One of the major issues with lyocell production is energy usage. It’s inline with polyester, if not a little higher. Pulp production is specifically energy intensive. However, with plants like Lenzing's, which derived 100% of their energy needs from renewable resources, the effects on the climate start to mitigate.
The best up-to-date research on viscose production comes from Utrecht University in the Netherlands. It looked at the entire Life Cycle Assessment (energy, global warming potential, toxicity, water usage and land usage) of Lenzing’s modal, viscose, and Tencel production and compared it to conventional cotton and polyesters. The study seems pretty relevant however, some technologies have changed since the 2012 publication but I don’t think the results would vary a ton.
I will refer to one Tencel and one Modal used in the study. We will also provide research on Tencel's new Thailand facility.
Tencel Austria (From Study): Derived from a mixture of eucalyptus and beech wood. Eucalyptus is sourced from plantation farms in South Africa (see wood for more information) and European beech wood. The process uses 70% natural gas and 30% biomass for its energy demands.
Modal Austria (From Study): Derived from European hardwood that is sourced within the region. The entire production process is done at Lenzing’s Austria plant where energy demands are met by the biorefinery. Almost 100% of the energy needs are derived from renewable resources.
Tencel Thailand (Not From Study): Derived exactly the same way as Tencel #1 but it will be processed at Lenzing’s new facility in Thailand. All energy demands will come from the biorefinery, similar to Modal Austria. The majority of the biorefinery inputs will come from black liquor or bark from pulp production and municipal solid waste incineration (MSWI).
The study did not look at the Tencel Thailand’s energy profile, as the plant was not built at the time of the study. However, I believe we can take the conclusions of Modal Lenzing’s energy profile and apply them to Tencel Thailand with one small change - Tencel Thailand will use a lot less caustic soda compared to Modal Austria. Refer to Tencel Austria’s caustic uses in the study.
Theoretically, Tencel Thailand will have the lowest overall energy profile compared to all regenerated fabrics, cotton and polyester. The amount of energy needed is still quite high however, the use of renewable resources and incineration of by-products derived from the production process benefit it tremendously.
All three regenerated fibers use less non-renewable energy compared to polyester while Modal Austria and Tencel Thailand use less non-renewable energy compared to cotton.
I would note that if this study was redone today, I am 100% sure all Lenzing fibers will have a better energy profile compared to the study, as Lenzing has become even more energy efficient.