AU2009238280A1 - Soil remediation system - Google Patents

Description

P/00/011I 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: 2 SOIL 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. 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 0 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, 5 - Semi-volatile organic compounds (SVOCs), . Polynuciear 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 20 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.

3 The most common process configuration involves a counter-current direct fired desorber, but there are numerous variants. The most common alternative is the co current desorber, which produces a hotter contaminated offgas stream. To avoid cooling these gases to enable fabric filtration, a cyclone is used to remove some of the dust 5 prior to thermal oxidation, followed by gas cooling then fabric filtration. In another variant the functions of the thermal desorber and oxidiser are combined by arranging to combust the contaminant gases within a metal jacketed combustion chamber within a rotary desorber. United States patent 5658094 discloses an arrangement in which heat 0 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 5 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 post-combustion waste gases are cooled and released into the atmosphere. 20 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, 25 including W.L. Troxler et al, "Treatment of non-hazardous petroleum-contaminated 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).

4 Table 1 Main Types of Thermal Remediation Advantages Disadvantages Direct fired rotary kilns - High rates of heat transfer. - Larger thermal oxidiser than for indirect fired. - Smaller desorber than indirect - Dilution strategies are usually required for fired. hydrocarbon contamination levels of >4%to - Simplest, most robust. avoid exceeding the LEL of desorber offgases. - Most flexible to variation in feed material and type and level of contamination. Indirect fired rotary kilns - May allow economic recovery of - Unsuitable for heavy contamination, especially hydrocarbons. of long chain or aromatic hydrocarbons (tars). - Lower dust losses from desorber. - Larger desorber. - Higher moisture soils severely impair capacity. Combination direct/indirect - Process simplification by using a - Inability to process large gas volumes. fired desorber, with integral single process step. - Lower peak soil temperatures will - prevent thermal oxidiser 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 5mm). - Increased maintenance (abrasion).

5 Remediation plants may be either stationary or mobile, with the larger, stationary plants being restricted to remediation of large heavily contaminated sites (e.g. large integrated steelworks sites), regional clusters of contaminated sites, or under circumstances where transport of contaminated materials is economic and not 5 hazardous. Key technical factors in thermal remediation include: . Solids temperature and contact time. . Soil moisture when treated. . Actual soil hydrocarbon contaminants present. 0 . Other contamination, e.g. chlorine compounds and heavy materials. . Extraneous rubble. Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art 5 could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. 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 embodiments of optimising energy usage and operating costs for a give soil 20 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 gasses, NOx and dioxin/furan emissions.

6 Summary of Invention The invention provides a process for remediating soil contaminated with hydrocarbons, including: desorbing the hydrocarbon contaminants from a bed of the soil by thermal 5 desorption in a desorption chamber and thereafter combusting the desorbed contaminants; wherein offgases from the combustion of the desorbed contaminants are rapidly quenched to below 200 0 C. The quenching may be carried out in less than 750ms. Advantageously, the 0 offgases are at a temperature above 600 0 C at commencement of the rapid quenching. The rapid injection may be by injection of ambient air, injected for example by an ejector powered by the offgases. Combustion of the desorbed contaminants may be effected in at least two stages, including a first stage in which the contaminants are combusted with a first 5 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. The invention further provides apparatus for remediating soil contaminated with hydrocarbons, including: 20 a furnace configuration in which a desorption chamber is defined wherein a bed of said soil is treated to separate the hydrocarbon contaminants from the soil by thermal desorption, and in which the desorbed hydrocarbon contaminants are combusted; and a quenching module coupled to said furnace configuration for recovering offgases from said combustion of the desorbed contaminants, which module is arranged 25 for rapidly quenching said offgases to below 200"C.

7 The furnace configuration may include at least two stages for the combustion of said desorbed contaminants respectively having first and second combustion air supply means, which stages include a combustion stage in which the contaminants are combusted with combustion air delivered by the first combustion air supply means and a 5 second stage in which combustion air is admitted by the second combustion air supply means for combustion of residual components and for controlling the offgas outflow temperature. Preferably, the desorption chamber is provided in a rotary kiln that is preferably inclined. The contaminated soil, which is advantageously optimally sized and prepared, 0 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 5 contaminants. Preferably, 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 chamber. Air admitted to effect such combustion may be injected at the burner end of the desorption chamber. The desorption chamber is preferably !0 provided by a high velocity desorber burner which provides a highly collimated stream of high temperature gases along the centre of the desorber. In a preferred operation of the first stage of the combustion of the desorbed contaminants, preheated near stoichiometric amounts of combustion air, preheated dedusted desorber offgases, i.e. desorbed contaminants, and auxiliary fuel are injected, 25 preferably via a nozzle mix burner. The fuel rate and preheat to this burner is arranged to give said adiabatic flame temperature of the mixture of 900 - 1200 0 C, and thus avoids localised high temperatures and high NOx 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 stage where cold or 30 preheated combustion air is injected into the hot gas stream to provide additional mixing 8 and oxygen for combustion of residual compounds, and to control the gas inlet temperature to the heat exchangers. Advantageously, combustion air for the desorption chamber and for the combustion of the desorbed contaminants is preheated by heat exchange with the 5 offgases. 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. The rapid quenching may be effected by the heat exchange with the combustion 0 air and/or as the desorbed contaminants. As proportion of offgases for the thermal oxidiser may bypass the heat exchange with the combustion air and the desorbed contaminants, and be subjected to the rapid quenching to below 200*C. Advantageously, the heat exchange configuration is directly installed in the hot 5 gas duct at the offgas outlet end of the combustion furnace, 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 aforementioned direct installation (preferably without radiation shields or excess metal temperatures). There may be an energy dump valve from the heat exchange configuration 20 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 temperatures above 500 0 C, 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 25 in either or both the offgas duct or by-pass duct to control hot gas flow through both the combustion air and contaminants heat exchanges.

