Sorption-enhanced processes for simultaneous waste utilisation, CO2 capture and hydrogen production

Sorption-enhanced processes for simultaneous waste utilisation, CO2 capture and hydrogen production

Dr Dawid Hanak, Associate Professor in Energy and Process Engineering, Cranfield University

Rapid development and commercialisation of energy efficiency measures, renewable energy production and low-carbon technologies, such as carbon capture and utilisation (CCU) and low carbon hydrogen (LCH), are critical to meeting commitments made by country signatories to the Paris Agreement for reducing greenhouse gas (GHG) emissions. In addition, companies are looking to remove fossil fuels from their supply chains and meet consumer-driven demand for greener products.

Waste-to-energy (WTE) coupled with sorption-enhanced processes is forecasted to be a promising approach for reducing GHG, the amount of waste sent to landfill and our dependence on fossil fuels. Yet, this technology is still at an early stage of development, and further R&D activities are necessary for its rapid commercialisation.

In addition, replacing fossil fuels with alternative energy vectors, such as LCH and hydrogen-derived fuels (i.e., ammonia, methanol, synthetic natural gas and synthetic aviation fuels), form a key part of technology-based strategies for decarbonising industry, transportation and heating. Over 50 countries globally have already devised a dedicated hydrogen strategy and are investigating how to obtain hydrogen at scale. Currently, nearly the entire global hydrogen supply (70 million tons per annum, tpa) still depends on the conversion of fossil fuels¹. Furthermore, the International Energy Agency (IEA) forecasted that hydrogen demand will grow to more than 200 million tpa by 2030 and 500 million tpa by 2050¹. As such a large amount of LCH is required, different sources and production methods need to be pursued.

Biomass is widely considered as an alternative feedstock to fossil fuels for energy and hydrogen production, given its low carbon characteristics. As a result, a significant amount of R&D has been undertaken over the past decades to develop conversion technologies such as gasification and associated syngas upgrading processes.

However, there are major challenges to using traditional virgin biomass, e.g., food crops, as the backbone for the hydrogen economy, not least the significant land and water requirement to produce it at the necessary scale and the potential competition with food supply chains. Therefore, more sustainable sources of biomass, e.g., non-food crops, crop waste, forestry waste and municipal solid waste (MSW) have also been considered as alternative feedstocks.

Notably, approximately 2 billion tpa of MSW are generated worldwide and this is forecasted to increase to 3.4 billion tpa by 2050³. In the UK, 7.2 million tons of MSW were produced in 2018⁴. The organic content of MSW varies between 40–70%, with developing countries being at the highest end of this range⁵. Approximately 4% of the UK GHG emissions are still associated with MSW, mostly due to its deposition in landfills⁶, which accounted for UK emissions of 12.8 million tons of CO2 equivalent (tCO_2eq) in 2020⁷.

MSW streams have an average energy content between 5–22 MJ_HHV,ar/kg^²,⁸, which is comparable to biomass (i.e. rice straw – 13.4 MJHHV,ar/kg; manure – 13.6 MJ_HHV,ar/kg²) and some fossil fuels (i.e. peat coal – 10.9 MJ_HHV,ar/kg; lignite – 6.8 MJ^HHV,ar/kg²) (Figure 1). Consequently, MSW can be converted into valuable fuel products using novel technologies, such as sorption-enhanced processes. These can efficiently convert MSW into thermal energy and/or clean fuels at near-zero CO₂ emissions, when linked with carbon capture, contributing to the decarbonisation of the energy and industrial sectors. This is explained in the proceeding sections.

Hydrogen production via conventional gasification and sorption-enhanced gasification

Although hydrogen production from MSW can be achieved via biological or thermochemical processes, the latter are most attractive because they offer faster process kinetics and lead to higher H₂ yields⁹. Among the main thermochemical routes, gasification offers better results in terms of H₂ yield when compared with pyrolysis¹⁰.

MSW conversion to H₂ via gasification occurs at temperatures between 700°C and 1200°C, which is necessary to support the endothermic gasification reactions. A gasifying agent, such as air, oxygen, steam or CO2 is normally required. Pure oxygen (up to 64 gH₂/kg biomass) rather than air (~15 gH₂/kg biomass) leads to higher H₂ yield and more concentrated product streams with higher calorific values because there is no nitrogen dilution¹¹. Steam gasification (up to 128 gH₂/kg biomass) leads to even higher H₂ yields when compared with O₂ gasification. This can be attributed to the decomposition of water, the water gas shift (WGS), and methane reforming reactions. The addition of steam also moves the equilibrium of the WGS reaction
(CO+H₂O<->CO₂+H₂) to the right, enhancing the H₂ yield (Table 1).

The products of biomass gasification are tars, chars, and syngas, with the typical syngas composition being 30–50%vol of H₂, 25–40%vol of CO, 8–20%vol of CO₂ and 6–15%vol of CH₄¹². It can be further upgraded in the WGS reactors and pressure swing adsorption (PSA) processes to produce H₂ with 99.9% purity. Yet, the endothermic characteristic of conventional gasification means that energy needs to be supplied from an external source. This is usually done via combustion of fuel, resulting in CO₂ emissions.

