Patent Application
20130174739
1. Field of the Invention
The invention relates to regenerable inorganic oxides, mixed oxides and/or layered double hydroxides for the capture of CO2 emissions from engine exhaust streams through repeated adsorption—desorption cycles under dynamic temperature fluctuations during the engine operation cycles.
2. Description of the Related Art
A current problem with tremendous implications at various levels is the CO2 emission from fuel combustion in different sectors of industry. Aiming to reduce these emissions, the concept of CO2 capture and storage (CCS) has been introduced in practice and important projects are under development or have been implemented already in industry. This concept deals with the CO2 capture from various sectors of industry, its transport through pipelines or tankers and its storage into deep saline aquifers, depleted oil and gas reservoirs or unmineable coal seams.
In the case of gas-fired power station we can talk about post combustion, pre-combustion or oxyfuel capture of CO2. In the first case, the CO2 evolved is captured just after combustion using amines columns. The pre-combustion capture process is based on two main steps which involve the generation of syngas (H2 and CO2) and the separation of CO2. A result is the utility of H2 as “clean fuel”. The oxyfuel capture aims to burn the fossil fuel in pure O2 which will generate a stream of CO2 and water.
Due to the rigorous international legislations regarding the CO2 emissions from engine gas exhausts, which aim a reduction of these emissions down to an average value of 130 g/Km by 2012-2015 and 95 g/Kg by 2020, the attention drawn into the direction of finding new solutions for this problem. An analogy to the big power plants can lead us to the idea of using amines for such purposes, but their major drawbacks are the high toxicity, physical form, operating temperatures and cost inefficiency making them inappropriate.
The present inventions aims to provide solutions to reduce the CO2 emissions from engine gas streams such as gasoline, lean gasoline, diesel type engines and fuel cells (both stationary and mobile) using regenerable inorganic solid sorbents to capture then CO2 in controlled way, rather than through the traditional methods such as improving fuel consumption and engine management systems (JP2005/016369).
In order to be suitable for such applications the materials of interest must posses some properties to make them useful in the pressure and temperature conditions offered by each type of exhaust streams. Adsorbents, which can be successfully used for CO2 reduction from engine exhausts due to the high flexibility of the processes involved here, are the inorganic solid materials such as alkaline or alkaline-earth metal oxides (CaO, MgO, Na2O, K2O) and promoted or unpromoted complex metal oxides (Li2ZrO3, Li2SiO3, Li4SiO4, Na2SiO3, Li2-xKxZrO3, Ca2SiO4) and hydrotalcites.
The applicability of each of these systems for CO2 reduction presents advantages and disadvantages, the later ones being easily approached by tailoring the composition of each system taken separately or a combination of them.
Thus, alkaline or alkaline-earth metal oxides posses high capacities and recyclability, but present the disadvantage of having high desorption temperatures (˜850° C.). This will make them high-energy consumers when recycled. Complex metal oxides, as ones mentioned above, present high capacities and operation temperatures, but slow kinetics which can make them difficult applicable when the aim is to capture the CO2 from continuous gas streams such as engine exhausts. Also, both previous oxides present the advantage of having high selectivity for CO2.
Among the complex metal oxides, lithium zirconate has attracted much interest for CO2 uptake. The mechanisms involved are well studied and are based on the formation of an external lithium oxide external shell when heated. As well as a physical adsorption on the surface at low temperatures this oxide will be converted into lithium carbonate when in contact with a CO2 containing steam at elevated temperatures (eq. 1). These materials are operational from room temperature, where only a physical adsorption of CO2 appears, up to 500° C. and can be reactivated at 750° C.
Li2ZrO3⇄(Li2O+ZrO2)
(Li2O+ZrO2)+CO2⇄Li2CO3+ZrO2
Due to the migration of lithium during the reaction, the preferred positioning of such a sorbant in the exhaust system would be after the catalysis sections to avoid potential poisoning of catalyst due to lithium. Alternative inorganic oxide systems which rely on chemical reaction to capture the carbon dioxide like lithium zirconate but do not contain potential catalysis poisons such as lithium (e.g. Ca2SiO4) could be positioned upstream of the catalyst sections where temperatures may be more favourable for the reaction.
Over the last decades hydrotalcite-type compounds or layered double hydroxides have attracted much attention due to their exceptional properties, which make them suitable for a wide variety of applications such as, catalysis, wastewater treatment, acidic gaseous streams cleaning, polymer stabilizers, flame retardants.
