Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2002-03-19
2004-02-24
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213200, C423S213500, C423S213700, C423S215500, C423S239100, C423S245300, C423S247000
Reexamination Certificate
active
06696031
ABSTRACT:
This invention relates to treatment of exhaust gas, in particular from internal combustion engines.
Certain types of such engines, especially lean-burn engines such as diesels, produce exhaust gas containing inter alia nitrogen oxides (NOx) and combustible particulate (soot). The exhaust gases from lean-burn engines are net-oxidising, making the reduction of NOx to N
2
more difficult than in a gasoline engine system where approximately equivalent amounts of oxygen and fuel are used. Removal of soot has been made practicable by the Johnson-Matthey Continuously Regenerating Trap (“CRT”™) process (EPA-0341832, U.S. Pat. No. 4,902,487, incorporated herein by reference), in which soot is collected on a filter and oxidised by NO
2
that has been enriched in concentration by a preceding step of NO oxidation.
This process, however, is applicable only to exhaust containing, after the preceding oxidation step, at least enough NO
2
to oxidise the soot. If an engine is operated with low NOx generation a technical problem arises. The low NOx can result form engine design, expedients such as Exhaust Gas Recycle (EGR), temporaty non-normal load or fuel or temperature conditions, deliberate less-lean engine operation or by injection of reductant into the exhaust gas. The prior processes may convert NOx to N
2
to a small extent, for example 3-8%. (Hawker et al. SAE paper 970182).
According to the invention a process for treating combustion exhaust gas containing HC, CO, O
2
, soot and possibly NOx that comprises the steps of:
i oxidising HC to CO
2
and H
2
O, and NO (if present) to NO
2
; and
ii oxidising said soot by reaction with NO
2
:
is characterised by the step of generating NOx by oxidising ammonia (as hereinafter defined) and introducing it into the exhaust gas upstream of step ii. Suitably soot is collected on a filter in step ii, but this is not presently regarded as essential if soot is collected or adheres to the walls or front face of a catalytic component or has an extended residence time in the equipments
Conveniently such ammonia oxidation is effected in step i, using therein one or more catalysts effective alone or together to promote oxidation of HC, CO, NO and ammonia. Preferably HC and CO are oxidised in a first stage within step i and NO is oxidised to NO
2
with the fed ammonia in a second stage. (Such a divided step i is the subject of a co-pending GB application 99.13300.1, part of PCT application GB99/03971).
The term ‘ammonia’ in relation to added reactants includes also other compounds that produce NOx in the oxidising reaction conditions. Thus for example amino- or amido-compounds can be used, for example hydrazine, urea, guanidine, biuret, cyanuric acid, lower alkylamines such as methylamines, and nitroxy compounds. Ammonia itself or any of these can be injected with other materials, such as: (where appropriate) a non-interfering acid such as carbonic acid; another fluid, suitably providing a liquid solution at ambient temperature that may be oxidisable; steam; air.
The introduction of NOx (“first ammonia addition”) may be continuous or intermittent.
Especially since the first ammonia addition increases the NOx content of the gas to a level above that due to the engine, the process preferably includes also, after step ii, a step of NOx removal. Several procedures are available for such NOx removal. The procedures are suitably based on catalytic methods, absorption methods, or a combination of both. Continuous catalytic decomposition of NOx to N
2
uses a lean-NOx catalyst possibly with adjustment of gas composition to equivalence or rich, or with injection of a NOx-specific reactant such as ammonia (using selective catalytic reduction, SCR). Absorption can be long-term (the absorber is eventually removed and regenerated) or short-term (the absorber is regenerated in-line). Regeneration can be achieved using temperature increase, or using the catalytic methods for decomposition of NOx to N
2
. The preferred method of NOx removal is use of an absorbent that contains and/or is followed by a catalyst for the NOx-reducing reaction. The addition of a NOx-specific reactant and the use of SCR to regenerate the absorbent is especially preferred. The provision of NOx specific reactant downstream of step ii will be referred to as “second ammonia addition”.
The source of ammonia for the second addition can be selected from the sources specified for the first addition, except for compounds containing oxidisable carbon radicals or reducible nitrogen radicals, since the second addition is to react with NOx to give N
2
.
Whereas either ammonia addition can be continuous or intermittent, the intermittent/intermittent combination is preferred.
The first ammonia addition may be controlled to provide incomplete reaction or may be in excess, thereby slipping enough ammonia to provide the second ammonia addition.
The composition, temperature and flow-rate of engine exhaust commonly changes with time, for example when establishing steady conditions after start-up, or during changes in power output or chance variation. It is therefore desirable to control the exhaust treatment process to meet such changes. Intermittent first ammonia addition and the resulting NOx addition may be made in response to a signal from sensor means indicating that there is for example
(i) inadequate NOx content in the gas leaving the engine; or
(ii) increase in soot filter pressure drop
which results because an engine has been operating for some time with inadequate NOx or exhaust temperature too low to combust soot on the filter, so that a build-up of soot takes place. The NOx addition is then made until the filter is substantially cleared.
If the oxidation of ammonia is effected over the step i catalyst, and if the engine is operated at varying levels of speed and/or load, such that the exhaust gas temperature varies, the first ammonia addition is made only when the temperature corresponds to formation of NOx from ammonia preferentially to reaction of ammonia with NOx. Since such intermittent ammonia produces an upward step in NOx content, the second ammonia addition will normally coincide in time with it or follow very soon after it. Such addition is controlled in response to detection of NOx exiting the absorber or to the (approaching) end of a time period designed for the absorber. This programmed regeneration period typically lasts 1 to 100 seconds.
The temperatures of the gas at the various stages of the process are controlled as follows:
(i) if the NOx addition is made by oxidising ammonia in step i, the temperature is desirably over 200° C., for example in the range 350°-500° C.;
(ii) for regeneration of a NOx absorber, the temperature should be for example 150°-300° C. This is easier to achieve if the second ammonia addition is intermittent, since intermittently higher temperature may be available in high speed engine running or can be obtained for example by oxidation of intermittently provided hydrocarbon in step i or on a pre-catalyst, typically for a time period of the same order as that of the second ammonia addition.
The catalysts and (if used) absorbent are suitably supported on a ceramic or metal honeycomb, coated with a surface-area enlarging washcoat comprising one or more of alumina, zirconia, silicon carbide or other, generally oxidic, material. Coated on the washcoat, in one or more layers, is the active catalytic and/or absorptive material, to be described in more detail below. The honeycomb has typically 50-400 cells per square inch, possibly more, eg up to 1200. The range 200-900 is of general application.
In the oxidation catalyst the active material comprises generally a platinum group metal (“PGM”), especially platinum and/or palladium, optionally with other PGMs, eg rhodium, and other catalytic or promoting components. The exact composition and structure of the oxidation catalyst is not critical to operation of the invention, and hence may be varied according to the requirements of the situation. A low temperature light-off formulation is generally preferred. Conventional manuf
Brisley Robert James
Twigg Martyn Vincent
Johnson Matthey Public Limited Company
Langel Wayne A.
RatnerPrestia
LandOfFree
Treatment of exhaust gas does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Treatment of exhaust gas, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Treatment of exhaust gas will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3316925