Method and apparatus for gas destruction

Abstract

A method for the destruction of a target gas, the method including: a) compressing at a first pressure a mixture of air and target gas to produce a compressed target gas mixture; b) destroying the target gas by combusting the compressed target gas mixture with diesel fuel in a forced-induction internal combustion engine, at a combustion pressure greater than the first pressure in the turbocharger, to produce an oxidised exhaust gas, the combustion occurring while maintaining a load on the diesel engine with a load bank; and c) processing the oxidised exhaust gas to produce a vent gas for venting to atmosphere where the vent gas includes substantially no target gas.

Claims

1. A method for the destruction of a non-combustible target gas, the method including: a) compressing at a first pressure a mixture of air and target gas to produce a compressed target gas mixture, wherein the target gas concentration in the mixture of air and target gas is kept below about 40 g/m.sup.3; b) destroying the target gas by combusting the compressed target gas mixture with fuel in a forced-induction internal combustion engine, at a combustion pressure greater than the first pressure, to thermally decompose the target gas and produce an oxidised exhaust gas, the combustion occurring while maintaining a load on the engine with a load bank; and c) processing the oxidised exhaust gas to produce a vent gas for venting to atmosphere where the vent gas includes substantially no target gas.

2. A method according to claim 1, wherein the internal combustion engine is a diesel-cycle internal combustion engine and the fuel is diesel fuel.

3. A method according to claim 1, wherein the combustion occurs while maintaining an electrical load on the engine with the load bank.

4. A method according to claims 1, wherein heat generated by the load bank is used to facilitate gas fumigant desorption where the target gas is a fumigant that is being destroyed after use in a fumigation process.

5. A method according to claims 1, wherein the heat generated by the load bank is used to raise ambient air temperature in a space being fumigated to allow a more rapid desorption of the fumigant from the fumigated product.

6. A method according to claims 1, wherein the target gas is methyl bromide, phosphine or sulfuryl fluoride.

7. A method according to claim 1, including the use of a pre-filtration system prior to or after the mixing of the target gas with air.

8. A method according to claim 1, including the use of fixed atmospheric and target gas sensors before compression to constantly measure the respective incoming gas concentrations.

9. A method according to claim 1, wherein target gas is mixed with atmospheric air prior to compression in a target gas mixer controlled by a process control system that controls the ratio of target gas to air to form the desired target gas mixture.

10. A method according to claim 1, wherein the desired target gas mixture includes a minimum 3.5% oxygen content.

11. A method according to claims 1, wherein the target gas concentration in the target gas mixture is kept below about 35 g/m.sup.3, or below about 30 g/m.sup.3, or below about 25 g/m.sup.3, or below about 20 g/m.sup.3.

12. A method according to claims 1, wherein the mixture of the target gas and air is compressed to a first pressure of between about 16 psi and 18 psi.

13. A method according to claims 1, wherein the compression of the target gas mixture occurs in a turbocharger.

14. A method according to claim 13, wherein the temperature of the target gas mixture will be increased to at least 550 C. in the turbocharger.

15. A method according to claims 1, wherein the target gas mixture is compressed in the combustion chamber (prior to combustion) to a second pressure within the range of 25 to 35 atm.

16. A method according to claims 1, wherein the combustion pressure in the combustion chamber during combustion is within the range of 50 to 65 atm to produce the oxidised exhaust gas.

17. A method according to claims 1, wherein flame temperatures in the combustion chamber during combustion reach between about 2,600 to 2,700 C.

18. A method according to claims 1, wherein combustion chamber temperatures are in the range of 600 to 700 C.

19. A method according to claims 1, wherein the oxidised exhaust gases exit the engine through a turbocharger and exit the turbocharger at a lower temperature.

20. A method according to claim 19, wherein the oxidised exhaust gases exit the turbocharger at a temperature below 570 C.

21. A method according to claims 1, wherein the oxidised exhaust gases are cooled to a temperature below 100 C. prior to processing step (c).

22. A method according to claims 1, wherein the processing of the oxidised exhaust gases includes a wet scrubbing stage that utilises a plurality of water-based scrubbers, together with a cooling system designed to maintain the scrubbing solution to an optimum temperature of about 50 C. to 60 C. and a desalination stage.

