Catalytic fixation of CO2: CO2 purity and H2 supply

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.

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C a t a l y t i c f i x a t i o n o f C O 2 : C O 2 p u r i t y and H2 s u p p l y J. N. Armor Air Products and Chemicals, Inc., 7201 Hamilton Blvd, Allentown, PA 18195 (USA) An analysis of factors affecting proposed solutions to the CO2 problem is provided. To consider CO2 as a feedstock, one has to consider the purity, reactivity, operational conditions, customer preferences, transportation costs, and availability of the CO2. Since CO2 is a global problem, local efforts to reduce CO2 emissions will have limited impact, except to convert waste CO2 to a more valued chemical product. Some have suggested that H2 offers a good approach to remove CO2, but use of H2 as the reducing agent must address the source and cost of the H2. Most H2 is produced by steam reforming of hydrocarbons which is also a source for CO2; thus use of conventional sources of H2 is not a practicable solution to destroying CO2. New non-fossil fuel routes to H2 production might enhance the use of H2 as the reductant, but initial production of chemicals, even if cost competitive, from CO2 is expected to have limited impact on worldwide CO2 emissions. Issues that impact Hz supply and cost will be discussed, since these may be a part of any CO2 solution. 1. INTRODUCTION An excellent summary [ 1] of the volume and sources of major atmospheric pollutants was published by the US Dept of Energy in 1994. Highlights of their report can be broken down into the types, volumes and sources of a variety of pollutants. In particular, for CO2, it is estimated that 160,000 million metric tons (mmt) are generated naturally, worldwide: 8,000 mmt from human derived sources, globally;165,000 mmt are absorbed by earth, with the balance being a global increase of-~3,400 mmt. There is some disagreement about the accuracy of the latter number since it is based on the difference of two large nmnbers. Further, a small group of scientists contend that global warming is not related to CO2, but to other factors such as water vapor. This manuscript will describe the solutions being considered for CO2 removal, the chemical and political limitations on use and reduction of CO2 levels, and the role of Hz in affecting a solution. Since Hz is such an important part of the potential solution, some introduction into the current supply, availability, and cost for Hz will be provided as well as alternative approaches to making more Hz. 2. PROPOSED SOLUTIONS TO CO2 BUILDUP

As described in an earlier publication [2], a number of solutions [refer to Figure 1] have been proposed to reduce this imbalance, including the establishment of a "carbon tax", minimizing CO2 emissions (already underway), demanding zero emissions of CO2 (solar, hydroelectric, wind, nuclear, or geothermal), burying CO2 by storing in deep in ocean pools or

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use it for enhanced oil recovery, absorbing it (already done with monoethanolamines), using it to produce clean C02 for carbonated beverages, etc, and finally considering it as a feedstock for valuable chemicals. - Carbon Tax Minimize Emissions Zero Emissions - Bury it [Oil recovery, oceans, MEA] - CO 2 as Feedstock -

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3,400 MMt

Figure 1. Potential solutions to offset the perceived imbalance in CO~ in our upper atmosphere 2.1. T a x e s

Some progress [3] has been made using taxes to enforce environmental regulations. For example, in the 1980s in France & Germany provided incentives to invest in waste water treatment. In the 1990s in USA, laws and taxes were enacted to reduce the impact of ozone depleting chemicals. In the 1990s revenue producing taxes were imposed on CO2 emissions in Sweden and Norway and on sulfur and NOx emissions in Sweden as well as on dumping and incineration in Denmark. Any tax on CO2 would have to be implemented worldwide to avoid upsetting the competitiveness already existing between nations of the world. 2.2. C 0 2 as a f e e d s t o c k

