedusted desorber off-gases, and auxiliary fuel.

16. Apparatus according to claim 15, wherein the means to inject is a nozzle mix burner.

17. Apparatus according to claim 16, wherein the fuel rate and preheat to the burner is arranged to give said adiabatic flame temperature of the mixture of 900 1200 0 C. Dated 16 June 2006 Freehills Patent Trade Mark Attorneys Patent Attorneys for the Applicant: Innova Soil Technology Pty Ltd