|Alstom has made significant progress in proving the feasibility of the chemical looping concepts, including completing the first self-sustained operation of a chemical looping combustion unit. Photo courtesy of Alstom.|
Shin G. Kang and John L. Marion
Alstom Power Inc.
For hundreds of years, coal has been an essential, abundant and low-cost energy source that has been a major contributor to global prosperity and energy independence. The development of chemical looping technology is motivated by the need to maintain coal’s contribution to electricity production in a clean, economic and environmentally sustainable manner. Alstom, a leading provider of energy solutions for all these generation sources, is actively investing in research and development to improve these technologies to help customers meet growing demands in a cost–effective and environmentally sustainable way.
Chemical looping is a breakthrough clean coal technology. It utilizes a solid oxygen carrier to provide oxygen to either a combustion or gasification process. This advanced process is transformational in terms of its overall efficiency and cost. Chemical Looping Combustion (CLC) for steam-power is one of the lowest potential cost (cost-of-electricity) technical approaches that Alstom has identified to date for coal power with carbon capture.
Alstom has been developing chemical looping technology over the last decade, starting with small laboratory scale technical feasibility testing, completing techno-economic analyses, reference plant studies, process models and developing design tools. Alstom has recently achieved a self-sustained operation of limestone-based CLC process at our 3-MWth pilot-scale test facility in Windsor, Conn. This is the first of such achievements for chemical looping process in the world. Alstom is also active in the development of a metal-oxide (ilmenite) system, at the stage of proving the concept and oxygen carrier at a 1-MWth test facility in Germany, with several European partners.
What is CLC?
Various carbon capture and storage (CCS) technologies are under development. They include pre-combustion, oxy-combustion and post-combustion capture technologies. Among these, oxy-combustion is one of the most promising and cost-competitive CCS technologies for new coal plants.
In this process, a high purity carbon dioxide stream is produced which facilitates CO2 storage or utilization by burning fuel in an atmosphere free of nitrogen. Conventional oxy-combustion processes rely on pure oxygen supplied by air separation units (ASUs). These are typically based upon cryogenic distillation of air, a process that requires the air to be chilled down to -280° F. These extreme operating conditions make ASU systems relatively costly to build and operate
CLC is an advanced technical approach that aims complete elimination of cryogenic ASUs while still realizing oxy-combustion for a high purity stream of CO2. Solids in powder form are used to separate oxygen from air and supply it to a reactor for combustion (or gasification) of fuels such as coal, petcoke, biomass or natural gas. In addition, the solid oxygen carrier transfers the heat required for some of the fuel conversion reactions.
Figure 1 shows a schematic diagram of a simplified coal-fired CLC process, where two circulating fluidized bed (CFB) reactors are interconnected to form a loop. In the air reactor, a solid oxygen carrier picks up oxygen from air through an oxidation reaction to form a solid oxide and leaves nitrogen behind. This chemical reaction is exothermic and releases heat into the air reactor.
|The chemical looping process allows for the use of coal for power while capturing carbon dioxide emissions. Courtesy of Alstom.|
The hot oxygen-carrying solid particles are then transported to a fuel reactor. Once here, they release oxygen and convert the coal stream in the fuel reactor into combustion gases. The solid carrier also carries heat needed for fuel conversion. This oxygen separation and supply step occurs at a temperature close to the fuel reduction temperature. As a result, this reduces thermodynamic penalty.
After the release of heat and oxygen in the fuel reactor, the solid oxygen carrier is recycled back to the air reactor for regeneration. The solid carrier continues to circulate in the two-reactor loop, repeating an oxidation-reduction cycle, or a “chemical loop.”
The chemical looping process is analogous to the cycle our blood stream (oxygen carrier in CLC) goes through in our body – red blood cells in blood absorbing oxygen from the lungs (air reactor) and then delivering it to muscles (fuel reactor) for consuming sugar (fuel).
The concept is powerful and flexible, and can be developed further for other high value-added products. By changing the amount of solid oxygen carriers relative to the coal fed to the fuel reactor, high quality syngas (CO, H2, and other light hydrocarbons) can be produced as feedstock for petrochemical/refinery, as well as power generation processes. Further processing of the syngas in the CLC process produces high purity H2 (hydrogen), which can be used for ammonia synthesis, petroleum refining, fuel cells or other applications.
