High capacity carbon dioxide sorbent

Abstract

The present invention provides a sorbent for the removal of carbon dioxide from gas streams, comprising: a CO2 capacity of at least 9 weight percent when measured at 22° C. and 1 atmosphere; an H2O capacity of at most 15 weight percent when measured at 25° C. and 1 atmosphere; and an isosteric heat of adsorption of from 5 to 8.5 kilocalories per mole of CO2. The invention also provides a carbon sorbent in a powder, a granular or a pellet form for the removal of carbon dioxide from gas streams, comprising: a carbon content of at least 90 weight percent; a nitrogen content of at least 1 weight percent; an oxygen content of at most 3 weight percent; a BET surface area from 50 to 2600 m2/g; and a DFT micropore volume from 0.04 to 0.8 cc/g.

Description

BACKGROUND

Fossil fuel combustion is increasing the atmospheric concentration of CO₂ and causing concerns over global warming. The electric power generation industry is one of the largest CO₂ emitters; roughly one third of the U.S. carbon emissions come from power plants. There is a need to find cost-effective ways to recover CO₂ from the flue gases of existing power plants and other large point sources, for example refineries and cement plants.

There is also a need to remove CO₂ from natural gas or biogas generated from anaerobic digesters to upgrade the methane concentration so that they can be used in a pipeline, to generate electricity or to run natural gas vehicles, for example.

The methods for post-combustion flue gas CO₂ separation from existing power plants include chemical and physical solvent processes, chemical absorption and physical adsorption using solid sorbents, CO₂ selective membranes and cryogenic separation. Chemical absorption using aqueous solutions of alkanolamines such as monoethanolamine (MEA) is a common CO₂ separation method. However, amine absorption has large capital and operating costs. Large amounts of heat are needed to dissociate the CO₂-amine complex via steam stripping during the regeneration step, resulting in very high energy consumption (Nsakala et al., 2001). The liquid phase contactors and circulation systems are also complex and expensive. Using conventional amine scrubbing to capture and purify CO₂ from flue gas for sequestration could nearly double the cost of electricity and reduces the plant's output by 30% (Ramezan et al., 2007).

Chemical absorbents also suffer from poor efficiency because of the large heat input required to break a chemical bond to regenerate the sorbent. Physical adsorbents that bind the CO₂ through a weaker interaction may help to reduce the regeneration energy input; however, the commercially available adsorbents have a number of drawbacks, including low capacity at the desired operating temperature, interference from water vapor and flue gas contaminants and poor cycle life.

Coal accounts for 56% of U.S. power generation and its contribution to future energy supply is expected to increase since U.S. has 25% of world's coal reserves (Tonks 2007). Coal-fueled Integrated Gasification Combined Cycle (IGCC) systems are environmentally superior to pulverized coal (PC)-fired boilers not only because they are more efficient at producing electricity, but also they can be equipped with more cost effective technologies for CO₂ capture and pollution control.

In an IGCC, it makes sense to capture the CO₂ early in the process (before the turbine), where it is concentrated and at high pressure, since it is far easier and less expensive to remove impurities when they are at high pressure and concentrated in the hot coal gas than when they are at atmospheric pressure and have been diluted by more 10:1 in the combustion turbine. The challenge for a CO₂ sorbent is that it must have high capacities and more importantly high stability at temperatures in the range of 100-300° C. Typically amine modified sorbents are not stable under these conditions.

On-site or on-farm manure to energy conversion is a highly attractive option for managing animal manure (as well as other agricultural bio-waste). Animal manure is an energy-rich opportunity fuel with a heating value of 8,500 Btu/lb (on a dry ash free basis). Large farms and feedlot operations produce large quantities of manure in a small area. Anaerobic digester units capable of producing pipeline quality methane installed at each farm or dairy could be instrumental in eliminating the difficulties associated with the transport of manure off site. Further, they could be a source of heat and electricity, while significantly reducing disposal costs and operating expenses for the farms and feedlots. If consumed properly, use of manure-derived fuels can also protect against environmental problems such as groundwater leaching and greenhouse gas emissions (methane emissions) associated with land filling of the farm wastes. The use methane that would otherwise be emitted into the atmosphere is an untapped resource of energy that has a significantly greater global warming potential (GWP) than CO₂. GWP is a measure of how much a given mass of greenhouse gas contributes to global warming over a specific time period compared to same mass of CO₂ over the same period (GWP of Methane over 100 years is 25 while that of CO₂ is 1).

