1. Introduction

Recently, hydrogen from low-carbon routes, has garnered attention as a high-density energy vector with low greenhouse-gas production emissions and no emissions at its point of use. The proposed sustainable route to produce Hydrogen is the gasification of biomass waste feedstock coupled with pre-combustion capture and long-term geological storage of carbon dioxide. This system, known as Bioenergy with carbon capture (BECCS), effectively removes CO₂ from the natural carbon cycle and is considered a technology that will play a pivotal role in keeping global warming well below the recommended 2°C since the pre-industrial period.

This research work focuses on the techno-environmental analysis of a complex product system that a) produces hydrogen as the main product b) captures and permanently sequesters carbon dioxide, a by-product and c) utilises municipal solid waste (MSW) feedstock, thereby diverting its fate from landfill or incineration.

This research highlights the use of biomass waste, resulting in lower costs and added environmental benefits, and its efficacy of producing grid quality hydrogen. However, the central theme of this work is an investigation into carbon capture technologies for the Waste-to-H₂ process. These include four that are already commercially available – i.e., amine scrubbing via monoethanolamine (MEA), a Benfield-like process, which uses potassium carbonate as solvent, Selexol® and Rectisol® - and Sorption-Enhanced Water Gas Shift (SEWGS). MEA and Potassium carbonate rely on chemical absorption. Selexol® and Rectisol® use physical solvents (dimethyl ethers of polyethylene glycol (DEPG) and refrigerated methanol respectively). SEWGS is a novel low-TRL technology that combines the water–gas shift (WGS) reaction with the selective adsorption of CO₂ on a potassium promoted hydrotalcite sorbent at high temperature and pressure.

2. Materials and methods

A sophisticated set of models to predict the performance of different pre-combustion CCS technologies, as well as the balance of plant on a fully integrated biohydrogen (Bio-H₂) system was developed using Aspen Plus simulation software. The facility is designed and modelled to convert approximately 110,000 tonnes per annum of municipal solid to approximately 50 MWh of grid-quality hydrogen. Upon collection, the waste is sorted and pre-treated. The obtained refuse derived fuel (RDF) is fed into a two- stage process for steam-oxygen gasification and tar-reforming to produce a clean syngas. The hydrogen fraction of the clean syngas is then increased via a series of catalytic water gas shift reactors. The carbon dioxide is separated using typical carbon-capture technologies and is then liquefied for permanent storage. Pressure swing adsorption (PSA) is employed to achieve the purity required for H₂ use for domestic heating, while the tail gas is burnt in a gas engine to generate heat and electricity.

Following the guidelines of the ISO 14040 and ISO 14044 standards, a Life Cycle Assessment (LCA) methodology was applied to investigate the environmental and carbon performance of greenhouse gas removal via capture of biogenic CO₂ from waste, while producing Bio-H₂. The multi-functionality of such a system lends itself to complexities arising from the choice of system boundaries, functional unit, and assumptions to integrate mature scale elements of the process with other sections at a lower technology readiness level. The function unit chosen here was 1 tonne of grid-quality Bio-H₂ produced.

3. Preliminary Results and discussion

An extensive analysis of the system was performed to assess the most appropriate carbon capture & storage technology for a BECCS system that produces hydrogen from waste. To this end two system boundaries (one focusing on stages affected by the technology change and another considering the complete plant) and different functional units were analysed. Although assumptions and results varied based on functional unit chosen, the ranking of technologies remained the same. Direct emissions are released on-site. Indirect emissions are associated with the supply chain and end-of-life treatment (where applicable) of chemicals and energy in UK. The negative climate change impacts represent avoided greenhouse gas emissions associated with the generation of excess electricity and the permanent sequestration of CO₂ from biogenic sources_. This isrelated to the biogenic fraction of waste that varies between 50 and 80% in weight. The results shown are given in terms of 1 tonne of grid-quality Bio-H₂ from the stages of wet MSW fed into the plant to the generation of Bio-H₂ and compression of CO₂, considering an average 65%biogenic carbon.

Carbon capture technologies based on physical adsorption (i.e. Selexol, Rectisol and SEWGS) outperform those technologies based on chemisorption (MEA and Potassium carbonate) in the climate change category. For chemisorption, high thermal energy requirements in the carbon capture stage to regenerate the solvent is the major contributor to climate change. Similarly, gas compression is the largest contributors for physical adsorption technologies. Major emission savings are realised due to the permanent sequestration of direct CO₂ process emissions.

The results obtained for the climate change category are corroborated by a comparative analysis across all environmental categories. Overall, MEA and Potassium carbonate solvents underperform while SEWGS consistently generates the lowest environmental impacts.

4. Conclusions

The work highlights the importance of BECCS as a negative emissions technology and the further environmental benefits of using a physical absorption technology for carbon capture, particularly SEWGS. This research work provides a real-world application of LCA to a BECCS system producing Bio-H₂ from waste and addresses some methodological complexities of prospective LCAs in order to progress from pilot/demo scale towards commercial scale production.