Outdoor Prototype Results for Direct Atmospheric Capture of Carbon Dioxide

Abstract

At Carbon Engineering, we have built a prototype air contactor with 10 m3 of packing volume, and have absorbed carbon dioxide from ambient air for over 1000hours of outdoor operation. Our prototype was built to: a) test our cross-flow, pulsed-liquid, PVC packing-based contactor design in an operational environment; b) to evaluate fan and liquid pumping energy requirements of our design; c) to assess technical and safety risks involved with liquid solution loss through entrained “drift” droplets, and; d) to examine for packing or solution fouling by atmospheric particulates. In this paper we present our results on the liquid pumping and fan energy requirements of our prototype, and show preliminary analyses we have conducted on liquid loss through “drift” droplets and NPE fouling.Our operation with pulsed liquid flow allows us to fully wet the surface of our structured packing with a brief period of full flow, then to cut flow for a duration of several minutes to let the liquid slowly react away as CO2 is absorbed. Using this technique we have cut average liquid flow to 10% of full manufacturer's specifications while retaining an average of over 80% of the capture rate observed at full flow. This method, under patent application, has allowed us to reduce our mgh fluid pumping energy requirement for the contactor to <40MJ/ton-CO2 (<11 kWh/ton-CO2). Here we report this energy requirement without accounting for pump efficiency, which we nominally assume to be ηpump=90%.Our prototype contactor has utilized high-performance XF12560 structured packing from Brentwood Industries (Reading, PA), manufactured from PVC rather than a stainless-steel. This choice has given us several advantages, chiefly that it costs far less on a per-surface-area basis, and that is provides a much lower pressure drop on a per- surface-area basis. We have previously shown data that suggests these plastic packings absorb CO2 with equal performance to traditional stainless-steel designs once wetted properly [1]. The high-performance and low pressure drop characteristics of our XF12560 product have allowed us to achieve PV work requirements of <350MJ/ton-CO2 (<100 kWh/ton-CO2). Again, this value does not reflect fan efficiency, which we commonly use as ηfan=55% in our analyses. This current value for PV work requirement is higher than we had previously reported, due to a specific cause of under-performance in our prototype which is discussed in this paper. Despite this, we are in fact very encouraged by the results of our first attempt at long-duration outdoor contacting, and we have numerous design improvements stemming from our prototype operation that will be included in the next iteration of our full-scale contactor design.Extensive air sampling was carried out during our contactor operation to measure the loss of liquid solution via “drift” droplets. During normal pulsed-flow contactor operation, 6 hour air samples taken from the contact outflow and measured for our K+ ion tracer, showed OH concentrations to be <5% of the OHSA regulated indoor respiratory requirement. After our 1000hours of integrated CO2 absorption time, we have as of yet been unable to observe any significant performance decrease in the absorptive properties of our working solution or in the efficiency of our structured packing product.