The organic matter in the manure and other agricultural waste by-products can undergo a biological breakdown process in the absence of oxygen and be converted to “biogas”. Biogas is most commonly produced by anaerobic digestion or fermentation of biodegradable materials. Anaerobic digester units are widely accepted and used in farms, food processing facilities and dairies across the nation, and the biogas produced by them is commonly referred to as anaerobic digester gas or ADG. This type of biogas is similar to natural gas but is diluted with large amounts of CO₂ (up to 45%), and therefore possesses less energy per unit volume than pipeline methane (natural gas). ADG is often used in a combined heat and power (CHP) gas engine to generate electricity and heat, which is used to meet the demands of the farm, dairy or food processing facility. The electricity demand varies greatly depending on the size of the facility and the nature of the process. Typically, energy requirements for a dairy farm range from 300-400 kWh per cow per year, to over 1500 KWh per cow per year depending on the type of barn used to house the cows, the type of lighting and cooling systems, the waste handling system, and the type of water heating (Ludington, 2004). However, anaerobic digestion of the manure collected per cow per year can produce up to 13,500 standard cubic feet (SCF) per year of methane (as biogas) with an energy content of 4,000 kWh. Frequently, the energy production capacity of the farm or feedlot is far greater than its total energy requirements. Hence, the excess energy or biogas or the manure (farm waste) needs to be exported. The most convenient and least capital intensive approach is to upgrade the biogas produced to “pipeline methane” specifications and put it into the natural gas distribution network where it can subsequently be used either for domestic heating or electricity generation.

Pressure swing and vacuum swing adsorption systems are useful for small to medium scale air separation to produce very high purity oxygen. These air separation systems use conventional physical adsorbents such as zeolites, carbon molecular sieves, activated carbons etc. Unfortunately, these sorbents are not suited for methane purification because they do not have the necessary selectivity for CO₂ over H₂O, and biogas is a humid gas. Hence, these sorbents are not suited for CO₂ removal or for producing pipeline methane from biogas because they have a water adsorption capacity that is too large for high moisture applications. As a result there is a need for a CO₂ sorbent with a high CO₂ capacity and low water capacity to improve CO₂ separations (for methane purification, for example).

U.S. Pat. No. 7,541,312 teaches a porous carbon characterized by a volumetric pore size distribution having two peaks, a first of said peaks being between 0.5 and 1.0 nm and a second of said peaks being between 1.0 and 5.0 nm. The porous carbon may have a volumetric capacitance in an organic electrolyte of at least 40 F/cm³, an average pore diameter between about 2 nm and about 30 nm, a surface area of at least 900 m²/g, and/or a density of at least 0.4 g/cm³. A method for making such a carbon includes a) curing a mixture comprising a carbohydrate, a dehydrating component, and a nonmetallic cationic pore-forming agent and b) carbonizing the cured carbon under conditions effective to provide a porous carbon having a surface area between about 100 m²/g and about 3000 m²/g. The dehydrating component and nonmetallic cationic component may comprise two moieties of one compound.

U.S. Pat. No. 7,288,136 teaches an improved method of treating an amine to increase the number of secondary amine groups and impregnating the amine in a porous solid support.

U.S. Pat. No. 3,491,031 teaches a method to create a CO₂ sorbent by treating activated carbon with gaseous alcohol amines such as MEA. It utilizes a wet-chemical stripping method employing MEA to remove the adsorbed CO₂ and regenerate the sorbent.

U.S. Pat. No. 6,547,854 teaches a method to create solid CO₂ sorbents by the treatment of an acidified or basified solid substrate with a substituted amine salt.

U.S. Pat. No. 6,364,938 teaches a method to create CO₂ sorbents by the incorporation of amine groups into a polymer substrate or backbone. The method is applicable to low load situations such as human breathing environments.

U.S. Pat. No. 5,876,488 teaches a method to create CO₂ sorbents by dispersing aqueous amines in polymeric materials. Such sorbents are limited in application to human breathing environments and at ambient temperatures of 25° C.

U.S. Pat. Nos. 5,620,940, 5,492,683 and 5,376,614, teach methods to create CO₂ sorbents by using amine-polyols on chemically inert supports. Sorbent desorption methods employ heat and/or reduced pressure.

U.S. Pat. No. 4,810,266 teaches a method to create CO₂ sorbents by treating carbonized molecular sieves with alcohol amines.

All of these references contain at least one of the following limitations: low carbon capacity, high cost, temperature limitations, high heat inputs for regeneration, and high water sorption. For all of these applications, there is a need for a low cost sorbent that has high CO₂ capacity, low water uptake (capacity or sorption), with long cyclic life and low energy input for regeneration (a low ΔH of adsorption).

BRIEF SUMMARY OF THE INVENTION

To solve the above problems the carbon sorbents of the present invention have one or more of the following features: high CO₂ capacity, low water capacity, high temperature stability, high selectivity, long cycle life and low energy input for regeneration.

An embodiment of this invention is a sorbent for the removal of carbon dioxide from gas streams, comprising: a CO₂ capacity of at least 9 weight percent when measured at 22° C. and 1 atmosphere; an H₂O capacity of at most 15 weight percent when measured at 25° C. and 1 atmosphere; and an isosteric heat of adsorption from 5 to 8.5 kilocalories per mole of CO₂. In a preferred embodiment the isosteric heat of adsorption is from 5.1 to 7.3 kilocalories per mole of CO₂. In another embodiment the sorbent further comprises: a CO₂ capacity of at least 6 weight percent when measured at 60° C. and 1 atmosphere. Alternatively the sorbent comprises a CO₂ capacity of at least 4 weight percent when measured at 22° C. and 0.15 atmosphere; or at least 2 weight percent when measured at 60° C. and 0.15 atmosphere.