Layered double hydroxides are crystalline structures resulted by partially replacement of Mg2+ cations in the brucite type layers by cations having the same valence, but different sizes or superior valent cations. These will generate an excess of positive charge, which will be accompanied by the presence of various anions, together with water molecules, at the level of interlayer space. When heated to elevated temperatures, both water and anions are released, the corresponding mixed oxides being generated. If the thermal treatment is conducted at temperatures below 600° C. these mixed oxides have the capacity to reform the initial structure when in contact with an anionic source. This phenomenon is known as “memory effect” and it is the milestone of the application directions of these types of materials. When heated above 750° C. a mixture of spinel and MgO can be obtained, this process being irreversible. TG/DTA curves recorded for these types of materials indicated that the dehydration step usually appears between 100-300° C. and is associated with the removal of both physi- and chemisorbed water from the interlayer space. This first mass loss is followed by a second process, which corresponds to the dehydroxilation of the octahedral layers. This second mass loss is not very well defined being overlapped with the removal of the interlayer anions and occurs at temperatures ranging between 350-500° C.
Hydrotalcites are interesting materials as they ensure a very fast uptake of any acidic component from a gas stream having a high affinity for CO2 in normal conditions. Their main drawback is their relative small capacity compared to other inorganic sorbants such as lithium zirconate, but this can be improved by doping with other elements such as potassium, magnesium or cocktails of various metal oxides. In terms of anions up-take these types of materials can operate both uncalcined and calcined, in the first case an anionic exchange being possible while in the second case the memory effect of these materials is involved. Another way to improve the capacity of these materials is the possibility to increase their charge density and implicitly their basicity by incorporating in the brucite-type layers elements having superior valences such as Zr4+ or other tetravalent cations.
The CO2 capture mechanisms can involve either the ionic exchange of the inter-layered anionis, other than carbonate, or the reformation of the layered structure using the “memory effect” principle.
These types of materials have been successfully used for removing of nitrogen oxides and sulphur oxides from car exhaust gas by storage reduction. The US patent 2009/0257934 makes a comparison between various transition and rare-earth cations containing hydrotalcites for nitrogen and sulphur dioxide removal. It has been proven that cerium containing hydrotalcites, for example, posses a high capacity for SO2 storage, while the materials made using sulphated precursors possess better capacities for NO2 storage.
Also, patent AU2010202087 describes the reduction of gasoline sulphur using catalysts obtained by calcinations of various hydrotalcites containing only divalent and trivalent cations.
US 2004/0081614 A1 patent describes the applicability of Mg/Al—type hydrotalcite for CO2 up-take and catalyst in a sorption enhanced water gas shift reaction to produce pure hydrogen. In the same line, US patent 2010/00112895 presents the applicability of Cu-containing hydrotalcites as chromium free catalysts for low temperature conversion of carbon monoxide and water into carbon dioxide and hydrogen. In both these applications the nature of the contacted stream is different to the proposed application in that the complexity of the exhaust stream is higher than the one of a H2/CO2 stream, while the temperature fluctuations are high allowing concomitant adsorption/desorption processes.
U.S. Pat. No. 5,616,430 describes a fuel cell system, which allows the capture of CO2 by zeolites as sorbents. The major drawback of these systems is their sensitivity to water which requires to be removed from the system prior the adsorption. Also, these systems are not operational in the temperature ranges which are characteristic to gas exhaust. As it has been shown in the above-mentioned patent the activity decreases with the increase of the temperature, and, as other literature states (Separation and Purification Technology 2002, 26, 195-205), they become inactive when heated at temperatures above 230° C.
The Japanese patent JP8206432(A) claims a material having a general formula of [M2+1-xM3+x(OH)2]x+An-x/n and mH2O to purify the exhaust gas streams by volatile inorganic matter hydride, halogen gaseous components and CO2 by activation at 200° C. for 2 hours. The mechanism involved here is different to the current application in that it is physic—sorption, (formation of weakly bound carbon species) and hence makes the materials inapplicable for the dynamical condition of a combustion process.
RU2359751 (C1) patent claims an synthetic procedure for ZnO/Zr(OH)4 preparation which can be for carbon dioxide decontamination of the atmosphere in hermetical objects. The technique exploits the ability of zirconium hydroxide to form carbonates (ZrO(OH)CO3) when in contact with CO2 in hermetical environments (humidity free). The material can be regenerated by contacting it with humidified air. The proposed material has limited applicability and would not be suitable for the dynamical conditions associated with a combustion exhaust stream where humidity is present.
