THE KVAERNER MEMBRANE CONTACTOR : LESSONS FROM A CASE STUDY IN HOW TO REDUCE CAPTURE COSTS

As the field of carbon sequestration and management matures, it will become increasingly important to bring new technology to the marketplace. In this paper, we review one such successful effort, development of the Kvaerner membrane contactor, in order to learn lessons about what is required to commercialize technology. We identify seven ingredients for success. INTRODUCTION A major barrier to the implementation of carbon management and sequestration strategies is the significant cost associated with the separation and capture of CO2 from flue gas. While there are many proposals on how to address this challenge, most ideas are never developed, let alone implemented. In this paper, through the use of a case study, we look at how ideas are generated and what i t takes to put them into practice. The case study presented here is the development of the Kvaerner Membrane Contactor (Falk-Pedersen et al., 2000). For the purposes of this paper, we will focus on natural gas combustion. However, the technology can also be applied to coal-fired combustion processes. The standard process to separate and capture CO2 from turbine exhaust gases is using an amine process. In an absorber, the exhaust gas is bubbled through the amine solution, which preferentially absorbs the CO2. The amine solution is then heated and almost pure CO2 is driven off in the stripper. The amine solution is then cooled and recycled to the absorber. This process is commercial, but has several drawbacks in the application to CO2 sequestration. The process is very energy-intensive and too expensive for most applications today (Herzog, 1999; David and Herzog, 2000). In addition, the size and weight of the equipment is quite large. While this does not pose a problem for many sites, it does pose major problems for offshore applications. Many approaches have been suggested to address the barriers to CO2 sequestration posed by separation and capture. One path suggests improved solvents to replace today’s amine solutions. Another suggests combustion of the fossil fuels in oxygen, rather than air, thereby drastically changing the exhaust gas composition to simplify the separation process. Yet another approach suggests steam reforming the natural gas, followed by the water-gas shift and CO2 separation from the high pressure synthesis gas, leaving mostly hydrogen to go through the gas turbine. Given all these options, it is very instructive to understand why Kvaerner chose to develop the membrane contactor. DEVELOPMENT OF THE MEMBRANE CONTACTOR Phase 1 – Problem Definition In 1991, the Norwegian government instituted a carbon tax in the North Sea of approximately 50 US dollars per tonne of CO2 emitted to the atmosphere. A major source of these emissions was the exhaust gas from the gas turbines that powered these offshore operations. Since the CO2 tax on these emissions accounted for about 20% of the operating cost on a platform, there was a great incentive to reduce them. Motivated in part by this carbon tax, Kvaerner held some initial discussions in 1992 with several Norwegian oil companies concerning the reduction of CO2 emissions from oil and gas operations in the North Sea. The initial scope was quite broad, covering improved efficiency, new power generation schemes (e.g., oxygen turbines), as well as removing CO2 from flue gases of conventional gas turbines. After some initial scoping studies, it was determined that pursuing a strategy of using a chemical (e.g., amine) scrubbing process to capture CO2 from the turbine exhaust was the most promising. Note that it was at this very early stage of the project that the exhaust gas scrubbing path was chosen over oxygen combustion or steam reforming the natural gas. The commercial amine process was not well suited for offshore operations, primarily due to the significant weight and size of the process equipment. To adapt the amine process for offshore use, Kvaerner formed a Joint Industrial Project (JIP) with several oil companies and the Norwegian government to undertake the major R&D effort that was required to get these features down. Phase 2 – Solution Definition The R&D program focused on largest components in the process, namely the waste heat recovery unit, the absorber, and the stripper. Also, the recycling of exhaust gas to increase the CO2 partial pressure was investigated (this work actually began in Phase I). The result of this later study was that a recycle up to 40% of the exhaust gas shows promise in theory, but it means that the gas turbines must be modified. Gas turbine modifications were beyond the scope of this project, so the final design had no exhaust gas recycle. Work on the waste heat recovery unit was done in conjunction with SINTEF (see http://www.sintef.no) in Norway. The goal was to come up with a compact design for offshore applications. Eventually, Kvaerner decided not to continue working on this unit because it was not in their main business stream. Instead, they allowed ABB to lead this effort. The end result is that ABB now offers a commercial version of this compact waste heat recovery unit (Pål Kloster, 1998), with the first unit just being put into operation by Norsk Hydro (Oseberg field in the North Sea). One approach in addressing the issues associated with the absorber and stripper was to investigate alternate packings. However, the constraints were fairly stringent. A certain amount of area was required for mass transfer and velocities were limited due to entrainment considerations. One packing that did show promise was Higee (vapor/liquid contacting in high acceleration fields within rotating packing). Another approach identified by Kvaerner for improved absorbers and strippers was the membrane contactor (see Fig. 1). Here, membranes are used to increase mass transfer areas in a given volume and to avoid some of the problems associated with vapor/liquid contacting. The membrane itself does not perform the separation, that job is still done by the amine (see Fig. 2). Others had tried using membrane separators in past, but they were not a commercial success. Nonetheless, an R&D effort was started, with work conducted at TNO in the Netherlands. A large number of membrane types were tested, but the amines destroyed some and wetted others (causing blockage). Only one type of membrane worked, PTFE (Polytetrafluoroethylene). Based on the above research, engineering studies were conducted. It was concluded that a process containing a membrane absorber with a PTFE membrane from W. L. Gore and Associates GmbH, a Higee stripper, the compact waste recovery boiler, and aquifer disposal (similar to Sleipner) was feasible. Therefore, it was decided to proceed to the next phase to see if the components performed as required. In Out Amine without CO2