Life cycle assessment of high value activated carbon production based on mass and functional performance metrics

Abstract

The production of activated carbon (AC) from biomass holds substantial environmental potential, but its impact varies widely depending on the synthesis methods employed. However, unreliable experimental data results in inconsistent life cycle assessments (LCA), often dependent on generic or highly localized information. Most available data focuses solely on production metrics, neglecting crucial performance-based indicators. This study conducts LCA for a conceptual AC production facility designed to produce 1 kg of AC per batch of coconut shell (CS), particularly examining potassium hydroxide (KOH) and sodium hydroxide (NaOH) activation routes. Environmental impacts (EIs) are evaluated using two functional units-mass-based and adsorption-based-and span eighteen metrics, including six key ones: net energy, climate change (CC), ozone depletion, fine particulate matter formation, marine eutrophication, and metal depletion. CC (kg CO₂ eq.) for 1 kg of AC production is 1.255 for KOH and 1.209 for NaOH, while energy requirements (in MJ) are 28.314 for KOH and 27.063 for NaOH. Notably, the pyrolysis step emerges as the most energy-intensive and significant contributor to carbon emissions. Per the adsorption-based unit, the KOH-led pathway shows a higher adsorption capacity of 729 g/kg versus 662 g/kg for NaOH, requiring less AC per kg of dye adsorbed. Consequently, the KOH pathway achieves 5% greater energy efficiency and 6% lower carbon emissions than the NaOH pathway. Synthesized ACs outperform commercial AC in all metrics, especially in energy use and carbon emissions. The study proposes improvements, such as alternative drying methods, to mitigate EIs and emphasizes the need to consider both production efficiency and functional performance to guide sustainable AC production and application.

Keywords: Activated carbon; Adsorption; Biomass; Carbon footprints; Climate change; Environmental impacts; Life-cycle assessment; Net energy; Pyrolysis.

Fig. 1

System boundary and process flow for activating CS into AC.

Fig. 2

(a) XRD patterns, and (b) FTIR spectra for KOH and NaOH activated carbon.

Fig. 3

EIs of AC: KOH versus NaOH activation.

Fig. 4

EIs comparison: KOH and NaOH pathways with commercial AC.

Fig. 5

EIs of steps involved in the production of AC using KOH.

Fig. 6

EIs of steps involved in the production of AC using NaOH.

Fig. 7

Comparison of EIs between KOH vs. NaOH activation pathways based on per kg of dye adsorbed.

Fig. 8

Comparison of KOH activation pathway: oven drying versus sunlight drying normalized values.

Fig. 9

Comparison of NaOH activation pathway: Oven drying vs. Sunlight drying normalized values.

Fig. 10

Sensitivity analysis of AC with variation in KOH and NaOH.