23. A method for the destruction of a non-combustible target gas, the method including: a) continuously controlling the ratio of atmospheric air to target gas to maintain the target gas concentration in a target gas mixture below about 40 g/m3; b) compressing at a first pressure the target gas mixture to produce a compressed target gas mixture; c) destroying the target gas by combusting the target gas mixture with fuel in a forced-induction internal combustion engine, at a combustion pressure greater than the first pressure, to thermally decompose the target gas and produce an oxidised exhaust gas, the combustion occurring while maintaining a load on the engine with a load bank; and d) processing the oxidised exhaust gas to produce a vent gas for venting to atmosphere where the vent gas includes substantially no target gas.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will now be described in relation to a preferred embodiment as presented in the schematic flow diagram of FIG. 1, and a worked example of that embodiment.

DETAILED DESCRIPTION

(2) Illustrated in FIG. 1 is a schematic flow diagram showing a preferred embodiment of a target gas destruction method and apparatus, referred to hereafter as a Gas Destruction Unit (GDU) in accordance with the present invention. The central functions are identified broadly as follows: Box Atarget gas inlet 12, with target gas filtering Box Batmospheric air inlet 14, air filtering, target gas and air mixing, forced induction diesel engine Box Cload bank, with useful heat outlet 16 Box Dscrubber system, with vent gas outlet 18 to atmosphere Box Eoptional cooling, with useful heat outlet 20 Box Foptional desalination

(3) Referring now to each of these central functions in turn, in Box A there is provided a filter assembly where target gas from, for example a fumigation chamber, enters the GDU via a flexible tube inlet 12, in this embodiment containing high volume removable filter elements to trap solids and excessive moisture.

(4) Located in Box B with the atmospheric air inlet 14 are air and target mixing valves and fixed sensors for oxygen, explosive atmospheres, and the target gas, to constantly measure respective incoming gas concentrations and relay data to digital/analogue read outs/gauges, and in the case of the oxygen sensor, provide an electronic signal to a Process Control System (PCS) to ensure that the engine of Box B (see below) receives adequate oxygen to permit normal combustion irrespective of the oxygen levels in the target gas being sourced, which for some target gases will be quite low.

(5) Air compensation valves (ACV) are also located on or adjacent to the engine in Box B, controlled by the PCS and actuated by the sensors described above. The valve aperture is totally closed under normal operating conditions, and will open and close proportionally in real time to adjust the air/target gas mixture to compensate for any corresponding lack of oxygen in the incoming target gas.

(6) Further in relation to Box B in FIG. 1, the internal combustion engine in this embodiment is a diesel engine. Suitable diesel engines are of course of varying cubic capacity, which is largely dependent upon the size of fumigation chamber they will be required to evacuate, and the time allotted to do so. The diesel engine of this embodiment includes forced induction via a turbocharger that compresses the mixture of gases to a first pressure between about 16 and 18 psi, to boost the combustion temperature by increasing the amount of air in the combustion chamber available for combustion, resulting in peak flame temperatures in the order of 2,600 C. to 2,700 C. and an ultimate combustion temperature range of from 600 to 700 C. at a combustion pressure in the combustion chamber after ignition of preferably about 60 atm, operating at a constant 1500 rpm.

(7) The chemical equations for the combustion chamber are thus:

(8) TABLE-US-00001 Main Products into Compound Scrubber System (Box D) Diesel H.sub.2O + CO.sub.2 Hydrocarbon + O.sub.2 + Heat Methyl Bromide H.sub.2O + CO.sub.2 + HBr CH.sub.3Br + O.sub.2 + Hydrocarbon + Heat

(9) The combination of compression-related air temperature increase, and the combustion of diesel fuel, thermally decomposes/combusts the target gases in the combustion chamber of the diesel engine in a discrete combustion process that is safe to operate when dealing with atmospheres below their respective Lower Explosion Limits (LEL). The effect of pressure on the combustion of gases is such that doubling the pressure, doubles the rate of chemical reaction.

