Green energy can be thought of as renewable power, energy storage, synthetic fuels (particularly sustainable aircraft fuels), and industrial steam/heat. The energy transition involves a number of technologies, but particularly carbon capture. The first question is whether sequestration is available. If not, renewable fuels and energy will be the primary path. If sequestration is available, carbon capture will likely be deployed. Renewable fuels include green hydrogen, biomass, biowaste, and renewable natural gas.
The EU does not allow growing a crop to convert to fuel. In the EU, there is a waste hierarchy which leads to energy recovery after reduce, reuse, and recycle. For a boiler, the most straight forward approach is to substitute hydrogen for natural gas. The DOE target is $1/Kg hydrogen. That translates to about $ 7/MMBTU. However, transportation and delivery of hydrogen is difficult. Delivered cost can be much higher. Some hydrogen will be made from grid power, but currently grid power is more carbon intensive than natural gas. Methanol may turn out to be a better solution for transportation fuels. Low cost solar or wind is usually “behind the fence”. However, they are still intermittent. Distribution is still a significant cost. To get “green steam”, an electric boiler driven by renewables could be applied. Intermittency is still a problem. Heat pumps can also be applied, but that doesn’t change the intermittency problem. Thermal storage can smooth out the problem. B&W is using sand as the storage medium. With carbon sequestration available, carbon capture technologies become applicable. There will likely be a capture business, a transportation business, and a sequestration business.
For capture, there is post combustion capture (amine scrubbing), oxygen firing, and fuel processing. Amine scrubbing is essentially commercial (although the largest unit is around 100 MW). Oxygen firing has been demonstrated. However, the boiler must be essentially leak proof. A package oxyfired boiler is already slightly pressurized, so leakage is not as much of a problem. Electric power can be generated to drive a small ASU as well as the CO2 compression station. The usual fuel can be used and the CO2 sequestered. Finally, B&W is developing a chemical looping system called BrightLoop. The system consists of three vessels. The lower vessel is a fluid bed with a solid particle that is an oxygen carrier. Air is introduced in which the particle absorbs oxygen. The depleted air exhausts. The particles are separated and sent to a moving bed. The particles transport to a higher vessel where fuel is introduced. The fuel takes the oxygen from the particle and produces a relatively pure stream of CO2. The CO2 can be sequestered. The system can also be operated to produce hydrogen. A pilot plant was built at the DOE/Southern Co test center combined with KBR on the gasification side. A 15 ton/day hydrogen plant is planned. The oxygen carrier is an iron oxide particle engineered to operate between two oxidation states of iron. There are other substances added to help control the oxygen uptake and oxygen pickup by the fuel.
– Brian Higgins, The Babcock and Wilcox Company
