Sandia LabNews

Clean coal leads the way to a hydrogen economy for US


The path to the hydrogen economy leads through some familiar territory. Although there are many long-term options for providing hydrogen as a future fuel, coal is a leading contender in the near term.

That’s the view of Chris Shaddix (8367), principal investigator for clean coal combustion at Sandia’s Combustion Research Facility. While some day we may be able to produce hydrogen by breaking up water molecules in association with the high-temperature heat from nuclear power reactors, or through renewable energy technologies, right now the most cost-effective way to produce hydrogen is with coal, Chris says.

Chris and his colleagues are involved in a number of experiments to optimize the combustion of coal to produce the most energy and the least possible pollution. While traditional coal combustion produces many harmful emissions, modern plants can meet environmental regulations for burning coal cleanly, Chris says. This can be costly to utility companies, but the cost of competing fuels — particularly natural gas — have climbed to the point where burning clean coal is competitive.

Figure in the possible benefits of sequestration of carbon dioxide emissions from the stacks (see Lab News, Jan. 20 stories beginning on page 1) and coal looks very promising for generating both electricity and hydrogen to provide a bridge to that future technology. “Utilities are starting to invest in coal,” says Chris.

Two approaches

Two different approaches to burning coal are now under study. One combines coal with pure oxygen. The second, called gasification, burns coal only partially to create a fuel-gas. The first approach, called oxy-combustion, is driven by concern over emissions of CO2 and other pollutants. The burning of coal in oxygen is a near-term solution that with current knowledge can produce exhaust streams that are close to pure CO2, says Chris. Harmful pollutants like nitrogen oxides, sulfur compounds, and mercury are virtually eliminated.

The oxy-combustion approach is favored by companies in Japan, Canada, Germany, and elsewhere where pilot plants are under construction. “Because the US didn’t sign the Kyoto accord, companies here are not as interested,” says Chris. “They tend to favor gasification technologies, which offer higher efficiency and low pollution formation.”

One of these technologies, called steam reformation, combines the coal with steam in a hot environment to produce a “syngas,” composed mostly of CO and hydrogen. Once the syngas is produced it can be burned directly in a combustor — such as a turbine — to produce power. Or the syngas can be further reacted with more steam to shift the remaining CO to CO2 and to produce more hydrogen. The CO2 can be sequestered and the hydrogen can be used in lots of places: to power a car in an engine or fuel cell, to power a turbine to produce electricity, or to power a turbine to fly an airplane.

DOE has already demonstrated this process in two pilot projects. The next step is for the US to combine coal gasification with hydrogen production and CO2 sequestration, says Chris. At the same time, several commercial proposals are afoot in the US for private utilities to build these plants without government support.

Working with the National Energy Technology Lab, Morgantown, W. Va., the CRF is focused on understanding the chemistry and physics of coal combustion, using its state-of-the-art diagnostic capabilities and modeling expertise. “We apply computational models of reacting particles to the data to understand why we see the results we see,” says Chris.

Alejandro Molina (8367), a Sandia postdoc working with Chris, lights a flat-flame burner plate in the CRF’s small-scale lab for coal studies and adjusts the amount of coal particles fed through the burner. A two-foot-tall chimney around the burner protects against disturbances inside the lab and enables researchers to analyze the combustion.

He shows a visitor a bright zone, just above the burner, where initial combustion occurs. A longer vertical track of flame is known as the char oxidation zone. “To optimize coal combustion for carbon sequestration, it is very important to understand how fast it burns and releases energy,” Alejandro says. Burning coal with air, which is predominantly (79 per cent) nitrogen, creates the problem of separating CO2 and nitrogen before sequestration. “If you use pure oxygen instead of air, you get water and CO2, so you only have to condense the water and you have 100 percent CO2.”

One problem with this oxygen approach has been a high flame temperature, he continues, which could rapidly destroy the metal burner materials. One solution is to recycle cooler CO2 into the burner to cool the flame temperature. “The question is: what is the right proportion of oxygen and CO2?”

Alejandro has been working on these experiments for about two years in the small-scale lab, but work is now under way to bring two other CRF facilities into the research. A gasification lab will help the researchers study the behavior of coal gas under pressure. And while the small-scale work focused on particle behavior and fundamental-scale measurements, says Chris, a large-scale lab will focus on gas issues within the reactor.

Two new reactors

The gasification lab, expected to be operational by this summer, includes a two-inch tube within a pressure vessel. “Gasification is slower than combustion, so it is done under pressure to increase the reaction rates,” Chris explains. The new apparatus is instrumented for laser diagnostics and sample collection and includes electrical heaters to preheat the gases so they flow through a vertical center section where data can be collected.

The third reactor is a two-story flow reactor that will help the team study the oxygen-coal combustion with recycled CO2. The unit includes a six-inch-diameter reactor tube running downward below a 75-kilowatt thermal heater. Specially designed hardware injects highly refined coal particles into the top of the reactor tube. As the reaction moves down the tube, equipment allows sampling and laser diagnostic testing.