|Alstom has constructed a pilot-scale test facility to demonstrate using chemical looping and is planning on building larger facilities. Photo courtesy of Alstom.|
Alstom has significant experience in studying and developing advanced combustion and gasification processes for coal based power generation. In the mid-1970s, Alstom was involved in the development of coal gasification. A 120-ton per day pilot plant (equivalent to 12-15 MW) was built and operated for three and a half years at Alstom’s Connecticut site. This technology was further refined and used in a number of demonstration plants in the world. Subsequently, in the mid-1980s, Alstom pioneered the introduction of Circulating Fluid Bed (CFB) technology. Since then, CFB unit sizes have increased from demonstration scale (15 MW) in the 1980’s to nearly 400 MW today, and efforts are underway to supply 600 MWe+ CFB units with ultra-supercritical steam conditions. This technology shares many of the same solids handling, circulation and control challenges found within chemical looping technology. Alstom initiated a new effort in gasification in the mid-1990s with the aim of leveraging existing CFB technology. The objective was to develop a process that could produce syngas for gas turbines without an oxygen plant. The conceptualized process used a solids recycle loop to transfer the necessary oxygen to the system as well as an oxidizer and fuel reactor, a version of chemical looping. In the late 1990s Alstom launched a subsequent effort to investigate what we now call the chemical looping process and discovered its potential to be the lowest-cost coal power generation with carbon capture. Based on this early evaluation and supporting reaction experiments, Alstom has been pursuing the chemical looping process through a systematic development approach that has culminated recently in the achievement of auto-thermal operations. This achievement is only possible in a suitably-sized pilot facility which can generate enough heat from reactions to overcome thermal energy loss to the environment. Alstom’s commitment to pilot testing is based on a sound technical foundation from smaller-scale reacting and non-reacting experiments and on the company’s commitment to an innovative, cost effective and environmentally sustainable use of coal.
Through our innovation, we recognized that the choice of solid oxygen carriers dictates the design, performance and, most importantly, the economics of the overall CLC process. Recognizing this, Alstom has thoroughly evaluated and screened a number of oxide/sub-oxide systems in terms of their cost, commercial availability, oxygen carrying capacity, attrition behavior, toxicity, transport properties and attrition behavior. Many highly-engineered materials currently being designed by researchers are costly and require a dedicated supply chain system, whereas other materials are abundant, naturally occurring and, therefore, available in most of the locations a CLC power plant might be built. Based on these considerations, Alstom is focusing mainly on two: ilmenite and limestone-derived calcium sulfate.
The ilmenite (iron-titanium oxide ore)-based CLC process is being developed by Alstom in partnership with Chalmers University, Sweden, and Technical University Darmstadt, Germany, and with financial sponsorship from the European Commission Research Fund for Coal and Steel. Ilmenite is a low-cost, abundant material and, because of its density, can be readily separated from ash by-products and reutilized. The development effort is at the stage of proving the concept and oxygen carrier at a 1-MWth test facility in Germany.
Limestone-based chemical looping combustion (LCL-C™) technology is being developed in Alstom’s labs in Connecticut with the support from the U.S. Department of Energy National Energy Technology Laboratories (DOE/NETL). Limestone is even more abundant and low in cost. The process uses calcium sulfate (CaSO4) as an oxygen carrier while employing two “fast” CFB reactors. In this limestone-based process, the LCL-C™ process chemistry is very similar to that in commercial CFB boilers. The solid oxygen carrier, CaSO4 is produced in the LCL-C™ system as a result of calcination of limestone, followed by sulfation when it reacts with the sulfur released from the fuel. The materials in the LCL-C™ process (CaO, CaSO4 and CaCO3) are also commonly present in CFB boilers. The supply chain network of the raw material (limestone) and the byproduct (gypsum) is well developed throughout the world.
A unique feature of the LCL-C™ system is that a near-zero emission power generation plant can be built around the concept while continuing to utilize fossil fuels. In addition to CO2, emissions of other major pollutants such as Nitrogen Oxide (NOx), Sulfur Oxides (SOx) and ash are also controlled. Almost no thermal NOx is formed resulting in significantly low NOx emissions and SOx emissions. The limestone-sulfur reaction step inherently built into the process, the LCL-C™ process, does not require a dedicated sulfur emissions control system. Other pollutants such as mercury, heavy metals, and VOCs can be easily removed from the product stream as Alstom’s innovative Air Quality Control System (AQCS) concept is incorporated.
Alstom is now conducting a large pilot test, which has accumulated over 300 hours of operation to date and sustained auto-thermal operation conditions for more than 50 hours.
Next steps and Product Vision
|This isometric drawing shows a depiction of a 550-MW utility-scale power plant using the chemical looping process. Photo courtesy of Alstom.|
Alstom has developed this transformative technology from initial paper studies up to lab-scale prototype where main technology assumptions have been verified. Additional planned prototype testing will continue to refine and improve the process performance with the goal of achieving an optimized design suitable for a demonstration plant.
Alstom plans to bring chemical looping technology to commercial realization and is working to begin Pre-FEED and FEED studies aimed at beginning construction on a large pilot-scale (10- to 50-MWe) demonstration. Alstom has the product vision of first commercializing chemical looping for power generation at industrial scale (150 MWe) before fully scaling up to 600 MWe size and ultra-supercritical conditions.
In parallel, Alstom also is exploring the syngas and H2 options for the petrochemical and refinery Industry. This synergistic effort is expected to expedite introduction of the novel, breakthrough technology to the market.
There is no doubt that CLC technology could be a game changer for coal power generation. Alstom has made significant progress in proving the feasibility of the CLC concepts at various levels with much success, the most notable being the completion of the first-in-the-world chemical looping combustion unit with self-sustained operation. Alstom has also verified the economic benefits of chemical looping process over other carbon capture technologies.
Power Engineerng Issue Archives
View Power Generation Articles on PennEnergy.com