9 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. Brief Description of the Drawings 5 Figure 1 is a block flow diagram of an apparatus incorporating embodiments of the principal aspects of the invention; and Figure 2 is a diagram depicting combustion of desorbed contaminants in the desorber kiln. Description of Preferred Embodiments 0 The illustrated system includes a pair of furnaces 20, 30, being a slightly inclined countercurrent rotary kiln 20 for effecting thermal desorption and a 2-stage 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 5 pre-heated for delivery to the lower, burner ends of desorber kiln 20 and oxidiser 30 by respective combustion 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 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 20 burner end of thermal oxidiser 30 via contaminant vapours supply line 45. Preheating may be to a temperature in the range 350-500*C. Sized and otherwise prepared soils requiring remediation are transferred at a 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 25 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.

10 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 5 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 0 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 1 1/2 - 2%) of the desorber off-gas. For this situation, the 5 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 increases to a value above the LEL, .0 combustion is controlled in the kiln both to 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. 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 0 C, typically comprise 25 50% steam, 5% carbon dioxide, 44% nitrogen, and approximately 0.5-1% 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 45. 30 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 11 from the thermal desorber. Typically the gases are heated and combusted at 1000 1200*C for approximately 10OOms. To minimise NOx 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 (P) and, ii) the post-combustion zone 5 (Q). 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 0 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 11000C. Gas temperatures above 11000C will lead to decreased heat exchanger life. 5 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 50. The combination of these features allows direct installation of heat exchanger 40 in the hot gas duct at the outlet of thermal oxidiser 30, without the need for radiation shields and !0 without incurring excess metal temperatures. This saves weight and cost. Dump valve 50 allows venting of excess pre-heated air from the leading tube bank M during operation. This dumping allows accurate control of the. process energy balance with varying moisture and hydrocarbon contamination levels. In addition, this facility decreases manufacturing costs for the heat exchanger by allowing the use of lower 25 alloy steels, and increases heat exchanger life. 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 30 the heat exchanger stages.

12 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 5 increase throughput by minimising the gas volumes processed. These features also allow maintenance of metal temperatures above 500*C to minimise dioxin formation from PCB or salt-contaminated soils, but below 700 0 C 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 600 0 C to further minimise 0 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 5 complemented by reduced CO 2 and NOx 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. The drawing also illustrates several modules for further off-gas treatment downstream of heat exchanger 40. These modules may be variously provided 20 according to the characteristics of the contamination. For low chlorine containing soils, there is no further off-gas treatment and a short refractory line stack 60 is utilised. This approach minimises water and electrical energy consumption. For high chlorine or PCB containing soils, where the risk of dioxin or hydrochioride containing gases is high, a scrubber section 62 is used to quench the off 25 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.

13 For gases of intermediate chlorine compound content, an ambient air quenching module is used, wherein a large volume of ambient air is injected at 65 into stack 60 to rapidly quench (within less than 750ms) the off-gases to below 200 0 C. Such a module might comprise, for example, a fan sucking in ambient air or an ejector powered by the 5 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 (32)

1. A process for remediating soil contaminated with hydrocarbons, including: desorbing the hydrocarbon contaminants from a bed of the soil by thermal desorption in a desorption chamber and thereafter combusting the desorbed 5 contaminants; wherein offgases from the combustion of the desorbed contaminants are rapidly quenched to below 200*C.

2. A process according to claim 1, wherein said quenching is carried out in less than 750ms. 10

3. A process according to claim 1 or 2, wherein said offgases are at a temperature above 6000C at commencement of said rapid quenching.

4. A process according to claim 1, 2 or 3, wherein said combustion of the desorbed contaminants is effected in at least two stages, including a first stage in which the contaminants are combusted with a first supply of combustion air at a substantially 15 adiabatic temperature in the range 900 - 1200*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.

5. A process according to any one of claims 1 to 4, wherein combustion air for said desorption chamber and for said combustion of the desorbed contaminants is preheated 20 by heat exchange with said offgases.