To consider CO2 as a feedstock, one has to consider the purity, reactivity, operational conditions, customer preferences, transportation costs, and availability of the CO2. There is a tendency by some to think that CO2 from a powerplant stack can simply be used to supply the same CO2 for chemicals production, such as the production of H2 by reaction of CO2 with CH4. In fact this cannot be done without a lot of purification. In the example to produce H2, CO2 reforming of CH4 will doubtless need very pure CO2, that is one will have to remove SO2 or NOx from the process stream. This will add cost to the CO2 for the necessary purification steps. Another way to look at the issue of purifying CO2 is to examine how CO~ is produced today. The Caloric catalog [4] provides a diagram of the many unit operations. Thus, one can see that CO: produced from a powerplant is recovered as high purity liquid CO2 which can be converted to cylinder grade gas for use in carbonated beverages. Typically one has to absorb the CO2 in an aqueous amine such as monoethanolamine. If the presence of water is unacceptable that has to be stripped out of the process stream. Steam is used to recover the CO2 from the amine solution. There are filters and scrubbers in the process before liquid CO: is produced. In addition there may be the need for compression of the CO2, which is a very energy intensive process operation thus adding more cost to the CO~. [One must remember that because CO2 is so stable, it will not be sufficient to react it at one atmosphere; it will probably have to be pressurized to enhance its reactivity.]

143 In addition one must also remember that where the CO2 is produced may not be the place where it is needed. It probably will need to be transported [via vehicles or pipeline] to the process operation. Transportation also adds more cost to any feedstock. In another process related issue, the CO2 will be needed on a continuous basis for the production of chemicals. This is because most commodity scale chemicals are produced around the clock at the same level of productivity. Chemical plants run efficiently when running full out. This continual need for CO2 is not consistent with the major anticipated source of CO2 which is from power plants. Power plants do not run at a constant output; power is produced during peak user periods and the plants reduce power production at night. This means that CO: production is reduced at night. Thus one gets a non-uniform production of CO2 which is unacceptable for commodity chemicals production. J. Rostrup-Nielsen pointed out [5] in 1994 that it is "questionable whether C1 chemistry can contribute significantly to solve the greenhouse problem created by CO2." For example the present world production of acetic acid is about 5 billion pounds per year. If one were to use CO2 + CH4 to produce acetic acid this would amount to the CO2 emission from only ONE 500 MWatt coal fired power plant- "a small drop in a big bucket." 3. OTHER NON TECHNICAL ISSUES RELATED TO CO~ CONVERSION The perceived CO2 problem is a global one, not a local one. This means that unlike NOx removal, localized removal of CO2 will not provide significant reduction in worldwide CO2 levels in the upper atmosphere, unless all countries are equally participating in rigid CO: emissions control. The undesirable cost of any added CO: emission control will have to be passed onto the consumer. It is anticipated that emerging nations will resist controls to growth; naturally, they will be more interested in doing whatever they can to enjoy the comforts of more prosperous societies without additional cost. Their rate of growth will be high and new laws will probably impact new construction greater than existing production facilities; thus they may be expected to bare a greater proportion of the CO2 reduction. The passage of international laws will require a good deal of compromise and negotiation. Naturally there will be local issues and pressures applied to politicians to minimize the cost burden to any one country. At the same time, environmentalists will be demanding strict reduction of CO:, thus we can expect governments around the world to be swayed by the "politically correct" lobbies. In addition, considerable uncertainties about the origin of any greenhouse effect will delay implementation of any globally binding agreements. Politics and business influence these efforts to legislation and changes. Since the commercial energy section impacts wide regions of any economy, therefore, its hard to control a single business. It's not simply a matter of focusing on the oil companies who supply the fuel, the automobile companies who produce the cars, the power companies (who are just trying to meet the demands of the consumer for increasing levels of power generation), or the water companies trying to quench the thirst of populations living in arid regions of the world- it's a collection of vested business and consumer interests that vary around the globe. Trying to tackle this uniformly will not be easy, if at all possible on a global scale. Indeed, one general approach that at least make a dent in the pollution of our planet is energy conservation. I personally believe this offers greatest impact and has a realistic chance

144 of making some impact. Currently, economic pressures (cheap fossil fuel), not legislation control. Since CO2 is a global problem, local efforts to reduce CO2 emissions will have limited impact, except to convert waste CO2 to a more valued chemical product.