An embodiment of the present invention is a carbon sorbent in a powder, granular or pellet form to remove carbon dioxide from gas streams, comprising: (a) a carbon content of at least 90 weight percent; (b) a nitrogen content of at least 1 weight percent; (c) an oxygen content of at most 3 weight percent; (d) a BET surface area from 50 to 2600 m²/g: and (e) a DFT micropore volume from 0.04 to 0.8 cc/g. In additional embodiments of the invention, the sorbent further comprising: an H₂O capacity of at most 15 weight percent measured at 25° C. and 1 ATM. In a further embodiment the carbon sorbent comprises an isosteric heat of adsorption from 5.0 to 8.5 kilocalories per mole of CO₂, more preferably from 5.1 to 7.3 kilocalories per mole of CO₂.

In another embodiment the carbon dioxide capacity of at least 9 weight percent, if measured at 1.0 atm and 22° C.; a carbon dioxide capacity of at least 4 weight percent, if measured at 0.15 atm and 22° C.; a carbon dioxide capacity of at least 6 weight percent, measured at 1.0 atm and 60° C.; The sorbent of claim 1 further comprising: a carbon dioxide capacity of at least 2 weight percent, measured at 0.15 atm and 60° C.

In one embodiment the high capacity carbon dioxide sorbent is a porous carbon. The porous carbon incorporates “N” into the carbon structure to provide selective adsorption of CO₂ over H₂O. These porous carbons, which may have mesoporosity and/or microporosity, can be prepared from organic precursors, these precursors are selected from the following group, but are not limited to only these precursors; sugar (sucrose), corn starch, simple sugars such as fructose (high fructose corn syrup), and dextrose. The carbon preparation method uses ammonium compounds. The ammonium compounds include ammonium sulfate, ammonium bisulfate, ammonium carbonate, and ammonium phosphate dibasic. Other useful nitrogen compounds are urea and melamine.

The sorbent has a mesoporous and/or microporous carbon structure grafted (bonded or covalently bonded) with Lewis base functionalized groups to remove CO₂ via physical adsorption. The relatively strong Lewis acid-base interaction enables effective operation of the sorbent in the presence of the high levels of methane and moisture such as in anaerobic digester gas. However, because the Lewis base adsorption sites and the CO₂ do not form a true covalent bond, the heat input to regenerate the sorbent is lower than alternatives. For example, in one embodiment the heat input to regenerate the sorbent is about 5 to about 8.5 kcal per mole of CO₂, which is much lower than that observed for either chemical absorbents (e.g., 29.9 kcal/mol CO₂ for sodium carbonate) or amine-based solvents (e.g., 14.2 kcal/mol CO₂ for monoethanolamine) or the zeolites, which need very high vacuum for regeneration.

In another embodiment, the relatively strong Lewis acid-base interaction enables effective operation of the sorbent at high flue gas temperatures (60-150° C. range). However, because the Lewis base adsorption sites and the CO₂ do not form a covalent bond, the heat required to regenerate the sorbent is lower, and more favorable than the heat required to regenerate sodium carbonate or monoethanolamine.

In an embodiment, the carbon sorbents can be made from sugars and cornstarch as carbon precursors. To increase the CO₂ loading nitrogen compounds are incorporated into the carbon. This is done by adding nitrogen containing compounds to the carbon precursor before carbonization. Sorbents of this invention can be made using sucrose, dextrose, high fructose corn syrup (HFCS) and cornstarch. The nitrogen is bound tightly to the carbon and cannot be easily removed. These carbons are amorphous: such carbons do not have long-range crystalline order, which excludes other forms (allotropes) of elemental carbon such as diamond, graphite, graphenes, fullerenes or nanotubes.

In another embodiment one or more ammonium compounds are added to the precursors to catalyze the char formation. For example, heating a dextrose/cornstarch mixture to 200° C. gives a black char with one or more of the below listed ammonium compounds compared to light brown char without the ammonium compound. This is method is effective with reducing sugars such as dextrose and fructose. Useful additives are (but not limited to) ammonium sulfate, cetyltrimethylammonium bromide, ammonium carbonate, ammonium acetate, ammonium citrate, ammonium oxalate, ammonium formate, ammonium hydrogen citrate, ammonium hydrogen oxalate, ammonium chloride, ammonium bromide and ammonium phosphate. Urea and melamine

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Nitrogen functionalities on the carbon surface.

FIG. 2. CO₂ loading versus nitrogen content at 180° C.

FIG. 3. CO₂ loading versus oxygen content at 180° C.

