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Power without pollution: The Zero Emission Coal Power concept

Energy vs. economic output
Figure 1: Natural carbon pools vs. anthropogenic carbon dioxide emissions. Emissions are shown in blocks of 600 gigatons, that being 100 times the quantity currently emitted each year as a result of human activity. For further explanation, see the caption to Figure 2 in Ziock et al. from which this figure is taken.

Eighty-five percent of the energy used worldwide is obtained through the combustion of fossil fuels: coal, oil and gas. The consequent rate of carbon dioxide emission to the atmosphere exceeds by a factor of three the rate of absorption by vegetation, soils and the ocean (1). Thus, if emissions attributable to human activity were to remain at the present rate, the atmospheric concentration of carbon dioxide would continue to rise, initially at a rate of about 2 parts per million per year, but slowing as the rate of absorption increased with the increase in atmospheric concentration until an equilibrium concentration were reached. Models show that if current carbon dioxide emissions were reduced by two-thirds, atmospheric carbon dioxide concentration would not stabilize until reaching twice the preindustrial concentration.

World population and economic output per capita are both increasing. Moreover, energy use and economic output are tightly linked (Figure 2), which means that both world energy use and carbon dioxide emissions are increasing rapidly. The climatic consequences cannot be estimated with certainty, but if an increasing rate of atmospheric carbon dioxide emissions is maintained for a number of decades, there will be a manyfold increase in the equilibrium atmospheric carbon dioxide concentration, which is likely to have major impacts on both climate and ecosystems. It might also significantly influence human performance.

Energy vs. economic output
Figure 2: Per capita economic output versus energy consumption (From Ziock et al.)

To prevent a harmful accumulation of atmospheric carbon dioxide without curbing world economic growth, one or more of the following conditions must be met: (i) economic output per unit of energy consumed must increase faster than total economic output; (ii) fossil fuels must be largely replaced by alternative energy sources; or (iii) carbon dioxide produced in the combustion of fossil fuels must be sequestered from the atmosphere.

Although economic output per unit of energy consumed shows a long-term upward trend, the improvement currently fails to match the increase in total output (2). There is widespread interest and significant investment in the development of non-fossil energy sources, but without large-scale adoption of nuclear power, fossil fuels are likely to supply the bulk of the world's energy needs for the foreseeable future. Achieving condition (iii), i.e., preventing a substantial proportion of the carbon dioxide from fossil fuel use entering the atmosphere may, therefore, be the best hope for preventing environmental catastrophe without economic stagnation.

There are many interesting approaches to sequestering carbon dioxide from the atmosphere. They include injection into geological formations or the deep ocean, incorporation into stable minerals, e.g., by the conversion of olivine or serpentine to magnesium carbonate and silica, or by recycling through photochemically or biologically generated synthetic fuels (3). Among many proposals discussed at the first National Conference on Carbon Sequestration (4) sponsored by the U.S. Department of Energy, the Zero Emission Coal Power concept reported by Hans Ziock, and K.S. Lackner of the Los Alamos National Laboratory and D.P. Harrison of Louisiana State University, is among the most intriguing (1).

The process involves anaerobic gasification of coal to produce hydrogen without release to the atmosphere. The hydrogen can be used as a non-polluting transportation fuel, or to generate electricity by means of a highly efficient solid-oxide fuel cell. As an end-product, a stream of pure carbon dioxide is generated, which is well suited to mineral sequestration, or other means of permanent disposal. In outline, the four-stage process for generating electricity is as follows (5):

Gasification: Pulverized coal reacts with hydrogen and water vapor leaving solid ash residue in the reactor vessel. Other contaminants including mercury, nitrogen oxides and ammonia are removed as solids or liquids in purge streams for appropriate treatment and disposal. The major reactions are:

C + 2H2 yielding CH4 (methane),

C + H2O yielding CO + H2, and

CO + H2O yielding CO2 + H2

Carbonation -- Reformation: Methane from the gasification process is reformed to hydrogen and carbon dioxide, and the carbon dioxide is fixed as calcium carbonate by reaction with lime. The heat of the carbonation reaction drives the reformation of methane to hydrogen.

Solid-oxide fuel cell power generation: Hydrogen from the reformer is combined with atmospheric oxygen to produce electricity and water.

Calcination: Waste heat from the fuel cell decomposes calcium carbonate from the carbonation reaction to regenerate calcium oxide and pure carbon dioxide for disposal by mineral carbonation or other means.

Not only does this design for an electricity generating plant permit the use of the most abundant and cheapest fossil fuel without atmospheric emissions of any kind, but it has the potential to achieve an overall efficiency close to 70%, twice that of a conventional coal-fired electrical generating station and comparable with the overall energy-use efficiency of a state-of-the-art natural gas cogeneration plant. To learn more about the prospects for the commercial realization of this concept we interviewed Dr. Hans Ziock who's comments appear below.

