Want Clean Coal? Try This Idea

Non-thermal plasma is showing great promise in solving the coal plant exhaust dilemma. EPA based test protocol indicated that non-thermal plasma carbon conversion (C²) now is extracting 75% of the carbon dioxide, SOx, NOx and CO out of the plant effluent. The surprising statistic is that the parasitic draw for this operation is less than 4%. Based on these testing results, the University of Iowa College of business ran up assessment of the cost per performance of the equipment and found that the period of investment payback is less than four years. The same study indicates a strong financial advantage to installing carbon conversion equipment over converting coal plants to natural gas.

Plasma Carbon Conversion Technology

Plasma chemistry has attracted much attention with reports of cost effective conversion and extraction of CO2. The NTP approach can be performed at low temperature and normal atmospheric pressure. NTP has been successfully applied to CO2, NOx & SO2 from the flue gases. NTP is a mechanical process that removes pollutants by using cold plasma generated using the arc plasma process in a series of devices that convert the three pollutants back to their native species. CO2 is converted to carbon and oxygen. Likewise, SOx and NOx are converted the same way back to their components. This process uses no chemicals and consumes a reasonable amount of energy (3.5% parasitic draw), about 80% less than competing technologies like chemical looping and amine absorption. As a byproduct, it produces oxygen and carbon, which can be fed back to the boiler to increase combustion efficiency by as much as 12% to offset the parasitic power demand or sold as a way to develop a new revenue stream.

NTP technology compares favorably with its main competition, gasification, amine adsorption and chemical looping. While the estimates for capital cost for NPT technology are not as well developed as the other technologies, it is estimated that costs of approximately $300/kwh are very favorable. The other technologies have costs of thousands of dollars per kilowatt. Power consumption for NTP is also favorable compared to the other existing technologies at 5-6% vs 33%. NTP also has a significantly smaller footprint when compared to gasification, amine adsorption and chemical looping. The most significant advantage of NTP, however, is its suitability to existing boiler installations versus the requirement to build new to get the most out of the other technologies. The smaller footprint, the lower cost and the chance to recover some of the parasitic energy consumption by re-burning the gas makes NTP a viable choice.

How It Works

The technology, which resembles a series of small electrostatic precipitators, functions by cooling combustion gas below dew point and subjecting it to an intense energy environment called arc plasma generation to create a plasma that initiates molecular breakdown of the pollutants. The process then re-adsorbs the protons and electrons in the plasma, collecting the carbon in a number of small ESPs in the system and discharges the gas from the system. This gas, now containing oxygen, can be ducted back to the front of the boiler or to the cold side of the air preheater to become a quasi oxy-combustor system. This can happen because the availability of protons and electrons in the plasma gas allows breakdown and buildup of molecular species utilizing the protons and electrons as elements in this process. This is an extreme oversimplification of what is happening, but is a good place to start. The novelty of this process is in how the plasma is created. There are many ways to create plasma. The most expensive and complicated of all of these is thermal plasma like that used in chemical looping and gasification systems. In addition to the expense of this process (high energy consumption) there is a serious challenge controlling what the plasma does. The heat is so high that molecules continue through the desired reactions and fail to stop there, creating other molecules, some of which are not particularly desired. The other processes frequently involve lasers and or microwave energy to achieve the correct plasma states. These technologies work but can get expensive due to the components used.

NTP technology is unique in the way it generates the plasma gas. Like microwave and laser, but unlike thermal processes, there is very little rise in gas temperatures. The entire process takes place at 160 degrees F or less. The plasma is generated in a stainless steel chamber fitted with probe assemblies that contain probes in electrically isolated collars creating high voltage arcs that are directed at the center of the flowing gas at voltages as high as 30,000 volts, but more typically at 15,000 volts. In addition, there are fixed wave-guides that are used to control the process to maximize pollutant removal through breakdown and reformation. This is called arc plasma generation- it is simple, rugged and reliable for the purpose.

This is the NTP arc-plasma generator. The particulate matter that might normally be present has already been removed by other solids-capture equipment (further upstream in the gas) and condensation of the gas from 650 degrees F to 160 degrees F to allow the process to function.

The bulk plasma remains at or around the 160 degree temperature, but molecules in the gas are subjected to high temperatures caused by the arc generating assemblies and can reach thousands of degrees. The cool temperatures of the bulk plasma are highly desired in that they act to quench the continuing reactions, stopping the desired end products from being reformed into yet something else. This is why low temperature plasma is preferable.

The NTP process converts CO2 and, by extension, NOx and SO2, by using an interesting process that borrows from a number of different technologies. It basically creates plasma, which is defined as an activated gas stream rich in protons and electrons. The NTP energy states are such that existing molecules in the gas stream get broken up and new molecules form. This can happen because the availability of protons and electrons in the plasma gas allows breakdown and buildup of molecular species utilizing the protons and electrons as elements in this process. The novelty of this process is in how the plasma is created. He patented process results in low parasitic draw and, therefore, low operating costs.

The claims put forth by the technology supported by testing and pilot installations include:

1) Non-thermal plasma process application to fossil fuel burning power plants reduces CO2 by 50% to 70%  It reduces SO2 and NOx by 70% to 80%.

2) Process produces 10% to 15% waste carbon as by product which can be reused as fuel or producing activated carbon with market value.

3} Process has been able to recycle the treat effluent gas to serve as a co-fuel through the cold air intake and improve the efficiency of the operation by an estimated 8 to 12% which contributes to the short return on investment period suggested earlier.

There appears to be a way to have coal generated power but not the carbon release. This could signal the beginning of a new life for the coal industry as it finds a way to re-create itself in the low carbon era.