FIG. 4. CO₂ Isotherms at different temperatures (T=180, 240 and 300° C.) and the fitted Langmuir-Freundlich isotherm parameters.

FIG. 5. TDA's CO₂ sorbent's working capacity under cycling in simulated synthesis gas at 240° C. and 500 psig.

FIG. 6. Sorbent CO₂ removal rate under cycling in simulated synthesis gas.

FIG. 7. CO₂ breakthrough profile from the long-term cycling experiments.

FIG. 8. Water sorption.

FIG. 9. Heat release due to CO₂ adsorption on AMS-19 as measured in DSC.

FIG. 10. Heat of adsorption of CO₂ on AMS-19 as measured in DSC/TGA.

FIG. 11. Sorbent performance under vacuum swing adsorption cycles in simulated flue gas at T=60-70° C., Pads=18 psia, Pdes=3 psia, GHSV=250 or 2,000 h⁻¹.

FIG. 12. CO₂ loading versus nitrogen content at 60° C.

FIG. 13. CO₂ loading versus oxygen content at 60° C.

FIG. 14. Adsorption isotherms on AMS-127 at 23° C.

FIG. 15. Extended multiple cycle tests with AMS-127 in a two-bed vacuum swing cycling system showing methane product purity. CH₄=60%, CO₂=40%, (dry basis), H₂O=sat. at 22° C., GHSV=62.5 h⁻¹; T=ambient, P_ads=19.0 psia, P_des=0.2 psia, L/D=8.

DETAILED DESCRIPTION OF THE INVENTION

In the summary of the invention above and in the Detailed Description of the Invention, and the claims below, and in the accompanying drawings, reference is made to particular features of the invention. It is to be understood that the disclosure of the invention in this specification includes all possible combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment of the invention, or a particular claim, that feature can also be used, to the extent possible, in combination with and/or in the context of other particular aspects and embodiments of the invention, and in the invention generally.

The term “comprises” and grammatical equivalents thereof are used herein to mean that other components, ingredients, steps, etc. are optionally present. For example, and article “comprising” (or “which comprises”) component A, B, and C can consist of (i.e. contain only) components A, B, and C, or can contain not only components A, B, and C but also one or more other components.

The term “at least” followed by a number is used herein to denote the start of a range beginning with that number (which may be a range having an upper limit or no upper limit, depending on the variable being defined). For example, “at least 1” means 1 or more than 1. The term “at most” followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending on the variable being defined). For example, “at most 4” means 4 or less than 4, and “at most 40%” means 40% or less than 40%. When, in this specification, a range is given as “(a first number) to (a second number)” or “(a first number)-(a second number)”, this means a range whose lower limit is the first number and whose upper limit is the second number. For example 25 to 100 mm means a range whose lower limit is 25 mm, and whose upper limit is 100 mm.

In this patent application the term “mesoporous” means having pores with diameters from 2 and 50 nm. The term “microporous” means having pores with diameters smaller than 2 nm. “BET” and “BET surface area” means Brunauer-Emmett-Teller theory (Bunauer et al. 1938) and “DFT” means Density Functional Theory (Lastoskie et al. 1993).

TABLE 1
Carbon formulations from sugars and cornstarch.
				Ammo-		Sul-
				nium		furic
Carbon				Com-	Corn-	Acid
Formu-		Sugar	Ammonium	pound	starch	(wt.
lation	Sugar	(wt. %)	Compound	(wt. %)	(wt. %)	%)
AMS-11	Sucrose	81.9	None			18.1
AMS-19	Sucrose	71.0	Sulfate	13.3		16.0
AMS-126			Sulfate	12.8	71.2	16.0
AMS-128	Sucrose	75.8	Bisulfate	24.2
AMS-170	Dextrose	32	Sulfate	20	48
AMS-171	Dextrose	38	Bicarbonate	4.8	57
AMS-172	Dextrose	36.4	Bicarbonate	9.1	54.5
AMS-173	Dextrose	33.3	Bicarbonate	13	50
AMS-184	Dextrose	38.1	Urea	4.8	57.1
AMS-185	Dextrose	38.1	Melamine	4.8	57.1
AMS-186	Dextrose	39	Urea	2.4	58.5
AMS-187	Dextrose	39	Melamine	2.4	58.5
AMS-200	Dextrose	36.4	Melamine	9.1	54.5

The general procedure for making granular carbon is as follows: (Step 1): Mix ingredients (i.e. sugar, HFCS, fines, cornstarch, and ammonium compound) in a mixer. (Step 2): Heat the mixture to 220° C. for 1-2 hours. (thermal step 1). (Step 3):

Remove the char and break up with a jaw crusher. (Step 4): Screen the material. The granular (−4 to +20 mesh) char is collected and the fines are mixed in with the material in Step 1. (Step 5): Carbonize the char in a rotary kiln under nitrogen at 300-1000° C. (thermal step 2). (Step 6): Optionally activate the carbon in a rotary kiln under a flow of carbon dioxide and/or steam at 800-1000° C. (thermal step 3).