4. WHERE DO WE GET THE HYDROGEN WE NEED? I sense another false impression is that a solution to the removal of CO2 is to just use H2 to reduce it back to CO or CH4. This just is not an acceptable solution, except in some micro economies around the world. The strong pressure for cleaner fuels has forced refineries to become net consumers of H2, whereas, 20 years ago, they were producers of surplus H2. As the article by P. Courty and A. Chauvel [6] indicates, H2 demands will continue to escalate into the next century which is expected to result in a substantial demand for H2 which cannot be matched by existing supply. The strong demand is driven by need of refineries to meet existing legislation for removing S and N from fuel. This is also aggrevated by the lower net H2 production [due to the reduced demand for adding aromatics to enhance octane number of fuels]. Independent of all this pressure from the refineries, H2 also offers some distinct advantages as a future fuel which may put much more demand pressures on H2, since it is a clean fuel when combusted and no CO2 is produced when H2 is derived from non-fossil fuels. Fortunately, the earth possess a huge H2 reserve [our oceans], if only we could figure out how to convert water to H2 in a cost effective manner. All these market and technology forces will keep the price of H2 relatively high, and it is probably unreasonable to use it for destroying vast amounts of CO2.

4.1. Cost of H2 to produce gasoline One estimate for the cost for pure H2 is -~$1.50 for 1000 std cu fl, which coverts to $ 0.00127/mole H2. If we assume that for CO2 + H2 to gasoline, we can represent gasoline as [(CH2)x with x=7]. This means that one will produce 2 moles water/mole CO2. This means that one must consume 2x moles H2 for every x moles CO2 to make (CHz)x, plus x moles of H2 to make water. Thus there are 3 moles of H2 for every mole of CO2. This means that (CH2)7 requires 21 moles of H2. This stoichiometry means $0.027/mole gasoline. If gasoline sells for $20/barrel, and we assume that for the density of gasoline we can use the density of methylcyclohexane, this converts to $0.015/mole gasoline as methylcyclohexcane. This means that gasoline from CO2/H2 is more than two times the current market price. This unacceptable price difference between gasoline and the cost of H2 is driven by 2 issues: gasoline is terribly cheap and the cost or credit one assigns to CO2. One might expect some relief in this cost if one could use a cheaper source of He, such as from an off gas process stream. Alternatively, if there were a credit [or tax] on CO2 emissions, that would help to reduce the large difference in costs. As pointed out earlier CO2,will not be free, since it will cost something to purify and pressurize it for suitable reactivity. However, many nations are proposing a tax on CO2 emissions or providing CO2 with a credit price which will add some cost incentive to CO2 conversion to chemicals. Getting money for disposing of CO2 to chemicals will depend on the world wide acceptance of such a philosophy, the chemical to be produced, market pressures, and commercially acceptable catalytic processes.

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5. A ROLE F O R H2 TO R E M O V E COs?

Some have suggested that H2 offers a good approach to remove CO2, but use of H2 as the reducing agent must address the source of the H2. Most H2 is produced by steam reforming of hydrocarbons which is also a source for CO2; thus use of conventional sources of H2 is not a practicable solution to destroying CO2. New non-fossil fuel routes to H2 production might enhance the use of H2 as the reductant, but initial production of chemicals, even if cost competitive, from CO2 is not expected to have significant impact on worldwide CO2 emissions [5]. 5.1. Production of H2 also Produces COs There are two primary sources of commercial production of H2 [other than by-product H2 from dehydrogenation, etc]. They are SR [Steam Reforming] and the partial oxidation of heavier hydrocarbons. SR uses a variety of hydrocarbon sources. Both approaches convert the carbon components to CO2, but a large portion of H2 is derived from added steam. The amount of CO2 generated depends [7] upon the hydrocarbon feedstock. Most of the current chemical approaches to H2 production also produce CO2 as a by-product; however, SMR coproduces much less CO2 than partial oxidation. Therefore, it does not make sense to use H2 to remove CO2 when more CO2 is produced whenever one makes H2. There is a very small need for making CO/H20 or CH4 from COdHz, and we already have ample catalysts for these reactions.

