Carbon Dioxide's Unsettled Future

With world population climbing, and energy demand along with it, countries are trying to figure out how to minimize the global-warming consequences of carbon-based energy. The challenges are enormous: Because of differences in energy resources, nations around the world have different abilities to shift away from fossil fuel and to adapt technologies that reduce CO₂ emissions. And many of those technologies are not moving as fast as they could be because of uncertainty in public policies to reduce CO₂ emissions. These are the take-home messages from a conference held to stimulate ideas and form collaborations to quicken the pace of development and implementation of CO₂-emissions-reducing technologies.

“Many scientists and engineers recognize that energy production and controlling greenhouse gas emissions are our biggest technology challenges today,” chemical engineer Frank Zhu told C&EN. “If we continue business as usual, we can’t imagine how CO₂ emissions are going to impact the planet.”

Zhu is a senior technical fellow at UOP, Honeywell’s subsidiary and a leading technology provider for the global refining and petrochemical industry. He and other leading international experts on CO₂ capture and storage technologies met in Vail, Colo., last month at CO₂ Summit: Technology & Opportunity. The conference was sponsored by Engineering Conferences International along with the American Chemical Society, the American Institute of Chemical Engineers, the Chinese-American Chemical Society, and the Chemical Industry & Engineering Society of China.

Countries around the globe are pursuing three CO₂ solution pathways: CO₂ reduction, CO₂ rejection, and CO₂ dilution, Zhu pointed out in conference-opening remarks. To “reduce” CO₂, countries can cut emissions by improving the efficiency of vehicles, electricity generation, and industrial processing, he said. To “reject” CO₂, countries can develop ways to burn coal cleanly and use technologies to capture and store CO₂ to keep the greenhouse gas out of Earth’s atmosphere. And to “dilute” CO₂, countries can reduce their use of fossil fuel and increase use of carbon-neutral biofuels and alternative sources of energy such as solar and wind.

“We aren’t running out of fossil fuels yet, but in a carbon-constrained world, we are limited by the threat of climate change to which hydrocarbons we produce,” said Joseph B. Powell, chief scientist for chemical engineering at Houston-based Shell Global Solutions. “There is no one silver-bullet option,” Powell said. “We have to go down many paths.”

Shell has some carbon-capture demonstration projects under way and uses CO₂ to assist oil and natural gas recovery, Powell said. But the company is starting to move toward a more diverse mix of energy sources than just oil and natural gas. To that end, he said, Shell is pursuing several commercial efforts with business partners: It has teamed up with Iogen to produce ethanol from cellulosic materials. It is collaborating with Virent Energy Systems for thermal processing of sugars and eventually cellulosic material into gasoline and diesel fuel. It has a joint venture, Cellana, with HR BioPetroleum to use marine algae to produce triglycerides for diesel. And it is working with Codexis to develop enzymes that convert biomass into fuels and chemicals.

The public would like to have clean energy from these new technologies, but at the same price as petroleum-based products, Powell said. “We can’t implement new technologies for free, but our challenge is to do it at the lowest cost possible and with as limited a footprint as we can—new energy technologies have to be cheap, clean, and convenient.”

George A. Richards, focus area leader for energy system dynamics at the U.S. Department of Energy’s National Energy Technology Laboratory, agreed: “We need more energy, and it needs to be affordable, especially for countries with developing economies,” he said. “Like the U.S., these countries are going to use the fossil-fuel resources available to them first. It’s imperative we develop CO₂ capture and sequestration technologies that will allow us to do that. And that means we can’t abandon fossil-fuel energy research solely in favor of renewables.”

A mechanical engineer, Richards outlined CO₂ capture, storage, and use technologies that DOE labs are developing and testing in collaboration with academic and industrial partners, including power producers, petrochemical companies, process engineering firms, and industrial gases providers, all of which were represented at the conference.

One focus is on capturing CO₂ by passing power plant flue gas through an adsorption column containing an amine-based solution, Richards explained. Heat is used to separate the CO₂ from the amine solution, which is recycled to trap more CO₂. The separated CO₂ can be compressed and injected into the ground locally or transported via pipeline to a final destination.

But this strategy requires lots of heat. “The energy cost adds up,” Richards said. “Depending on the details of the plant, with existing technology, about 20 to 30% of the energy produced by a power plant would be needed to recover the CO₂. This energy penalty makes for bad economics.”

