In: Mechanical Engineering
Design an ECBM system with methane recovery, electricity generation, and carbon sequestration for a coal field with 17 billion m3 methane capacity and a yield of 280 m3/tonne CO2 sequestered. The methane (assume it is pure CH4 and ignore impurities) is delivered to an advanced technology-generating plant where it is mixed with a stream of pure oxygen to deliver electricity for the grid, CO2 for sequestration, and water as a by-product. Of the energy available in the methane, 40% is delivered to the grid in the form of electricity. The remainder is dissipated in losses in the plant and to supply energy needed for the operation (methane, CO2 and water pumping, CO2 injection, and so on). All CO2 from the generating plant is injected into the coal field, so that there are no emissions of CO2 to the atmosphere. Additional CO2 is transported by pipeline from other sources located at a distance. The plant is planned to operate for 40 years at a constant output, after which all the methane will have been withdrawn. How much electricity does the plant produce per year? What is the net amount of CO2 sequestered, after taking into account the fraction that is derived from carbon that was under the ground in the first place?
Sequestration options are presented in this chapter in increasing order of complexity, but also in increasing order of potential for reliably and permanently sequestering carbon in unlimited quantities. Geological sequestration may provide the optimal balance of high potential volume of sequestration and manageable life cycle cost, and at the time of this writing, it is also the option receiving the greatest research effort. All discussion in the next several paragraphs assumes that our R&D efforts can achieve mature sequestration technologies that function reliably and that meet required cost – per-tonne CO2 targets.
Indirect sequestration is limited by the amount of earth surface area available for planting of trees or possibly exploitation of smaller organisms that perform photosynthesis. It may be possible to increase the yield of CO2 sequestered per square meter of area by using genetic engineering or creating some process to assist with the conversion of plant or tree activity into sequestered carbon, but the potential is limited. Nevertheless, in the short run it should be exploited vigorously, not only because of low cost but also because of ancillary benefits from forests.
Some questions remain at the present time about the permanence of geological CO2 sequestration, including the possibility of leaks from some reservoirs if the practice is used on a large scale, and the concern over contamination of drinking water or increases in seismic activity. If these issues are resolved favorably, then there may not be a need to pursue CO2 conversion. On the other hand, the issue of the “carbon legacy” lasting for thousands or even millions of years is difficult to rule out with absolute certainty. If CO2 conversion is sufficiently cost competitive, it may be desirable to prioritize it in order to completely eliminate any concern about negative consequences of geological sequestration that would be impossible to detect until long after human society had committed itself to the latter.
Energy penalties are important for all sequestration options. Due to high energy content in fossil fuels per unit of carbon combusted, it is often possible to sustain a significant energy penalty during the sequestration life cycle and still create an energy system that delivers electricity to the consumer at a competitive cost per kWh with no net CO2 emissions to the atmosphere. If, however, energy penalties are excessive, too much of the energy available in the fossil fuel is eaten up in the sequestration process, and fossil fuels become uncompetitive with nuclear or renewable alternatives.
The role of nonfossil resources in carbon sequestration raises interesting possibilities. In some cases it may be possible to use nonfossil energy sources to power any component of the sequestration process (air capture, separation, transmission, or injection of CO2, as well as recycling of CO2 into hydrocarbon fuels for transportation). On the other hand, depending on the efficiency and cost of the processes involved, it may be preferable to use the nonfossil resources directly in the end-use applications, rather than directing them toward the carbon-free use of fossil fuels. Also, per the preceding discussion, CO2 must be sequestered in real time as it is generated, while many renewable options function intermittently. Therefore, use of these intermittent resources for carbon sequestration would require backup from some dispatchable resource. We pose these questions about fossil versus nonfossil energy resources here without attempting to answer them; a quantitative exploration is beyond the scope of the book.
In all consideration of options, the “end game” for carbon sequestration should be kept in mind. In the future, the maximum concentration of CO2 in the atmosphere, and the length of time over which that maximum exists, will determine the extent and effect of climate change, and not the absolute volume of CO2 that is emitted over the remaining lifetime of all fossil fuels on earth. Therefore, imperfect or “leaky” sequestration options may still be useful if they can help to lower the peak over the coming decades and centuries.
Lastly, carbon sequestration, and especially geological sequestration, may be applicable first in the emerging economies, where utility operators are rapidly adding new power generating facilities to keep up with burgeoning demand. Each new power plant represents an opportunity to launch the separation and sequestration of CO2 without needing to retrofit plants that are currently operating, especially in the industrialized countries, at large cost and part way through their life cycle. These sequestration projects might provide a very good opportunity for the clean development mechanism (CDM) under the Kyoto Protocol (or subsequent UNFCCC protocols), in which industrialized countries meet their carbon reduction obligations by financing the sequestration component of the new plants. Timing is important—for geological sequestration to be useful for the CDM, our R&D efforts must perfect the technology and make it available commercially before the current phase of power plant expansion is complete. After this time, carbon sequestration will require retrofitting of plants, which would greatly increase the cost per tonne of CO2 sequestered.
7-8 Summary
Carbon sequestration, as overviewed in this chapter, is the key technology for making fossil fuel use sustainable over the long term; without some means of preventing CO2 from reaching the atmosphere, an increase in its concentration is inevitable if we are to continue to use fossil fuels at current rates. Options for carbon sequestration are evolving and will change over time. In the short run, the planting of forests is a viable alternative, along with sequestration in underground reservoirs on a demonstration basis in order to sequester some CO2 while learning more about its long-term behavior in these reservoirs. Enhanced recovery options, such as enhanced oil recovery (EOR) or enhanced coal basin methane recovery (ECBM), provide a means of sequestering CO2 while generating a revenue stream from recovered oil or gas that is otherwise unrecoverable. Gradually, as opportunities for forestation or enhanced recovery are exhausted, geological sequestration in saline aquifers or in the carbon pool at the bottom of the ocean may expand in order to increase the volume of CO2 sequestered, provided earlier results are satisfactory in terms of minimizing leakage and preventing negative side effects. Geological sequestration may eventually sequester all CO2 generated from the combustion of remaining fossil resources. Further in the future, conversion of CO2 to inert materials or air capture of CO2 to reduce atmospheric concentration may emerge, although these techniques are still in the laboratory. For all sequestration options, we have an imperfect ability to measure how much CO2 will be sequestered and its long-term fate once sequestration has started, so further research is necessary to clarify the extent to which sequestration can safely and effectively be used to prevent climate change.