In one embodiment the carbon sorbent is made using a nitrogen containing compound, and a carbon source, for example cornstarch and sugars and optionally an acid. Representative formulations are shown in Table 1. In Table 1, “Sulfate” means ammonium sulfate, “Bisulfate” means ammonium bisulfate, “Melamine” means 1,3,5-Triazine-2,4,6-triamine, and “Urea” means diaminomethanal, also known as diaminomethanone, “HFCS” means high fructose corn syrup, “Sulfuric” means sulfuric acid (H₂SO₄) In Table 2 “wood” means hardwood chips.

TABLE 2
Carbon formulations from wood.
			Ammonium	Sulfuric
Carbon	Wood	Ammonium	Compound	Acid
Formulation	(wt. %)	Compound	(wt. %)	(wt. %)
AMS-127	71.2	Sulfate	12.8	16.0

Table 3 shows formulations using added activated carbon or carbon black.

TABLE 3
Carbon formulations made with activated carbon and carbon black.
Carbon				Ammonium	Corn-
Formu-		Sugar	Ammonium	Compound	starch	Carbon	Carbon
lation	Sugar	(wt. %)	Compound	(wt. %)	(wt. %)	Source	(wt. %)
ZD511	Dextrose	26.6	Melamine	4.0	48.0	Carbon	16.0
						black
ZD548	Dextrose	8.9	Bicarbonate	1.8	0	Activated	89.3
						carbon

Table 4 shows the elemental analyses for the carbon sorbents referenced in this application. The amount of nitrogen in the carbon sorbent is influenced by the type and amount of nitrogen compound in the initial formulation. The amount of nitrogen in the carbon is influenced by the initial formulation and the carbonization temperature.

TABLE 4
Elemental Analysis of Carbons.
	Nitrogen	Oxygen	Carbon	Hydrogen	Sulfur
Sample	(wt. %)	(wt. %)	(wt. %)	(wt. %)	(wt. %)
AMS-11	0.16	7.09	88.64	3.13
AMS-19	3.24	2.44	90.98	0.53	2.19
AMS-126	3.56	3.29	90.79	0.65	0.16
AMS-127	4.05	2.26	89.44	0.61	1.69
AMS-128	3.31	2.46	91.49	0.56	1.12
AMS-170	5.58	2.20	89.66	0.44
AMS-171	1.74	4.99	90.95	0.57
AMS-172	1.72	1.41	94.81	0.48
AMS-173	1.44	1.33	95.37	0.40
AMS-184	2.10	1.53	94.21	0.42
AMS-185	2.1	1.63	94.35	0.37
AMS-186	2.58	2.56	92.84	0.65
AMS-200	2.44	1.56	94.13	0.40

Table 5 shows the pore size distribution and surface area for sorbent formulations carbonized under nitrogen or activated with carbon dioxide. Heating the carbon under nitrogen gives a surface area up to 700 m²/g. By activating with carbon dioxide, the surface area can be up to a 2500 m²/g. BET surface area is Brunauer-Emmett-Teller theory (Brunauer et al. 1938) and DFT is Density Functional Theory (Lastoskie et al. 1993). Brunauer 1938 and Lastoskie 1993 are incorporated by reference herein.

TABLE 5
Surface Areas and Pore Volumes of Carbons.
	BET	DFT Total	DFT
	Surface	Pore Volume	Micropore	Process
Sample	Area (m2/g)	(cc/g)	Volume (cc/g)	Gas	pH
AMS-11	416	0.19	0.15		4.6
AMS-19	401	0.17	0.17	N2	6.0
AMS-126	396	0.17	0.16	N2	6.8
AMS-127	154	0.071	0.048	N2	10.6
AMS-128	263	0.12	0.092	N2	6.7
AMS-170	374	0.19	0.15	N2	7.0
AMS-171	516	0.27	0.21	N2
AMS-172	648	0.27	0.26	N2
AMS-172	1920	0.85	0.68	CO2	9.2
AMS-173	473	0.25	0.19	N2	7.3
AMS-184	2550	1.09	0.78	CO2	11.1
AMS-185	2230	0.95	0.77	CO2	10.8
AMS-186	237	0.10	0.095	N2	7.3
AMS-187	457	0.19	0.18	N2	6.6
AMS-200	1483	0.63	0.58		11.5

The elemental composition on the surface of the carbons, can be analyzed by X-ray photoelectron spectroscopy (XPS). For example, for AMS-19 the major elements that are detected are carbon, nitrogen, oxygen, and sulfur.

TABLE 6
Relative elemental composition of sample
surfaces as determined by XPS.
Sample	C (Atom %)	N (Atom %)	O (Atom %)	S (Atom %)
AMS-19	85	2.5	4.9	0.8

The sulfur on the surface of samples AMS-19 has a peak energy that suggests the sulfur is present as either S—C or S—S.