The penalty could be reduced with dry sorbents, Richards pointed out. Although they require a lot of solid material and constant heating and cooling to remove CO₂, their use is significantly more energy efficient than heating and cooling aqueous amine solutions, he noted.

One way to make a dry sorbent is by spray-drying the amine onto a clay sorbent, “which is literally dirt cheap to make,” he said. “These sorbents have demonstrated at least 8 wt % CO₂ uptake and stand up to more than 250 operating cycles, meeting our initial performance goals,” Richards said.

The energy penalty associated with regenerating wet or dry sorbents could be avoided altogether by implementing new combustion reactor technologies in power plants, Richards added. Two approaches, called oxy-fuel combustion and chemical looping, burn coal in a mixture of oxygen and recycled flue gas. After condensing water, the resulting exhaust gas is nearly pure CO₂.

Most projects to develop new CO₂ capture, storage, and use technologies are still government funded, Richards told C&EN. And they won’t move forward to commercial scale until they have been successfully demonstrated in pilot-scale operations.

“It’s not just a matter of solving technical issues. It is a matter of cost and social acceptance,” Richards said. “Cost remains a bottleneck for carbon-capture technology, and regulatory certainty is needed before investments will be made in large-scale sequestration.” In the U.S., climate and energy legislation to provide that certainty does not appear likely before midterm elections in November (C&EN, July 5, page 8).

Providing an Australian perspective, John N. Carras, a physicist and director of advanced coal technology at Australia’s Commonwealth Scientific & Industrial Research Organization (CSIRO), related that, like the U.S. and China, Australia depends heavily on coal, but for different reasons.

“Australia has a lot of coal: We are the world’s largest exporter of coal, and about 80% of the country’s electricity comes from coal,” Carras said. Not surprisingly, Australia is one of the world leaders in per capita CO₂ emissions. The country would like to reduce the approximately 25 metric tons of CO₂ it emits per person, Carras added.

Australia plans to increase the use of renewables significantly, which will help reduce emissions, Carras said. But renewables won’t be able to offset emissions from growing energy demand in the near term, he added. And unlike other developed countries, Australia can’t take advantage of nuclear power, because nuclear power has never been politically or socially acceptable there. That makes developing low-CO₂-emissions technology critical, Carras said.

Among those ideas being pursued in Australia are retrofitting an existing coal power station with oxy-fuel technology and four projects using amine-based CO₂ capture technology that are competing for billions of dollars in government funding. Australia also is working with a power company in China to test a pilot-scale amine-based CO₂ capture system. Most of these projects include CO₂ sequestration, he noted.

Globally, three sequestration technologies are actively being developed: storage in saline aquifers in sandstone formations, 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.

Overall, billions of tons of CO₂ must be captured and stored per year to stabilize atmospheric CO₂ at levels that should moderate global warming. Global capacity for sequestration is pegged at hundreds of billions of tons of CO₂, adequate for several hundred years of storage. Currently, only tens of millions of tons of CO₂—most of it from natural gas and not coal-fired power plants—are being squirreled away by demonstration storage projects and oil and natural gas mining operations.

Australia’s largest CO₂ storage project involves Chevron, ExxonMobil, and Shell and will capture and sequester CO₂ from the Gorgon gas fields in Western Australia, Carras noted. The plan is to inject some 4 million tons of CO₂ stripped out of natural gas per year over 30 years into a saline aquifer.

Smaller, but still critical, sources of greenhouse gas emissions are “fugitive emissions,” Carras explained. These are variable amounts of CO₂ and CH₄ stemming from coal mining, and in Australia, they make up 6% of the country’s greenhouse gas total, he said.

Spontaneous combustion is another source of fugitive emissions, Carras noted. Some open pit mines have areas where coal is smoldering and emitting CO₂, he said. “No one has been able to accurately determine the magnitude of these emissions, but we are trying to get an idea by using infrared thermal imaging, surface measurements, atmospheric monitoring, and modeling,” he said.

Like other countries, Australia is struggling to adopt a climate-change policy. The public broadly supports proposed emissions cuts, Carras said, and Australia is working on a carbon-reduction scheme. But the failure to reach an international agreement at the Copenhagen climate conference this past December has delayed legislation (C&EN, Jan. 11, page 27).