(10) In this embodiment, the diesel engine also supplies electrical power to the load bank of Box C and other ancillary electrically powered liquid pumps (not shown) and/or heating or refrigeration units (Box E) if they are utilised. The load bank provides a load to the diesel engine, in this embodiment an electrical load, which prevents cylinder bore glazing, and raises engine combustion temperatures via the increased fuel/air mixture volumes in the cylinder/combustion chamber necessary to power the load, and is a source of heat supply 16 to, for example, facilitate gas fumigant desorption in a fumigation chamber by raising the ambient air temperature in that chamber to allow more rapid desorption of the target gas (a fumigant) from the product that has just been fumigated. The heated air is ducted from the load bank to the fumigation chamber air inlet point via fixed or flexible tubing structures from outlet 16.

(11) In this embodiment, the load bank is an apparatus that applies a load to the engine by passing electrical energy through a series of metal resistor banks (which convert the electrical energy generated by the generator into heat during the process) all of which is preferably electronically controlled to take account of any other electrical load applied to the diesel engine, and reduce or increase resistance levels (load) to the diesel engine accordingly to ensure only the pre-programmed load is applied to the diesel engine.

(12) The exhaust gases from the diesel engine transport the heat and waste gases from the combustion process (which includes some highly toxic by-products of the target gas) via the exhaust pipe 22 to the subsequent processing units (Box D), and specifically to the first scrubbing chamber 24 located in the scrubbing tank 26. Heat from this source is used to raise the temperature of the scrubbing solution in the scrubbing tanks to between 50 C. and 60 C. to speed up the molecules within the scrubbing solution to facilitate molecular interaction between the exhaust gases and the scrubbing solution.

(13) In another embodiment, the exhaust gases may be exhausted from the combustion chamber through the turbocharger, which in turn drives the compressor turbine that pressurises the induction air to the diesel engine, and continues through a diverter valve that diverts the gas flow to either the scrubbing system of Box D or to atmosphere, depending upon the exhaust gas content.

(14) In this embodiment, incoming exhaust gases for Box D are cooled to approximately 100 C. (max) just prior to entry so as to minimise the incoming air temperature to avoid excessive evaporation, but remain above the condensation point of the gases to avoid formation of potent acids in the exhaust pipe. Cooling of the exhaust can be effected by using a thermostatically operated cooling fan to increase airflow over a naked (smooth) or finned exhaust pipe (not shown). The cooled exhaust air then enters the scrubbing system of Box D and is immediately quenched (sprayed) using a water based scrubbing solution which is drawn (pumped) from a scrubbing fluid reservoir occupying the bottom section. Once again, though, it should be appreciated that pre-cooling of this type is optional and need not be utilised.

(15) Box D includes the scrubbing chamber 24 and the scrubbing tank 26, which include a series of jets on multiple spray bars, such that the scrubbing solution cools incoming exhaust gases and initiates liquid scrubbing processes. The rapid cooling of the gas also inhibits nitric oxide and nitrogen oxide production, which are both significant global warming gases. Additionally, the scrubbing medium (water) interaction via the series of spray jets allows conversion of the exhaust gases into acids of the gaseous constituents. Other processes involved include physical contact with a calcium carbonate aggregate media to permit both phase transfer from gas to liquid (acid) and the resultant acid/base reaction to neutralize the acids and effect conversion to salts of the acid/base constituents. Heat exchange coils may or may not be installed in the tank 26 to control scrubbing liquid temperatures. A moisture trap (filter) is also located above the scrubbing tank medium to minimize the vapour loss from the tank.

(16) Vent gas 18 exits the scrubbing system of Box D, in this embodiment with the use of an optional fan 32 in order to maintain a negative internal air throughout the scrubbing system, and also through a particulate filter (not shown). Also, fixed sensors 34 for the target gases are located in the outgoing airstream and are linked to the engine control system to stop the diesel engine instantly if a target gas reading is detected by the sensor as being outside pre-set parameters.

(17) The optional Box E includes an external radiator bank that provides cooling energy to a heat exchanger coil located within the scrubbing fluid of the scrubbing tank 26 to maintain a constant 50 C. to 60 C. in order to prevent the scrubbing solution from boiling. As with the load bank, heat can also be ducted from either the refrigeration unit or radiator bank air outputs 20 to a fumigation chamber as described above, particularly for large scale fumigation chambers. If this extra heat source is not required, the heat is just dissipated to atmosphere.