TABLE 7
Relative compositions and most probable peak assignments for
carbon specieson sample surfaces as determined by XPS, C 1s region.
Carbon	C—C, C—H, C—S	C—O, C—N	C═O
Sample	(Atom %)	(Atom %)	(Atom %)
AMS-19	80	14	4.0

TABLE 8
Relative compositions and most probable peak assignments for oxygen
species on sample surfaces as determined by XPS, O 1s region.
Carbon Sample	O—C (Atom %)	O═C (Atom %)
AMS-19	53	47

The sample showed no evidence of quaternary or oxidized nitrogen. The peak energies suggest that the carbons have a mixture of nitrogen in the form of pyridine and pyridone/pyrrole (FIG. 1).

TABLE 9
Relative compositions and peak assignments for nitrogen species
on sample surfaces as determined by XPS, N 1s region.
Carbon	N—C pyridine	N—C pyridone/pyrrole
Sample	(Atom %)	(Atom %)
AMS-19	66	34

The sorbent can be regenerated by applying concentration swing, pressure swing and/or a thermal swing. Both the CO₂ adsorption and regeneration are rapid. The fast adsorption and regeneration allows short cycle times and high utilization of the sorbent, thereby reducing the system size and overall sorbent inventory. The sorbent works in the presence of moisture.

The amounts of carbon dioxide adsorbed on the carbons have been measured in a constant volume apparatus at various temperatures. It was found that carbons sugar derived carbons with high nitrogen content (AMS-19 and AMS-172) have higher capacities than carbons with little nitrogen (AMS-11).

TABLE 10
Amount of carbon dioxide adsorbed.
	Density	CO2 @ 1 atm (wt %)	CO2 @ 0.15 atm (wt %)
Carbon Sample	(g/cc)	22° C.	60° C.	22° C.	60° C.
AMS-11	0.33	7.36	4.67	2.93	1.56
AMS-19	0.30	10.85	6.65	4.77	2.54
AMS-126	0.19	12.40	8.15	6.35	3.59
AMS-127	0.26	9.4	4.82	6.17	3.00
AMS-128	0.13	11.10	7.77	5.48	2.97
AMS-170	0.49	9.56	5.39	4.08	1.92
AMS-171	0.28	12.36	6.88	4.54	2.06
AMS-172 (N2)	0.24	14.13	8.41	5.89	3.15
AMS-172 (CO2)	0.18	13.25	6.83	4.48	2.43
AMS-173	0.40	11.09	7.14	4.29	2.46
AMS-184	0.12	13.43	7.05	4.89	2.92
AMS-185	0.17	13.25	6.82	4.5	2.47
AMS-186	0.33	9.93	6.12	4.45	2.42
AMS-187	0.45	10.46	6.28	4.51	2.24
AMS-200	0.22	14.15	7.04	4.86	2.39

FIG. 8 shows the impact of oxygen content of the carbons on the water loading at 25° C. The lower the oxygen content, the less water is adsorbed onto the carbon. With an oxygen content of <3% wt. there is <15% wt. water uptake. With an oxygen content of <2% wt. there is <5% wt. water uptake. FIG. 2 and FIG. 3 show the impact of nitrogen and oxygen content of the carbons on the CO₂ loading at 180° C. The higher the nitrogen and oxygen content the greater the CO₂ capacity. However, in certain embodiments of this invention it is advantageous to minimize the oxygen content. Oxygen present in the carbon can promote gasification of the carbon (resulting in mass loss of the sorbent).

Sorbent AMS-19 had a capacity in excess of 20% wt. CO₂ at higher CO₂ partial pressures (180° C. and 600 psia CO₂ partial pressure) and the experimental data from the bench-scale tests were fitted with Langmuir-Freundlich isotherms as shown in FIG. 4.

Table 11 shows the water uptake/capacity for various sorbents of this invention.

TABLE 11
Water uptake/capacity for various sorbents of this invention.
		Oxygen Content in Sorbent	Water Uptake
	Sample	(% wt.)	(% wt.)
	AMS 19	2.44	13.54%
	AMS 126	3.29	23.5%
	AMS 127	2.26	12.1%
	AMS-185	1.63	0.37%
	AMS-172	1.41	0.49%