“Energy is central to everything that we do, which creates challenges for policy development,” Carras observed. “It is tough for countries to get it right. Some 20 years ago, we held a conference in Australia with the notion that we need to start doing something to address global emissions. Although a lot of progress has been made, the message is still the same today.”

China is one country where much of the work to control CO₂ emissions will take place. China recently surpassed the U.S. as the world leader in CO₂ emissions, and by 2030 the country is projected to emit nearly twice as much CO₂ as any other country.

“China has a lot of coal, but not much oil or natural gas,” commented Ke Liu, a chemical engineer and vice president and chief technology officer of China’s National Institute of Clean & Low-Carbon Energy (NICE), in Beijing. “We have to take advantage of our rich coal resources to meet energy demand,” Liu noted. “But if the world is trying to improve the environment and control global warming, it doesn’t help if China is not participating. To do that, China needs to step up R&D of low-carbon technologies to reduce CO₂ emissions.”

NICE, which opened this past December, is unusual as a national institute, Liu said. First, it is funded by a private company, Shenhua Group, the largest coal company in the world and one of the CO₂ conference’s sponsors. Second, it is led by an international board of directors drawn from leading global experts in coal technologies. Liu himself brings experience working on oil and natural gas for ExxonMobil, coal gasification for General Electric, and hydrogen fuel cells at other companies.

NICE’s board is defining a strategy for coal power generation that includes clean-coal technology to reduce pollutants, carbon capture and storage, CO₂ use as a feedstock chemical, coal conversion to natural gas, and coal conversion to synthesis gas to produce electricity and make liquid fuels and chemicals, Liu explained. The institute will help global companies quickly set up pilot operations in China to demonstrate new technologies, commercialize them, and make them available globally through licensing.

According to Liu, China plans to increase use of compressed or liquefied natural gas generated from biomass and coal because the country doesn’t have many natural gas reserves. Biomass can’t be grown in large enough quantities to displace fossil-fuel-based transportation fuels on its own, Liu said. But using China’s rich coal reserves to make natural gas makes sense, he noted, because natural gas produces 45% less CO₂ when burned than coal.

Coal- and biomass-derived diesel and natural gas could help displace petroleum-based transportation fuels, Liu noted. In particular, natural gas-liquid fuel hybrid cars would be inexpensive and a more practical future solution for China than electric vehicles or fuel-cell-powered cars, he said.

“Natural gas is an affordable, low-carbon platform for China,” Liu said. “It can be used to generate electricity, for heating and cooking, and for transportation. It’s a direction China should follow.”

At the other end of the coal spectrum is Brazil, which is an international model for renewable energy for its development of bioethanol and biodiesel. According to chemistry professor Jussara L. de Miranda of the Federal University of Rio de Janeiro, Brazil obtains 46% of its energy from biomass-derived fuels and hydroelectric power but only 6.2% from coal. Although biobased ethanol is carbon-friendly, ethanol production still produces a lot of CO₂, she said. As a consequence, once the anticipated international CO₂ emissions-trading schemes are worked out, it might be beneficial for Brazil to trap and sequester CO₂.

Miranda’s group is thus searching for low-cost ways to make sufficient quantities of metal-organic framework materials that can selectively capture CO₂. The researchers also are devising catalysts specific for C₁ chemistry to use the captured CO₂ as a feedstock to make a variety of small organic molecules.

“We are trying to establish a new paradigm for CO₂, thinking of the greenhouse gas not as garbage that we have to bury, but as a challenge for chemists to develop new technology to convert CO₂ into useful products that have both environmental and economical advantages,” she said.

Like most of the attendees at the CO₂ summit, DOE’s Richards believes many different energy technologies, from coal to solar, will be integral parts of the future energy mix. It’s not possible to know politically or economically which ones will play leading roles. “Predicting the future is easy, but predicting it correctly is more difficult,” he quipped. “I can say with confidence that we need more energy, and we want to manage the CO₂ emissions.

“And like many people, I am on the edge of my seat waiting to see how the Gulf of Mexico oil leak is going to go,” he continued. “No matter what happens, this disaster is going to be part of the future of energy debate in the U.S., and I expect it to impact policy decisions going forward.”