Biogas Reforming as a Precursor for Integrated Algae Biorefineries: Simulation and Techno-Economic Analysis

Abstract

Biogas is a significant by-product produced in algae processing and may be used for many different applications, not only as a renewable energy carrier but also as a chemical intermediate in integrated algae-based biorefineries. In this work, the reforming of biogas to H2/CO2 mixtures (referred to as SynFeed) as feed for the direct hydrogenation of CO2 to methanol is investigated. Two conventional processes, namely steam methane and autothermal reforming, with upstream CO2 separation from raw biogas are compared to novel concepts of direct biogas bi- and tri-reforming. In addition, downstream CO2 separation from SynFeed using the commercial Selexol process to produce pure H2 and CO2 is considered. The results show that upstream CO2 separation with subsequent steam methane reforming is the most economic process, costing 142.48 €/tSynFeed, and taking into consideration the revenue from excess hydrogen. Bi-reforming is the most expensive process, with a cost of 413.44 €/tSynFeed, due to the high demand of raw biogas input. Overall, SynFeed from biogas is more economical than SynFeed from CO2 capture and water electrolysis (464 €/tSynFeed), but is slightly more expensive than using natural gas as an input (107 €/SynFeed). Carbon capture using Selexol comes with costs of 22.58–27.19 €/tCO2, where approximately 50% of the costs are derived from the final CO2 compression.

Renewable Fuels from Integrated Power- and Biomass-to-X Processes: A Superstructure Optimization Study

“…The pure CO 2 stream can be mixed with low-pressure CO 2 from carbon capture to provide raw material for thermochemical synthesis, or it just can be vented to the environment. SMR, ATR and TRIR are similar in that they include a series of reforming steps, namely pre-reforming at 500 • C, main reforming to syngas as well as two-stage water-gas-shift reaction at 350 • C and 210 • C to convert CO and H 2 O to H 2 and CO 2 [31]. In the end, the raw SynFeed is available at approximately 23 bar and cooled down to 40 • C for compression to 70 bar [31].…”

“…SMR, ATR and TRIR are similar in that they include a series of reforming steps, namely pre-reforming at 500 • C, main reforming to syngas as well as two-stage water-gas-shift reaction at 350 • C and 210 • C to convert CO and H 2 O to H 2 and CO 2 [31]. In the end, the raw SynFeed is available at approximately 23 bar and cooled down to 40 • C for compression to 70 bar [31]. Differences in the process design arise in the main synthesis reaction.…”

“…Differences in the process design arise in the main synthesis reaction. The SMR unit uses an externally fired isothermal reactor at 890 • C, where heat supply is provided by combustion of a share of the inlet bio-methane [31]. Hot flue gas is used in heat recovery and steam generation to produce excess electricity.…”

“…Hot flue gas is used in heat recovery and steam generation to produce excess electricity. ATR and TRIR both utilize partial oxidation using pure oxygen instead of air at temperatures around 1000 • C [31]. Due to internal heat supply, no excess heat is produced.…”

“…The raw SynFeed has to be upgraded to meet the H 2 /CO 2 ratio of the different thermochemical synthesis process options. Therefore, excess hydrogen from SMR is separated by pressure swing adsorption (H 2 -PSA) and excess CO 2 from ATR and TRIR is separated using physical absorption with Selexol as sorbent [31]. H 2 -and CO 2 recovery factors of the given technologies are 89% and 90%, respectively.…”