The heat of adsorption can be quantified with laboratory techniques. Using AMS-19 as a specific example, we measured the heat of adsorption in a differential scanning calorimeter (DSC) and combined it with the amount of CO₂ adsorbed under similar conditions from thermogravimetric analyzer (TGA) to calculate the heat released (heat of adsorption) for adsorption of one mole of CO₂ on our sorbent. We placed about 10 mg sample in the DSC and carried out multiple cycles at a set temperature (for example different tests at 30, 60, 120, 180, 240 or 300° C.) changing the process gas between pure He and CO₂. The sample released heat on exposure to CO₂ and absorbed heat during desorption, providing the heat of adsorption in kcal/g of sorbent. We then repeated the same experiments in the TGA, we placed about 10 mg sample in the TGA and carried out multiple cycles at a set temperature (again for example at 30, 60, 120, 180, 240 or 300° C.) changing the process gas between pure He and CO₂. The weight change will provide the amount of CO₂ adsorbed per g of sorbent. Combining the heat of adsorption from DSC and the CO₂ capacity from TGA, we calculate the heat of adsorption per mole of CO₂, and it was between 4.5 and 5.6 kcal per mol. FIG. 9 shows the heat release due to adsorption as measured in DSC. FIG. 10 shows the heat of CO₂ adsorption as a function of temperature for AMS-19 as measured in DSC/TGA. In comparison, amine sorbents require three times more energy (˜14.4 kcal per mole CO₂) and the alkali sorbents that form a covalent bond with CO₂ needs five times more energy, 27 to 37 kcal for each mole of CO₂ (and water).

Table 12 shows the isosteric heat of adsorption data as calculated from CO₂ isotherms for other sorbent samples. The −ΔH_ads varies from about 6.4 to 8.3 kcal per mole of CO₂. These tests used a constant volume apparatus commonly known as Sieverts apparatus and routinely used for adsorption isotherm measurements.

TABLE 12
Isosteric heat of adsorption for sorbents
of the present invention.
              −ΔHads
Carbon Sample kcal/mol
AMS-11        6.41
AMS-19        8.29
AMS-126       7.25
AMS-127       7.30
AMS-128       6.49
AMS-170       6.89
AMS-171       6.49
AMS-172 (N2)  6.67
AMS-172 (CO2) 7.04
AMS-173       5.10
AMS-184       5.49
AMS-185       6.88
AMS-186       6.46
AMS-187       6.42
AMS-200       7.65
Vapure410     6.49

The sorbents of this invention may be used, for example, for CO₂ removal from anaerobic digester gas. This sorbent can be used in a vacuum swing or pressure swing adsorption process to achieve pipeline quality methane.

The sorbents of this invention may be used for CO₂ removal from flue gas. This sorbent can be used in a pressure, vacuum, temperature or concentration swing adsorption process.

Example 1

A long-term cycling experiment was conducted in a 1.3 L sorbent reactor under representative conditions using simulated synthesis gas. Sorbent AMS-19 was used. This test completed over 10,000 cycles and the sorbent maintained a stable performance (FIG. 5). The CO₂ removal rate is provided in FIG. 6. The CO₂ breakthrough during from cycling is shown in FIG. 7.

During these tests, in some experiments, the desorption (regeneration) conditions were varied by changing the desorption pressure and purge volume etc. For cycles 2100-2163, this test increased the adsorption bed temperature to 260° C. This increased the working capacity of our sorbent from 9% at the baseline conditions up to 12.4% wt. CO₂. For cycles 2164-2310 this test decreased the regeneration pressure to 50 psi, 100 psi, and 150 psi. These regeneration pressures all resulted in a capacity of about 15% wt. CO₂. For cycles 2672-2716, the normal adsorption cycles ran with dry nitrogen regenerations at 150 psi. This resulted in the highest capacities, about 20% wt. CO₂. For cycles 2977-3014, normal adsorption cycles ran with dry nitrogen regenerations at 117 psi. This resulted in capacities of about 16% wt. CO₂. For cycles 3050-3096, baseline conditions ran, but with an added 5-minute dry nitrogen purge at 500 psi. This resulted in capacities of about 11.5% wt. CO₂. N₂/CO₂/H₂O ran from cycles 3614-4425. This resulted in very similar capacities to baseline conditions (8-10 wt. % CO₂). In the cycles between 5,500 to 7,000 cycles the CO₂ capacities and removal efficiency appeared to be lower; this was due to drift in the analyzer. Once re-calibrated, the analyzer measured sorbent performance that was back to original capacity of about 8% wt. CO₂ at standard test conditions.

Example 2

Using AMS-172 as a specific example, 9.1 wt. % ammonium bicarbonate, 36.4 wt. % dextrose and 54.5 wt. % cornstarch were thoroughly mixed in a standard Kitchen Aid mixer. To the powder was added 16 wt. % water. The material was dried in a Despatch box oven at 110° C. for 12 hours, followed by 200° C. for 4 hours. The char was removed and ground with a Bico Chipmunk jaw crusher. The crushed material was screened in a Sweco sifter and the −4 to +20 mesh cut was collected. The material was devolatilized at 400° C. under nitrogen in a Bartlett and Snow continuous rotary kiln. The carbon was heat treated further under nitrogen in a Bartlett and Snow continuous rotary kiln at 900° C. for 1 hour. (BET surface area 648 m²/g).

