mills and bio-waste to power plants. There are various ways of integrating the PtX plant into the CO₂ provider facility. An example would be to route the steam demand from the carbon capture and methanol plant into the pulp mill or a bio-waste to power plant, aiming to minimise its cost and loss in power production. Another example of process integration would be dealing with side streams; for example, oxygen is freely produced in PtX as a by-product. Although oxygen has a well-established market, it is quite often a saturated market; hence, finding new off-takers may be challenging, especially if the market becomes flooded with oxygen from many MW (or GW) scale PtX projects. One option is to find a nearby off-taker, like the pulp mill itself. Here, knowing intimately how a pulp mill operates creates a leading advantage for the project by monetising this side-stream that otherwise would have been vented or sold at a marginal price. This is another example of how partnering with an engineering specialist in PtX and associated industries can give you the extra edge in your project’s viability. Case study: viability of integrating an e-methanol facility in a pulp mill A pulp mill was considering revalorising ca. 70 MW of surplus electricity by producing e-methanol. The three main operating units to deploy are: the hydrogen plant (electrolysis- based), carbon capture and conditioning, and the methanol synthesis loop. Figure 3 shows a representative diagram of the main units in an e-methanol facility. Biogenic CO₂ is abundant in pulp mills and, as such, was not a limiting factor in sizing the
e-methanol facility. Amine-based carbon capture technology for flue gas applications was selected in this project as a proven way of recovering CO₂. The content of CO₂ in the flue gas was around 15 vol%, whereas the purity of CO₂ recovered was more than 98 vol%, with the balance being mainly water. The front end of the carbon capture plant was designed to remove particulate matter, SOx, and NOx, although it was important to ensure there was no sulphur slip (from reduced species) in the recovered CO₂, which would irreversibly damage the catalyst if it slipped through to the methanol synthesis loop. The technology to remove reduced sulphur species is readily available; however, proper sulphur speciation is required for an adequate design. The flue gas from the recovery boiler comprises <10 ppmv of reduced sulphur, of which >80% is H₂S and the balance light sulphur organic species. The low-pressure steam needed to run the reboiler in the carbon capture section is taken from the main facility condensing turbine. Hydrogen production was the limiting factor in this case study (in fact, renewable power to produce hydrogen) and very likely in most projects of this type. Given the power profile available, alkaline water electrolysis was well suited. The hydrogen generated is at 99.95 vol% purity, the balance being mostly moisture since the deOxo catalytic unit depletes oxygen that slips with hydrogen. The oxygen by-product is sent back to the mill as a chemical consumable, creating a credit for the e-methanol facility. Hydrogen and CO₂ are mixed in an over- stoichiometric ratio. A sulphur guard upstream of the methanol converter removes any reduced sulphur that comes with the CO₂, extending
Stack
CO -depleted ue gas
Boiler feed water
Steam to distillation
CO
Hydrogen
CO scrubbers & gas cleaning
Separator
Hydrogen plant
MeOH AA grade Condensate
MeOH converter
Steam drum
Oxygen (back to mill)
Recovery boiler
Distillation
Power Demin water
Figure 3 Schematic representation of e-methanol plant adjacent to a pulp mill
www.decarbonisationtechnology.com
64
Powered by FlippingBook