1.4. Future Contributions of Chemical and Bioengineering
The solution of many of the pressing problems of society for the future (e.g., global warming, clean energy, manned missions to Mars) will depend significantly on chemical and bioengineers. In order to more fully explain the role of chemical and bioengineers and to illustrate the role of chemical and bioengineers in solving society’s technical problems, we will now consider some of the issues associated with carbon dioxide capture and sequestration, which is directly related to global warming.
Because fossil fuels are less expensive and readily available, we would like to reduce the impact of burning fossil fuels for energy, but without significantly increasing the costs. Therefore, it is imperative that we develop low-cost CO2 capture and sequestration technologies that will allow us to do that.
An examination of Figure 1.1 shows the sources of CO2 emissions in the United States. What category would you attack first? Electric power generation is the number-one source. Transportation sources are widely distributed. No doubt power generation would be the most fruitful.
Figure 1.1. Major sources of carbon dioxide emissions in the United States excluding agriculture
Carbon capture and storage (CCS) is viewed as having promise for a few decades as an interim measure for reducing atmospheric carbon emissions relatively quickly and sharply while allowing conventional coal-fired power plants to last their full life cycles. But the energy costs, the disposal challenges, and the fact that adding CCS to an existing plant actually boosts the overall consumption of fossil fuels (because of the increased consumption of energy to collect and sequester CO2, more power plants have to be built so that the final production of net energy is the same) all suggest that CCS is not an ultimate solution.
One interim measure under serious consideration for CCS that might allow existing conventional coal-fired power plants to keep producing until they can be phased out at the end of their full lives involves various known technologies. An existing plant could be retrofitted with an amine scrubber to capture 80% to 95% of CO2 from combustion gases; the CO2 would then be condensed into a liquid that would be transported and stored somewhere indefinitely where it could not leak into the atmosphere. If several hundreds or thousands of CCS systems were deployed globally this century, each capturing 1 to 5 metric tons of CO2 per year collectively, they could contribute between 15% and 55% of the worldwide cumulative mitigation effort.
However, the engineering challenges are significant. First, CCS is an energy-intensive process, so power plants require significantly more fuel to generate each kilowatt-hour of electricity produced for consumption. Depending on the type of plant, additional fuel consumption ranges from 11% to 40% more—meaning not only in dollars, but also in additional fossil fuel that would have to be removed from the ground to provide the power for the capture and sequestration, as well as additional CO2 needing sequestration by doing so. Current carbon-separation technology can increase the price tag of producing electricity by as much as 70%. Put another way, it costs about $40 to $55 per ton of carbon dioxide. The annual U.S. output of carbon dioxide is nearly 2 billion tons, which indicates the economic scale of the problem. The U.S. Department of Energy is working on ways to reduce the expenses of separation and capture.
By far, the most cost-effective option is partnering CCS not with older plants, but with advanced coal technologies such as integrated-gasification combined-cycle (IGCC) or oxygenated-fuel (oxyfuel) technology. There is also a clear need to maximize overall energy efficiency if CCS itself is not merely going to have the effect of nearly doubling both demand for fossil fuels and the resultant CO2 emitted.
Once the CO2 has been captured as a fairly pure stream, the question is what to do with it that is economical. In view of the large quantity of CO2 that must be disposed of, disposal, to be considered a practical strategy, has to be permanent.
Any release of gas back into the atmosphere not only would negate the environmental benefits, but it could also be deadly. In large, concentrated quantities, carbon dioxide can cause asphyxiation. Researchers are fairly confident that underground storage will be safe and effective.
This technology, known as carbon sequestration, is used by energy firms as an oil-recovery tool. But in recent years, the Department of Energy has broadened its research into sequestration as a way to reduce emissions. And the energy industry has taken early steps toward using sequestration to capture emissions from power plants.
Three sequestration technologies are actively being developed: storage in saline aquifers in sandstone formations [refer to S. M. Benson and T. Surles, “Carbon Dioxide Capture and Storage,” Proceed. IEEE, 94, 1795 (2006)], where the CO2 is expected to mineralize into carbonates over time; injection into deep, uneconomic coal seams; and injection into depleted or low-producing oil and natural-gas reservoirs.
Preliminary tests show that contrary to expectations, only 20% maximum of CO2 precipitates form carbonate minerals, but the majority of the CO2 dissolves in water. Trapping CO2 in minerals would be more secure, but CO2 dissolved in brine is an alternate disposal outcome.
Other suggestions for the reduction of CO2 emissions include permanent reduction in demand, chemical reaction, various solvents, use of pure O2 as the oxidant, and so on. See J. Ciferno et al., Chemical Engineering Progress, 33–41 (April, 2009), and F. Princiotta, “Mitigating Global Climate Change through Power-Generation Technology,” Chemical Engineering Progress, 24–32 (November, 2007), who have a large list of possible avenues of approach. The bottom line is that a solution for CO2 emissions reduction is not just a matter of solving technical problems but a matter of cost and environmental acceptance. Based on the nature of these challenges, it is easy to see that chemical and bioengineers will be intimately involved in these efforts to find effective solutions.