US6521026B1 proposes the utilisation of lithium zirconate and alkali and earth alkaline metal oxides as CO2 sorbents in a rotor-type system which consists in high and low temperature sections on its circumference. The system is designed to be utilised to clean up the CO2 emissions from combustion boilers exhaust streams. In the proposed system, the CO2 is discharged internally on a heat exchange principle, generating a second gas stream. The method might be suitable for steam generating boilers, but would have limited suitability for some internal combustion engines from stationary and mobile sources as they are energy intensive. In the car exhaust systems for example the dynamicity is different, especially not constant, so it would be difficult to use such a system as the periods when the CO2 can be desorbed would be more or less frequent.
JP2005016369 describes pressure fluctuations in the exhaust as a feedback combustion controller. This controller determines the amount of intake air that is required for the combustion of fuel, and is therefore different to the proposed application. The current application is about true storage of CO2 for the purpose of reducing the CO2 emissions for environmental purposes, whereas JP2005016369 discusses CO2 as an indication of combustion efficiency and or regulated emission control.
The flexibility in the design of hydrotalcites lends them to be potentially bi-functional sorbants, removing both carbon dioxide and additional gases from exhaust systems such as NOx or SOx either fully from the system, or to be released again at certain temperature ranges to aid other steps within the exhaust system e.g. SCR or NOx traps. The lower temperature range for operation compared to other inorganic sorbants means they could be particularly suitable for cold start applications, or lower operating temperature systems such as diesel engines. Their extremely fast kinetics may also lend them to lower CO2 concentrations applications.
The invention relates to regenerable and water tolerant inorganic oxides and mixed oxides for the capture of CO2 emissions from engine exhaust streams through repeated adsorption—desorption cycles, the aim being to reduce the CO2 emissions by 10% or more.
The present invention deals with the applicability of various solid sorbents to reduce the CO2 emissions from engine exhaust systems. The invention can be applied to any engine exhaust system, including but not limited to: gasoline, lean gasoline, diesel passenger car, diesel truck, stationary engine with 50 Hz or 60 Hz electrical frequency, or a SOFC data (Solid Oxide Fuel Cell), powered by methane. The concept proposed here is to use solid sorbents, which can be positioned along the exhaust systems to capture partially or totally the CO2 evolved during the fuel combustion. Preferably the overall CO2 reduction will be greater than 5 wt % and even more preferably by 10 wt % or more. The sorbent system would preferably be replaceable and recyclable. This allows for the capture and storage of CO2 until the kit can be replaced with an empty one, regenerated, or when the stored CO2 can be offloaded onto an external system, for example transferred into a pressurised storage system.
The inorganic regenerable sorbent material can be housed at varying points within the exhaust system depending on the specific engine and the specific temperature requirements of the given material. It can be present as pellets or granules in a canister through which the exhaust gas if passed, or coated onto a substrate, in the latter case a fine particle size may be required to obtain optimal adhesion. The positioning will also allow for onboard regeneration by switching between high and low temperature gas flows to alternate between sorption and desorption cycles. It is anticipated that sensors will control the gas flow and temperature to switch between sorption and desorption cycles. The positioning will also have to take into account the presence of additional catalytic systems, to ensure no potential for poisoning of downstream catalysts.
The optimal system is likely to have more than one capture system so that simultaneous adsorption and desorption cycles can take place, the weight of sorbent can be minimised, and the captured CO2 can be stored on board either on sorbent, or in additional storage devise, such as pressurised cylinder, which can then be emptied or regenerated at set intervals for example during refuelling. It is expected that for mobile engines such as passenger cars or trucks, the maximum weight of inorganic sorbent at capacity will be no more than the weight of fuel for that given system. For stationary engines where weight and size issues are less of a constraint, greater quantities of sorbent can be used.
It is anticipated that for some applications a mixture of regenerable inorganic sorbents will be used to cover a wider operation temperature range, for example a low temperature sorbent with fast kinetics for cold start conditions, then a higher temperature sorbent for normal or extreme running conditions such as high speeds.
The preferred solid inorganic sorbent material would be one containing a zirconium compound greater than 1 wt %, preferably with zirconium oxide content between 1 wt % and 90 wt %. Even more preferably the used regenerable inorganic sorbent materials will be from the class of materials described as hydrotalictes, lithium zirconates or modified lithium zirconates, as the ones reported in WO2007/023294, either individually, combined or doped with additional promoters.
