Nov-2021

New low carbon methanol production approach

An innovative steam methane reformer-based methanol production process achieves significant reductions in atmospheric CO2 emissions.

Dan Barnett
BD Energy Systems

Article Summary

BD Energy Systems introduces TrueBlue Methanol, an innovative low carbon emission steam methane reformer (SMR)-based methanol production process to the industry. This process utilises proven techniques to achieve greater than 90% reduction in the emission of CO2 from the stack of the SMR furnace while producing methanol with an overall energy consumption that is favourably competitive with even the newest operating SMR-based methanol plants. The TrueBlue process can be implemented not only on grassroot and relocated methanol plants but as an upgrade to many existing methanol plants for any natural gas fed process configuration.

This process delivers a product CO2 stream using an amine-based CO2 removal system placed upstream of the methanol synthesis reactor. Doing so reduces the consumption of hydrogen in the methanol synthesis process, resulting in greater hydrogen availability for SMR fuel. Removal of hydrogen from the synthesis loop purge stream recovers hydrogen for use as SMR fuel, and recompression of the carbon containing tail gas allows recycle of most tail gas to the SMR feed. This recycle results in more complete conversion of incoming natural gas feed to synthesis gas, and use of hydrogen as the primary fuel effectively reduces SMR stack gas CO2 emissions to a very low level.

This article will present key flowsheet elements of the TrueBlue process and an overall performance contrast of the BDE process with conventional natural gas fed SMR-based methanol plants.

Worldwide methanol production1 is largely based on the use of natural gas feed, with ~65% of total methanol production based on natural gas, ~35% based on coal, and less than 1% currently based on renewables. Achieving significant reductions in atmospheric CO2 emissions from natural gas-based methanol production is made possible with the approach outlined here.

Conventional smr-based methanol production
For the purposes of comparison, the ‘conventional’ SMR-based methanol plant is defined in general terms as one having a modern high efficiency SMR, an ‘isothermal’ methanol synthesis reactor, and combustion of the synthesis loop purge gas in the SMR. Overall energy consumption of a conventional SMR-based methanol plant is in the range of 27.0-28.0 MMBtu of total energy (LHV basis) per short ton of high purity methanol product. Total energy here is based on total natural gas, feed plus fuel, as well as net electric power import for the methanol plant and associated utility units.

Use of natural gas (96-97% methane) as feed results in the production of more hydrogen than required for methanol synthesis. This excess hydrogen is typically purged from the methanol synthesis loop and burned as fuel in the SMR. This reduces the make-up natural gas fuel required for the SMR; however, required natural gas fuel make-up remains in the 7-8% range in terms of total required heat release. Further, the hydrogen containing purge gas from the methanol synthesis loop also contains methane, carbon monoxide and carbon dioxide. Considering only the SMR fuel, not including fuel to a gas fired boiler or a gas turbine, the emission of CO2 to atmosphere is in the range of 0.35-0.36 weight CO2/weight methanol product. So, for a 2000 tpd conventional methanol plant, the SMR stack gas CO2 emissions would be in the range of 700-720 tpd (255,500 to 262,800 tpy).

TrueBlue Methanol production
The BDE TrueBlue Methanol plant design is also SMR based with several significant additions when compared with the conventional plant. First, the SMR front end includes the use of pre-reforming and convection section reheat of pre-reformer effluent to effectively shift a portion of the required reforming reaction duty from the radiant section to the convection section.

Second, an amine-based CO2 removal system is added immediately upstream of the synthesis gas compressor to reduce the proportion of CO2 feeding to the methanol synthesis reactor. This effectively reduces the quantity of hydrogen consumed in the production of methanol, reducing the amount of reaction water produced, which enables recovery of that hydrogen for use as SMR fuel:

CO + 2H2 Û CH3OH + Heat  [2 moles of H2 consumed per mol of CH3OH produced]    
                                           
CO2 + 3H2 Û CH3OH + H2O + Heat  [3 moles of H2 consumed per mol of CH3OH produced]

Third, purge gas from the methanol synthesis loop is routed to a pressure swing adsorption (PSA) unit to separate out a major portion of the hydrogen remaining in that stream for use as fuel. The tail gas from the PSA unit, containing CH4, CO2, CO, inert components and unrecovered H2, is recompressed to allow recycle of approximately 90% of the tail gas back to the SMR feed stream. Approximately 10% of the PSA tail gas is routed to SMR fuel to limit the accumulation of inerts in the synthesis gas. The recovery of hydrogen as described, along with the small PSA tail gas flow, are enough to supply all the fuel required for the SMR, reducing the natural gas firing of the SMR to zero. The recycle of approximately 90% of the carbon-containing components of the purge achieves more complete conversion of incoming natural gas feed to synthesis gas.

These added elements extract CO2 from the plant as a product stream while reducing the SMR stack gas emissions of CO2 by 90-93% when compared with a conventional plant.

Overall energy consumption of the TrueBlue methanol plant remains in the range of 27.0-28.0 MMBtu of total energy (LHV basis) per short ton of high purity methanol product. Total energy here is based on total natural gas feed with zero natural gas fuel, as well as net electric power import for the methanol plant and associated utility units.

With the use of hydrogen as the primary fuel and make-up fuel being a portion of the PSA tail gas, there is no need for natural gas fuel firing. Comparing to conventional SMR-based plants, considering only the SMR fuel, not including fuel to a gas fired boiler or a gas turbine, the emission of CO2 to atmosphere is in the range of 0.032-0.035 weight CO2/weight.