6. H O W CAN W E G E T M O R E H2? With the building demand for Hz - and preferably relatively cheap H2- how are we going to produce H2 to meet the future generation's needs? Steam methane reforming is one of the preferred approaches with natural gas accounting for about 50% of the feed for H2 production. The lower levels of CO2 produced via the use of natural gas feedstocks will continue to make this an attractive feedstock for H2 production. There are some areas of opportunity to consider in modifying or displacing in future decades the current approaches to SMR. These include: the fact that steam Reforming (SR) is energy intensive, endoergic process and large quantities of CO2 are co-produced. Currently considerable H2 purification is necessary to meet the customer's demands, and H2 is needed at >10 atm pressure by most customers. In particular, refineries need H2 a t 500-2000 psi. A number of alternative approaches are being pursued worldwide to generate H2. Some of the more attractive processes include, 9 Oxidative dehydrogenation 9 C02 reforming without carbon formation: C02 4- CH4 - ~ 2 CO + 2 H2 9 Use of methane 9 Solar energy for water electrolysis 9 Selective Oxidation of CH4 9 Thermochemical water splitting combined with solar or nuclear sources of energy 9 Fuel cells [8] 9 Photoassisted water splitting 9 Biomass conversion

146 7. OTHER ISSUES IMPACTING H2 PRODUCTION

Just as with CO2, there are some issues that will restrict our technical approaches, and these need to be appreciated in considering alternative routes to H2 production [9]. The type of feedstock available [NG, heavy oil, etc] will have an impact on the preferred process approach; these feedstocks are controlled by regional issues and supplies out of the direct control of R&D. For a partial oxidation plant one will need a supply of 02 from a nearby air separation plant. Once again the needs of customer [pressure, purity, volume, etc] will have a strong influence on new plant construction. In some regions of the world, the cost of power is strongly influenced by governmental tax credits and subsidies which can make some fuels, technologies and feedstocks much more acceptable. Certainly, the availability of large amounts of capital can severely limit not only the decision to build a plant, but also the type of process chosen. One often forgets about what one does with all the Hz produced. Investments must be made in H2 storage, separation, and purification. Finally environmental regulations [the degree and breadth] will impact the process approach and costs. 8. CONCLUSION I believe that any large scale removal of CO2 will be impacted by the continuing huge, broad, and expanding production of CO2 vs. what one can do with it. The potential production of chemicals from CO2 is small and can only have a limited and localized impact on a global problem. With regard to the use of CO2 as a feedstock, process issues will prevail regarding purity and pressure limitations on CO2 value. In order to use H2 as a reducing agent for such a huge quantity of CO2 one will need CHEAP H2, which simply is not possible in a world market where Hz is in high demand and the price of Hz is set to match the demand. There is no simple solution to destroying all the excess CO2 now produced. I believe the only meaningful approach that we can take immediately to attack the issue of CO2 emissions with technology within the reach of today's knowledge is the insistence on greater energy efficiency in chemical processes, automobile production, power generation, etc. With regard to the production of H2, we need to continue to try to avoid or minimize the coproduction of CO2 in fossil fuel based plants. REFERENCES

1. "Emissions of Greenhouse Gases in the United States: 1987-1992," DOE/EIA Report # 0573, October, 1994, US Government Printing Office, Washington, DC. 2. J.N. Armor, Catalysis Today, to be published 3. M. Burke, Env. Sci. & Techn., 31 (1997) 84A. 4. Catalog "Make your own CO2", Caloric GMBH, Lohenstrasse 12, D-8032 Graefelfing, Germany. 5. J. Rostrup-Nielsen, Natural Gas Conversion II, H. E. Curry-Hyde and R. F. Howe, Eds., Elsevier Science Publishers BV, Amsterdam, The Netherlands, 1994, 25-41. 6. P. Courty and A. Chauvel, Catalysis Today 29 (1996) 3. 7. W. Scholz, Gas Separation & Purification, 7 (1993) 131 8. Chemical Engineering, August 1996, 46 9. J. Abrardo & V. Khurana, Hydrocarbon Processing, Feb. 1995, 43