Specifically the preferred classes of sorbent would be chosen from, but not limited to lithium zirconate and modified lithium zirconates and M(II)xM(III)yM′(III)z(OH)aAb.cH2O (I) and M(II)xM(III)yM(IV)z(OH)aAb.cH2O (II) type materials, wherein,
Before utilization the solid sorbents have been activated under specific temperature conditions. (essentially performing an initial desorption or regeneration cycle) Thus, the hydrotalcites have been heated at temperatures between 350-500° C., preferably for 0.5-24 hours, while lithium zirconates and modified lithium zirconates are activated by heating at temperature greater than 700° C., preferably at 750 ° C. for between 0.5-24 hours.
In accordance with another aspect of the present invention, the methods described here aim to investigate the synthesis and applicability of BxHyCO3 promoted (doped) above mentioned materials for CO2 reduction in engine exhaust streams, wherein, B═K, Na, Mg, etc., x=1-2, y=0-1 without contributing at the enhancement of the sorbent regeneration temperature.
FIG. 1—The bottom graph (a) of
FIG. 2—The bottom graph (a) of
FIG. 3—The bottom graph (a) of
FIG. 4—This figure shows an example set up for use of sorbent system in engine exhaust. This system works by having a temperature sensor in each sorbent bed. When the temperature starts to go outside the operating range of either sorbent then the controller will operate the valves accordingly.
FIG. 5—This figure shows how the hydrotalcite structure can be recovered after a sorption cycle at 500° C.
The assumptions used when modelling the processes disclosed in the examples are listed in Table 1. In the case of lithium zirconate; end use will depend on the position of the sorbent in the exhaust system. The chemical sorption process occurs between 450 and 700° C. with desorption above 700° C. The assumed rate of adsorption is 0.83 wt %·min−1 while the rate of desorption is 1.57 wt %·min−1. The capacity of the material is estimated being 29.87 wt %, as discussed in Chem. Eng. J. (2009), 146, 249. Synthesis of lithium zirconate is shown in WO 2007/023294. For the purpose of this model we have ignored any physisorbed low temperature CO2 uptake. Table 2 summarises the results for lithium zirconate in a range of different exhaust systems.
For hydrotalcites, use will depend on the precise vehicle application and available location within the exhaust system. The adsorption process occurs at lower temperatures from room temperature to 450° C. with desorption above 450° C. Two sets of assumptions have been used. The first assumption for melsorb 1679 which is a zirconium containing hydrotalcite, the rate of adsorption and desorption is equal at 8.3 wt %·min−1 (estimated at 10× faster than the lithium zirconates). The capacity of the material is estimated at 2.28 wt % measured at room temperature on TGA. The second is a non-zirconium hydrotalcite and has a rate of adsorption 5.7 wt %·min−1, desorption 2.85 wt %·min−1 and a capacity of 5.74 wt %, this data is based on room temperature TGA measurements.
Table 3 summarises the results for the given hydrotalcites in the same range of different exhaust systems as for lithium zirconate in Table 2.
If we consider a conventional gasoline passenger car on the NEDC as shown in
To remove the target level of 10% of the CO2 from the exhaust stream over the NEDC (206.2 g) then 732 g of sorbent will be required.
If the same lithium zirconate sorbent is placed in a position T2 (post 3-way catalyst) of the stoichiometric gasoline passenger car exhaust system, where the gas stream has temperatures less than in point T1, then to 10 remove 10% of the CO2 from the exhaust stream during this drive cycle (206.2 g) the amount of sorbent increases slightly to 781 g, while its ability to desorb during the cycle remains high.
In the case of a lean burn combustion mode gasoline engine the temperatures of the exhaust gas stream are lower than in the previous case and the positioning of the sorbent can be tailored to increase its efficiency for CO2 removal. Thus, as it can be observed from
To capture the target level of 10% of the released CO2 over the drive cycle (228 g), to minimise the weight of sorbent, the most efficient use would be the Zr-hydrotalcite at position TC_17, for which only 180 g would be required, however this would assume a method of onboard storage of CO2 in separate system to exploit maximum potential.
As suggested by
A combination of two types of sorbent could improve efficiency, one of them (like lithium zirconate) being placed in a chamber where the temperatures of the exhaust streams are higher and the second one (e.g. a hydrotalcite type material) placed in a second chamber where the temperatures are lower than say 450° C., this could be particularly important for cold start applications.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1015605.7 | Sep 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2011/051755 | 9/19/2011 | WO | 00 | 3/12/2013 |