An Interview With Dr. Hans Ziock

Dr. Hans Ziock, of the Los Alamos National Laboratory and an author of the Zero Emission Coal Power concept kindly provided these responses to our emailed questions:

naturalSCIENCE: What is the current scale of funding for the Zero Emission Coal Power research program?

Dr. Ziock: The research and development effort at Los Alamos National Laboratory through which the concept arose, is a Laboratory Directed Research and Development (LDRD) program which has provided approximately $1M per year to our work here. Additional research in this area is funded by the Zero Emission Coal Alliance (ZECA) Corporation. In addition, the US Department of Energy is providing funds through the National Energy Technology Laboratory for an ongoing collaborative effort on mineral carbonation, involving the Albany Research Center and Arizona State University.

naturalSCIENCE: Has a plant of any size been constructed to demonstrate the zero emission coal power concept?

Dr. Ziock: No integrated plant has yet been built, although individual component concepts have been tested at the pilot plant scale in the past. The 2nd reference (6) in the paper you are basing your article on (1) discusses a pilot plant used to test production of hydrogen from coal using calcium oxide. The Gas Technology Institute had tested hydrogasification at the pilot plant scale in the past. Both projects however had different end goals in mind. Solid oxide fuel cells are beginning to be tested in real applications (few hundred kW units) and Siemens Westinghouse recently announced their intention to build a solid oxide fuel cell production plant. These fuel cells however run on clean natural gas fuel and would need real changes to work with a "dirty" coal based gas stream. The initial goal of ZECA Corporation is the construction of a pilot plant to test the overall concept.

CO2 sequestration through mineral carbonation is still only at a bench top scale, not counting the vast quantities of CO2 that Nature has put away in that form. The issue here is finding a quick reaction mechanism that requires little or no input energy. Nature has demonstrated the no input energy part (but very slow) while we and our collaborators have demonstrated the sufficient speed part. The ongoing R&D is making progress on the combined solution.

I believe that the main technical challenge is the full integration of the different components that comprise the full concept. We anticipate a 5-year program to build a pilot plant, with the first 2 years being devoted to R&D. Process and equipment improvements and further integration work will continue during construction and operation of the pilot plant leading to the construction of a demonstration plant.

naturalSCIENCE: Has there been a study of the economics of zero emission coal power, and if so, what are the main conclusions?

Dr. Ziock: Nexant has done a preliminary economic analysis of a ZECA plant. I am attaching a
copy of a paper (see Ref. 4) of theirs presented at the 2001 Clearwater Conference.

naturalSCIENCE: Is there a realistic possibility of performing the hydrogasification process in situ, e.g., in unminable coal deposits?

Dr. Ziock: In principle it is possible, although it would certainly be a long way off. People have not yet demonstrated economic conventional underground gasification. I expect that by the time that one gets around to consider trying underground gasification in earnest, technology will have advanced sufficiently to make "unminable" coal minable so one will never get to underground hydrogasification.

naturalSCIENCE: How long do you think it might take to develop zero emission coal power for large-scale commercial use?

Dr. Ziock: This is a difficult question as it depends on a lot of external factors. Right now fossil fuel, especially coal, is dirt cheap and it is cheaper to burn coal inefficiently in old plants rather than build new plants. There is also the question of when CO2 emissions will begin to be restricted and what shape those restrictions will take, as well as what will happen with other airborne emissions. Finally there is always the question of funding and political priorities which will depend on all of the above. Energy security is clearly a wild card in all of this. A man-on-the-moon scale effort could probably produce the first commercial scale plant(s) in a decade, a slower approach to CO2 controls will probably push it out to about 2020.


REFERENCES

(1) Ziock, H.-J., K.S. Lackner and D.P. Harrison. 2000. Zero emission coal power, a new concept Proc. 1st US National Conference on on Carbon Dioxide Sequestration.

(2) National Energy Policy Development Group. 2001. Reliable, affordable, and environmentally sound energy for America's future. http://www.whitehouse.gov/energy.

(3) Mill, T. and D. Ross. 2000. Reductive sequestration of carbon dioxideProceedings of the First National Conference on Carbon Sequestration.

(4) 2000. Proceedings of the First (U.S.) National Conference on Carbon Sequestration.

(5) M. Nawaz and J. Ruby. 2001. Zero Emission Coal Alliance Project: conceptual design and economics. Proc. 26th Int. Conf. on Coal Utilization and Fuel Systems. Coal Technology Association, Clearwater, FL.

(6) McCoy, D.C., G. Curranand J.D.Sudbury. 1976. CO2 acceptor process pilot plant. Proc. 8th Synthetic Pipeline Gas Sympo. Chicago IL,October 18-20, American Gas Association Catalogue No.L 51176.

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