6. A process according to any one of claims 1 to 5, wherein said desorbed contaminants are preheated prior to said combustion by heat exchange with said offgases.

7. A process according to claim 6 when appended to claim 5, wherein said offgases 25 preheat the combustion air first and then the desorbed contaminants. 15

8. A process according to claim 5, 6 or 7, wherein said rapid quenching is effected after said heat exchange with said combustion air and/or said desorbed contaminants.

9. A process according to any one of claims 5 to 8 wherein the heat exchange is conducted in a heat exchanger having metal exchange surfaces and wherein the metal 5 surface temperatures are maintained above 500 0 C and below 700*C.

10. A process according to claim 9, wherein the heat exchanger is arranged for co current flow.

11. A process according to any one of claims 5 to 10, wherein the hot gas flow through both the combustion air and contaminants heat exchanges is controlled. 0

12. A process according to any one of claims 5 to 11, wherein excess preheated air is vented during treatment of soil with high hydrocarbon contaminant levels.

13. A process according to any one of claims 1 to 12, wherein desorbed contaminants in gaseous form are at least in part combusted within said desorption chamber by controlled admission of air into said chamber above said bed to effect such 5 combustion.

14. A process according to any one of claims 1 to 13, wherein said rapid quenching is by injection of ambient air.

15. A process according to claim 14, wherein said ambient air is injected by an ejector powered by said offgases. 20

16. A process according to any one of claims 5 to 12, wherein a proportion of offgases from the thermal oxidiser bypasses said heat exchange with said combustion air and said desorbed contaminants and is subjected to said rapid quenching to below 200 0 C.

17. Apparatus for remediating soil contaminated with hydrocarbons, including: 16 a furnace configuration in which a desorption chamber is defined wherein a bed of said soil is treated to separate the hydrocarbon contaminants from the soil by thermal desorption, and in which the desorbed hydrocarbon contaminants are combusted; and a quenching module coupled to said fumace configuration for recovering 5 offgases from said combustion of the desorbed contaminants, which module is arranged for rapidly quenching said offgases to below 200*C.

18. Apparatus according to claim 17, wherein said furnace configuration includes at least two stages for said combustion of the desorbed contaminants respectively having first and second combustion air supply means, which stages including a combustion 0 stage in which the desorbed contaminants are combusted with combustion air delivered by said first combustion air supply means, and a second stage in which combustion air is admitted by said second combustion air supply means for combustion of residual compounds and for controlling the offgas outflow temperature.

19. Apparatus according to claim 17 or 18 further including a separate furnace for 15 combustion of the desorbed contaminants; respective ducts for conveying combustion air to said desorption chamber and to the separate furnace; and a heat exchange configuration for preheating said combustion air by heat exchange with said offgases.

20. Apparatus according to claim 17, 18 or 19, further including a duct for conveying the desorbed contaminants from the desorption chamber to the separate furnace, and a 20 heat exchange configuration for preheating said desorbed contaminants by heat exchange with said offgases.

21. Apparatus according to claim 20 when appended to claim 19, wherein the heat exchange configuration is a series configuration so that said offgases preheat the combustion air first and then the desorbed contaminants. 25

22. Apparatus according to claim 19, 20 or 21, wherein said quenching module is arranged downstream, for said offgases, from said heat exchange configuration. 17

23. Apparatus according to any one of claims 19 to 22, wherein the heat exchange configuration is directly installed in the hot gas duct at the offgas outlet end of the separate furnace.

24. Apparatus according to claim 23, wherein the heat exchange configuration is 5 arranged for co-current flow.

25. Apparatus according to claim 23 or 24, wherein a leading tube bank of the heat exchange configuration incorporates variable tube spacing to facilitate said direct installation.

26. Apparatus according to any one of claims 19 to 25, further including an energy 10 dump valve from the heat exchange configuration for venting of excess preheating air.

27. Apparatus according to any one of claims 19 to 26, wherein the heat exchange configuration includes combustion air and contaminants heat exchangers which include a hot gas by-pass and damper system or by-pass duct to control hot gas flow through both the combustion air and contaminants heat exchangers. 5

28. Apparatus according to claim 27, wherein the heat exchanger for the contaminants is adapted to be made reversible depending on operating conditions.

29. Apparatus according to any one of claims 19 to 23, further including: a control device for controlled admission of air into said desorption chamber above said bed to effect in the said chamber at least partial combustion of said 20 desorbed contaminants in gaseous form; and a duct for conveying the products of said at least partial combustion to said separate furnace for further combustion therein.

30. Apparatus according to any one of claims 19 to 29, further comprising a duct that bypasses said heat exchange configuration to said quenching module for conveying a 25 proportion of offgases from the thermal oxidiser direct to the quenching module. 18

31. Apparatus according to any one of claims 17 to 30, wherein said quenching module is arranged to carry out said rapid quenching in less than 750ms.

32. Apparatus according to claim 31, wherein said ambient air injection device comprises an ejector powered by said offgases. 5