Evaluation of methane hydrate technology for the utilization of stranded natural gas

Abstract

Natural gas (NG) is a promising solution to reducing CO2 emissions from energy plants because it is the cleanest fossil fuel and an abundant energy resource, but it is largely under-utilized. About 40 – 60 % of NG is considered stranded because of low volume capacity fields, along with high transport and infrastructure cost for small and large size reserves’ utilization. The utilization of NG is further constrained with having relatively low energy density compared to other fossil energy resources. In addition, NG utilization technologies require stringent safety and environmental considerations being in gaseous form, which results in cost intensive operations. Among the various technologies for utilizing stranded NG, methane hydrates technology (MHT) could offer advantage for NG transportation. In this research, qualitative evaluation and comparison of typical technologies for stranded NG utilization was first carried out using focus indicators of; technology development stage, process complexity, gas volume capacity and storage, economic feasibility, environmental and safety merits. The observations revealed the key role of NG utilization as a low CO2 emission fuel and a prime contributor to the widely increasing global energy demand. Among the examined technologies; liquefied natural gas (LNG), compressed natural gas (CNG), gas to liquid (GTL), gas to wire (GTW), and pipeline technologies, the prospects of MHT were primly underlined with also its limitations on level of technology development in terms of research and commercialization. The need for further investigations on the feasibility of MHT aggregately for stranded small and large capacity NG reserves’ utilization was indicated. Commercial scale methane hydrate (MH) production simulation in a reactor from pure methane gas (pre-processed NG) and pure water at high-pressure condition was carried out and further processed to pellets for transportation of NG in three main project units; MH pellet production, transportation and regasification of the MH pellets. In this work, the MHT chain was studied with the focus on enhancing it for stranded natural gas utilization from small and large commercial reserves. For the production unit, a methane hydrate pellet production (MHPP) model was developed which comprised the reactor, hydrate slurry dewatering and pelletization, and pellet storage units based on pilot-scale system data in literature. The MHPP reactor model implemented in Aspen HYSYS® was used to simulate steady state operation of a jacketed continuous stirred tank reactor (CSTR) for MH production process with an adapted gas consumption rate correlation based on experimental study in literature. In various case scenarios, the developed MHPP model was used to size and investigate MH reactor and downstream combined filtration-pelletization machine with a base simulation of 9.16×10-3 m3 volume at 5.40 MPa and 285.15K iv for 10 kg hr-1 methane hydrate pellet production. On this basis, scale-up considerations were assumed in eight case scenarios for plant capacities employed for the evaluation of commercial gas reserve capacities. Therefore, from this study detailed commercial cost estimation protocol and data were obtained for the MHT chain based on developed MHPP model, sea transportation, and regasification framework for 0.3 - 566.0 bcm per year capacity reserves. This is applicable for the utilization of stranded NG from Nigerian Niger-Delta offshore region to the end-users’ market of Europe and Asian continents (10,000 km). Furthermore, a sensitivity analysis summary of the key parameters of MHPP reactor simulation was implemented in MATLAB with the HYSYS simulation data and revealed the significant effect of superficial gas velocity (gas injection rate) on the methane gas consumption rate, about double the effect each of stirring rate, pressure or subcooling. For the transportation unit, equations were developed based on consideration of sea transport leading to computation of the required number of ship bulker-carrier trips as well as round trip transport time associated with the MH production capacity and market distance. On this basis, the detailed costing of the MHT offshore/sea transportation was implemented. For the regasification unit including dehydrating system and compressor, a framework for the assumptions made relating to the main equipment and utilities required are presented. Finally, cost estimation and analysis of the MHT chain for stranded gas utilization was carried out, the results indicating that it is an economical option for stranded gas utilization for 2.8 – 566.0 bcm per year commercial reserve capacities (for 20 years project life) over 10,000 km market distances. In addition, the small-scale reserve category evaluation revealed that MHT shows the best economic viability for utilizing stranded gas compared to CNG for 2.8 – 25.5 bcm per year reserve capacities but does not seem viable for reserve capacities below that for 10,000 km market distance. CNG was observed to be the best alternative for small market distances of 2000 km for 0.3 – 2.8 bcm per year reserve capacity. For 28.3 – 566.0 bcm per year reserve capacity over 10,000 km market distance, MHT showed economic viability but LNG and CNG showed clear advantage over MHT below 7000 km and 5000 km respectively. As a result, LNG and CNG seem to be the best options for utilizing stranded gas of 28.3 – 339.8 bcm per year reserve capacities from Nigeria to European continent (less than 7000 km and 5000 km respectively).