Example 3

Using AMS-185 as a specific example, 4.8 wt. % melamine, 38.1 wt. % dextrose and 57.1 wt. % cornstarch were thoroughly mixed in a standard Kitchen Aid mixer and enough water added to form a paste. The mixture was added to Teflon trays and heated to 200° C. for 2 hours in a Despatch box oven. The char was removed and ground with a Bico Chipmunk jaw crusher. The crushed material was screened in a Sweco sifter and the −4 to +20 mesh cut was collected. The granular char was carbonized at 400° C. under nitrogen for 2 hours and activated with carbon dioxide at 950° C. in a rotary kiln for 4 hours (BET surface area 2230 m²/g).

Example 4

Using AMS-19 as a specific example, adsorption and desorption cycles are conducted in a 100 mL sorbent reactor under representative conditions using simulated flue gas. This test completed over 4,200 cycles and the sorbent maintained a stable performance. The CO₂ capacity of the sorbent during these cycles are provided in FIG. 6. The CO₂ breakthrough during from cycling is shown in FIG. 7. During these tests, in some experiments, the gas hourly space velocity (GHSV) was changed from 250 to 2,000 h⁻¹ and the water concentration in the flue gas was changed from 12% to 18%. The increase in space velocity increased the capacity of our sorbent from 1.7% at the baseline conditions up to 5% wt. CO₂.

Example 5

CO₂ and CH₄ isotherms for a carbon sorbent prepared from wood (AMS-127) were measured in a constant volume apparatus. FIG. 14 shows the CO₂ and CH₄ isotherms for carbon sorbent prepared from wood (AMS-127). The sorbent achieved a CO₂ loading of 10.9% wt. CO₂ at 1 atm partial pressure while the methane adsorption is only 1.15% wt. CH₄ at 1 atm partial pressure (CO₂/CH₄ selectivity of 3.45). This high CO₂ selectivity for the carbon results in a significant reduction in methane loss in the vacuum swing adsorption process. Vacuum swing adsorption-desorption cycles were carried out at two different space velocities with carbon sorbent prepared from wood (AMS-127) in a two-bed vacuum swing cycling system. The methane product purity as measured from the exit of the two-bed vacuum swing cycling system from the extended multiple cycle tests with AMS-127 is shown in FIG. 15. The AMS-127 sorbent also achieves very high methane purities under two-bed vacuum swing cycles.

Claims

1. A carbon sorbent for the removal of carbon dioxide from gas streams, comprising: (a) a CO2 capacity of at least 9 weight percent when measured at 22° C. and 1 atmosphere;(b) an H2O capacity of at most 1 weight percent when measured at 25° C. and 1 atmosphere;(c) an isosteric heat of adsorption of from 6 to 8.5 kilocalories per mole of CO2; and,(d) a nitrogen content of at least 2.2 wt %, which is present in the form of pyridine, pyridone or pyrrole.

2. The carbon sorbent of claim 1, wherein the isosteric heat of adsorption is from 6.0 to 7.7 kilocalories per mole of CO2.

3. The carbon sorbent of claim 1 further comprising: a CO2 capacity of at least 7 weight percent when measured at 60° C. and 1 atmosphere.

4. The carbon sorbent of claim 1 further comprising: a CO2 capacity of at least 4.8 weight percent when measured at 22° C. and 0.15 atmosphere.

5. The carbon sorbent of claim 1 further comprising: a CO2 capacity of at least 2.3 weight percent when measured at 60° C. and 0.15 atmosphere.

6. A carbon sorbent in a powder, a granular or a pellet form for the removal of carbon dioxide from gas streams, comprising: (a) a carbon content of at least 90 weight percent;(b) a nitrogen content of at least 2.2 weight percent, which is present in the form of pyridine, pyridone or pyrrole;(c) an oxygen content of at most 1.8 weight percent;(d) a BET surface area from 50 to 2600 m2/g; and,(e) a DFT micropore volume from 0.04 to 0.8 cc/g.

7. The carbon sorbent of claim 6, further comprising: an H2O capacity of at most 1 weight percent, based on the mass of the sorbent when measured at 25° C. and 1 atmosphere.

8. The carbon sorbent of claim 6, further comprising: an isosteric heat of adsorption of from 6.0 to 8.5 kilocalories per mole of CO2.

9. The carbon sorbent of claim 8, wherein the isosteric heat of adsorption is from 6.0 to 7.7 kilocalories per mole of CO2.

10. The carbon sorbent of claim 6, further comprising: a carbon dioxide capacity of at least 9 weight percent when measured at 1.0 atmosphere and 22° C.

11. The carbon sorbent of claim 6, further comprising: a carbon dioxide capacity of at least 4.8 weight percent when measured at 0.15 atmosphere and 22° C.

12. The carbon sorbent of claim 6, further comprising: a carbon dioxide capacity of at least 7 weight percent when measured at 1.0 atmosphere and 60° C.

13. The carbon sorbent of claim 6, further comprising: a carbon dioxide capacity of at least 2.3 weight percent when measured at 0.15 atmosphere and 60° C.

14. The carbon sorbent of claim 6, further comprising: a pH of at least 9.0.

15. The carbon sorbent of claim 14, further comprising: a pH of at least 11.0.