(18) Finally, optional Box F is a desalination unit that desalinates scrubbing fluid and returns desalinated water to the scrubbing tank 26 as well as drying and crystalizing concentrated brine into harmless salt variants of the target gas. This desalination stage is contiguous with the scrubbing stage, so there is a continuous desalination process occurring while scrubbing solution is pumped through spray nozzles. The spray nozzles (not shown) in Box F spray a salt laden solution onto a hot surface in order to evaporate off the water component of the solution, leaving the crystallised salts on the surface to be scraped off and collected in a hopper ready for disposal. In this embodiment with optional Box F, resultant steam is collected, condensed into water and returned to the scrubbing tank 26, with carbon dioxide also being diverted back into the scrubbing system.

(19) By way of worked example, the following system was tested, giving rise to the data of Table 1.

(20) The target gas used was 100% methyl bromide having a concentration of about 7,710 ppm (30 g/m.sup.3) and a thermal decomposition temperature of 537 C. The target gas/air mixture was set to achieve an incoming target gas concentration of between 2,500 and 5,200 ppm at ambient air temperature.

(21) The internal combustion engine was a 4 cylinder, 3.9 L, turbocharged diesel engine; the generator was rated at 415V/42 kVa/50 Hz (electronically or mechanically governed); the load bank was rated at 415V/25 kW/50 Hz with an automatic 3-stage load ramp; and the pump was a 3-stage centrifugal pump capable of delivering a flowrate of 200 LPM at 105 kPa.

(22) With the load bank maximum load set at 75 to 80% of the generator capacity and the generator operating at 1500 rpm, the turbocharger boost pressure was in the range of 16 to 18 psi, the compression pressure was between 25 and 35 atm, the combustion chamber temperature was at 550 C., the ignition pressure was between 50 and 65 atm, and the ignition flame temperature was between 2,600 and 2,700 C.

(23) The scrubber system was a 4-chamber, contra-flow, scrubbing tank having a water capacity of 880 L, 1.38 m.sup.3 of CaCO.sub.3 aggregate and a design air flowrate of 2.5 m.sup.3/min. The operational pressure range was 1 to 4 kPa, the operational temperature range was 30 to 60 C., and the operational pH range was 6.3 to 6.9. The oxidised exhaust gas exiting the diesel engine was the input to the scrubber tank and contained hydrogen bromide, carbon dioxide, carbon monoxide, nitrogen oxide, nitric oxide and diesel particulate matter (DPM). The outputs from the scrubber tank were carbon dioxide, water, diesel particulate matter (DPM) and calcium bromide salt.

(24) In relation to gas sensors, gas sampling and gas analysis, the sensors used were IR/Electrochemical sensors, 24V DC with high and low range and gas specific capability (set to sense for methyl bromide), with remote access and data logging. The gas sampling equipment was a Drager metered pump (X-act 5000) with sampling set at 10 minute intervals. Four Drager activated charcoal sample tubes were used for analysis and analysis was conducted by gas chromatograph in independent accredited laboratories. Exemplary amounts of methyl bromide detected, noting the initial methyl bromide concentration of about 30 g/m.sup.3 were:

(25) TABLE-US-00002 TABLE 1 Target Gas Sample Target Gas Target Gas Detected in % ID Concentration Concentration Vent Gas Target Gas No. (ppm) (g/m.sup.3) (ppm) Destroyed 2 7710 30.0 NIL 100.0 4 7710 30.0 NIL 100.0 6 7710 30.0 <1 >99.99 7 7710 30.0 NIL 100.0

(26) From the data of Table 1, the method and apparatus of the embodiment destroys up to 100% of the target gas by converting it into gases containing hydrogen and the base element of the chemical that was destroyed, in this worked example being bromine from methyl bromide, resulting in hydrogen bromide in the oxidised exhaust gas exiting the diesel engine.

(27) Finally, there may be other variations and modifications made to the configurations described herein that are also within the scope of the present invention.