Enabling Distributed Gas-to-Liquid (GTL) Opportunities at the Micro-Scale

Advancements in drilling technologies have provided beneficial energy sources but also contributed to significant flaring of natural gas. Similarly, more environmental pushes to deal with waste streams has increased the production of biogas. These sources of gas many times are smaller than most conventional conversion technologies at micro-scales of less than 1 MMSCFD. This increased availability of distributed small gas volumes has driven pushes for the development of small-scale technologies for its conversion to various products as a way to both monetize these streams and assuage environmental concerns from their emission. One of the biggest challenges in the development of small-scale technologies is overcoming the economic advantages resulting from the large scales in traditional processes. RTI has developed and demonstrated an internal combustion-engine based Microreformer as a syngas generator from natural gas. In this study, we evaluate the feasibility and economics of using this technology for monetizing stranded gas resources by producing liquid chemicals using two case studies demonstrated at pilot scale – methanol production and an advanced syngas-to-liquids (STL) technology.

In the first case study, RTI’s Microreformer technology uses commercially available internal combustion engine blocks, thereby countering the economies of scale with economies of mass manufacturing. RTI’s engine-based, impurity-tolerant Microreformer has been demonstrated at near-deployment scale, to process 50,000 SCFD natural gas feed. The use of air as an oxidant further reduces the cost of the system. We have demonstrated the operation of this air-blown non-catalytic partial oxidation (POx) system for over 250 hours cumulatively. Methane conversions > 90% were achieved, with very low oxygen slip (<0.5%) resulting in syngas with H2:CO of 1.6. Techno-economic analysis of the Microreformer showed that the cost of syngas at the small-scale was comparable to costs from conventional technologies at scales much larger in size.

Methanol production was integrated with the engine to demonstrate the viability of producing liquid fuels or chemicals. The use of air as an oxidant in the engine results in a diluted syngas stream. A 1 barrel/day (BPD) methanol skid was designed based on a commercial methanol synthesis catalyst and built to use a 10% slip stream of the engine exhaust. The methanol skid included gas conditioning and compression prior to a 2-stage synthesis reactor with interstage cooling. The process also included a hydrogen membrane to recycle tail gas back to the process. The data collected was used to validate techno-economic analyses of methanol production from the Microreformer at a 300,000 SCFD scale. The production cost was calculated to be ~$1/gal. This cost shows that while additional savings might be needed to make distributed methanol cost competitive with large-scale facilities, the Microreformer can be attractive for markets where gas resources are stranded.

A second example of a fuel production technology is an advanced syngas-to-liquids (STL) system RTI recently demonstrated at a pilot scale. Working with a vendor, a small-footprint 1BPD Fischer-Tropsch (FT) skid was constructed. The system is based on compact, heat exchanger reactors that consist of alternating channels of catalysts and heat transfer fluid, and hence provide excellent temperature control of the exothermic reaction. Downstream of the reactors are a set of product collection vessels, hot and cold traps, to condense organic hydrocarbons. The compact nature of the reactors and the excellent heat transfer play a crucial role in modularizing and lowering the capex of the small-scale FT system. While this FT skid was not directly integrated with the Microreformer, its operation was tested with a natural gas POx system at a partner’s test site. In particular, the system was tested with diluted syngas simulating air-blown POx at Standard FT synthesis conditions. High CO conversions were observed, and periodic organic product sampling revealed that 95% of the hydrocarbons had a carbon number below C-25 with a mean between 10-12. Distillation testing according to ASTM D-1160 showed that the majority of the FT product consisted of diesel and lighter hydrocarbons. The minimal wax production from the system further reduces plant complexity and reduces capex, and hence makes it attractive for stranded gas applications. Based on the operation of the 1 BPD pilot skid, RTI has developed detailed designs for a 12.5 BPD commercial-scale modular skid. Techno-economic analysis of the integration of the advanced STL process with the engine-based Microreformer has also shown promising results.

RTI’s Microreformer and other modularized technologies provide a unique, cost-effective technologies for the utilization of distributed micro-scale gas resources. In the case of the Microreformer, the use of a mass-manufactured engine greatly reduces the capex of the system. Low replacement costs and times mean that these systems can operate with a low opex and very little down-time if needed. RTI is currently evaluating this technology in several distributed gas markets in addition to flared/associated gas, particularly hog farms in Eastern North Carolina, where increasing environmental concerns with swine lagoons have led to widespread nuisance lawsuits. RTI is also assessing other biogas sources including waste water treatment plants, crop residues and landfills. These markets can have far-reaching impact, offering owners/operators/farmers an incentive to implement solutions that offer an additional revenue stream, while providing adequate waste management, and most importantly reinvigorating non-exportable manufacturing jobs in economically depressed rural America.