I don't think methane at ambient levels (2 ppm) can be combusted in a diesel engine. A typical diesel engine has a compression ratio of around 20, meaning that the air drawn in is compressed by a factor of 20. http://en.wikipedia.org/wiki/Diesel_engine#How_diesel_engines_work As a result, the temperature of the compressed air is raised to around 1000 degrees F, high enough for the diesel fuel to autoignite. Thus, no need for spark plugs for a diesel engine. Unfortunately, the ambient concentration for autoignition of methane is about 5% or around 50,000 ppm and at 1000 degrees F. In the diesel combustion chamber, it will be around 40 ppm, ambient nominally 2 ppm. So while a 5% methane in air fuel would probably burn in a diesel engine, lower levels would not.
This issue has been addressed in studies conducted for the USEPA as part of the Coalbed Methane Outreach Program, run out of EPA's Climate Protection Division. http://www.irgltd.com/Resources/Publications/US/Technical%20and%20Economic%20Assessment%20-%20Mitigation%20of%20Methane%20Emissions%20Coal%20Mine%20Ventilation%20Air.pdf The goal of this study was to determine ways to use methane from coal mine ventilation air as fuel at levels above 3000 ppm. One of the units tested was able to combust methane at levels as low as 800 ppm. As noted on page 3, "Below 4.5%, methane will not ignite or sustain combustion on its own without a constant ignition source, unless it can remain in an environment where temperatures exceed 1832 degrees F. Therefore, any conventional method proposed to use ventilation air as a fuel or even to destroy it, would require a net energy input in addition to the fuel value of the methane contained in the ventilation air." Internal combustion engines and solar gas turbines can burn low levels of methane as ancillary fuel, but won't operate on ventilation gas alone as the temperatures don't get high enough to combust the methane. To burn ventilation air containing methane as a primary fuel, a thermal flow and a catalytic reactor were evaluated. Both require an external source of electricity to provide the initial heat for combustion, with the catalytic system operating at lower temperatures of around 700-1500 degrees F vs. 1832 for the other one (page 19). So, it doesn't appear that it is possible to combust ambient levels of methane using a diesel engine or any other source using the methane as the primary fuel. Compressing the low levels of methane in ambient air to 1000 ppm, about the lower limit of the catalytic system, would require a compression ratio of 500. Even if this could be overcome, in my opinion, the extremely large volumes of air required would preclude the use of such a system to mitigate ambient methane. The better approach would be to limit the sources of methane emissions including feedbacks to prevent a runaway outcome for which there is also no mitigation technology. ----- Original Message ----- From: "Andrew Lockley" <[email protected]> To: "David Schnare" <[email protected]> Cc: <[email protected]>; "geoengineering" <[email protected]> Sent: Wednesday, January 28, 2009 9:56 AM Subject: [geo] Re: methane air capture > > You don't need a licence to squash air. I'm not proposing a fuel - > I'm proposing to drive the diesel engine with windpower. > > A > > 2009/1/28 David Schnare <[email protected]>: >> In the U.S., use of a compresion ignition engine requires certification >> of >> both the fuel and the engine (by EPA), and limits the amounts of priority >> pollutants that may be emitted from such an engine. These include NOx, >> SOx >> and particulates, all of which will emerge from the scheme you are >> discussing. >> >> In a regulatory state, nothing is as easy as it seems. >> >> David Schnare >> >> >> On Tue, Jan 27, 2009 at 8:33 PM, Andrew Lockley >> <[email protected]> >> wrote: >>> >>> You don't need a combustible fuel-air ratio provided that the >>> combustion doesn't need to be self-sustaining. Once the correct >>> temperature is reached, any methane present will oxidise. The >>> advantage of using a diesel engine is that it runs with minimal energy >>> input as the temperature can be changed without irrecoverable energy >>> input - the mix cools as it expands. I thought about using a jet >>> engine - essentially an adapted turboprop or high-bypass turbofan, but >>> I think it would be more lossy. >>> >>> I don't agree that you'd be processing 'a few hundred cc'. I envisage >>> building vast arrays of wind turbines, all connected to huge marine >>> diesel engines. >>> >>> Why would you need a catalytic convertor? The CH4 just oxidises to >>> H20 and Co2. I can see the benefit of a heat exchanger, and I already >>> thought of that. >>> >>> I covered the issue of hydroxl radical - it's created by ozone >>> photochemistry, so the best way to manipulate it seems to be by >>> delivering ozone to the stratosphere. >>> >>> A >>> >>> 2009/1/28 dsw_s <[email protected]>: >>> > >>> > Compression ignition requires a suitable ratio of fuel to air. Even >>> > if compression in a diesel engine perfectly removed methane from the >>> > air, you're not going to process the atmosphere a few hundred cc at a >>> > time. To remove methane from the air, I see two options: increase the >>> > amount of hydroxyl radical if there's enough methane to deplete it, or >>> > as you say build air-cooled CSP plants. For the CSP option you would >>> > want a counter-flow heat exchanger and a catalytic converter on the >>> > outgoing air. >>> > >>> > On Jan 27, 2:03 pm, Andrew Lockley <[email protected]> wrote: >>> >> If you fixed up diesel engine to a wind turbine, you'd get >>> >> compression >>> >> ignition of any methane residue in the atmosphere, even without >>> >> injecting any fuel. This would be expensive, but I think it would >>> >> work. >>> >> >>> >> An alternative would be to pump air through concentrated solar power >>> >> plants >>> >> >>> >> Any thoughts? We appear to need some bright ideas on methane >>> >> remediation pretty soon. >>> >> >>> >> A >>> > > >>> > >>> >>> >> >> >> >> >> -- >> David W. Schnare >> Center for Environmental Stewardship >> > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. 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