In contrast to conventional gasification, a sorption-enhanced gasification (SEG) process combines the solid fuel gasification process with simultaneous CO₂ removal to produce H₂-rich syngas (Figure 2). This is achieved by the presence of a CO₂ capture sorbent, such as calcium oxide (CaO), in the gasification chamber which is held at approximately 600–700°C. This temperature has been shown to maximise the CO conversion and CO₂ capture rate¹³. The so-called carbonation reaction (CaO+CO₂<->CaCO₃) is exothermic and releases enough heat to balance the heat demand needed to drive the endothermic gasification reaction. Consequently, the SEG process does not require an external combustor to drive the gasification process, avoiding the corresponding CO₂ emissions.

Moreover, the underlying premise of such a process is an enhancement of the H₂ yield by removing CO₂ from the gasification environment, shifting the equilibrium of the water gas-shift (WGS) reaction further to the right¹⁴. For example, considering the same feedstock composition and input, the study by Santos and Hanak¹⁵ revealed that SEG will result in a substantial increase in the H₂ yield (up to 80 gH₂/kg MSW), and thus concentration (Figure 3), compared to the conventional gasification process (up to 70 gH₂/kg MSW).

It is also important to highlight that the SEG process uses natural sorbents, such as calcium oxide (CaO). This sorbent is considered a promising catalyst and CO₂ capture material due to its high availability, low cost, and potential use of the spent material in the cement industry¹⁶. SEG would produce about 146 kg of spent sorbent per each tonne of CO₂ captured. However, stream can contain sulphur compounds (i.e., CaSO₄) and ash¹⁵ that may prevent its industrial re-use.

The main downside of the emerging SEG process is that the sorbent needs to be regenerated, in the so-called calciner, at a temperature above 900°C. The state-of-the-art process designs assume that high-grade heat to drive the calcination reaction is supplied via oxy-combustion of fuels directly in the calciner. This, in turn, results in an added energy penalty for producing pure oxygen of about 200 kWh/tO₂. Alternative options to supply high-grade heat to drive the calcination reaction, including indirect heat transfer from air-fired combustor17 and solid-oxide fuel cell¹⁸, were considered. However, such concepts are still in the early stage of development.

Another major challenge of using Ca-based sorbents for SEG is a reduction in the sorbent CO₂ carrying capacity (Figure 4) on cycling operation due to sintering, attrition and sulphation20. Consequently, the deactivated sorbent needs to be partially replaced by fresh sorbent to keep acceptable sorbent conversion in the carbonator. This further increases both the demand for high-grade heat and the operating cost. To reduce fresh sorbent requirement and improve the cyclic performance of SEG, the Ca-based sorbents can be enhanced via hydration (CaO+H₂O<->Ca(OH)₂), doping with salts or acids, and thermal pre-treatment. Synthetic sorbents were also shown to deliver much better cyclic performance²¹. However, the economic viability and commercial scalability of alternative sorbents have not yet been proven²².

Energy and environmental benefits

The core premise for integrating CO₂ capture and H₂ production is to enhance the energy performance of the gasification process while reducing the amount of CO₂ released into the atmosphere. Higher H₂ yields achieved through the enhancement of the WGS reaction will naturally result in syngas of a higher quality that, in turn, requires less purification effort. Yet, the actual energy performance of the process is highly dependent on the gasification operating conditions, such as the steam to MSW ratio that stands for the amount of steam needed to gasify a unit amount of solid fuel.

The primary function of the gasification process is to convert MSW into a high-purity hydrogen stream. The energy performance of such a conversion process is usually characterised by the ratio of the hydrogen energy produced and the amount of energy input with MSW. This performance metric is substantially influenced by the amount of steam used in the gasification process. For the conventional gasification process, the optimum operating point is at the steam-to-MSW ratio of about 1.2, which results in the H₂ production efficiency of 47.7%. This is because any further addition of steam would not improve the H₂ production efficiency, increasing the energy demand and water demand for the process. On the contrary, the H₂ production efficiency for the sorption-enhanced process has increased proportionally to the steam-to-MSW ratio, achieving 51.2% at the steam-to-MSW ratio of 2. This is because, in this case, MSW gasification takes place simultaneously with the WGS reaction, which promotes steam conversion to hydrogen. Despite the 3.5%-point increase in hydrogen production efficiency, the optimum steam-to-MSW ratio for SEG is about 1.8–1.9. Operation under higher steam-to-MSW ratios would result in an electricity deficit and SEG would need to draw electricity from the grid.

Given that both conventional gasification and SEG operate at elevated temperatures above 650°C, waste heat can be recovered to generate electricity that can then offset some (or all) of the process electricity requirement. This usually takes place in a conventional superheated steam cycle. Such a practice allows avoiding the need for electricity import from the grid and increases the overall energy efficiency of the process. However, the additional equipment required in SEG to supply pure O₂ for the calcination process and the compression of captured CO₂ results in an additional electricity requirement. Consequently, the net electric efficiency, and thus the amount of electricity exported to the grid, tends to be lower for SEG than for conventional gasification (Figure 5). Therefore, it is expected that SEG would result in an overall energy penalty of about 4% points¹⁵.

Regardless of marginally lower energy efficiency, the key advantage of SEG over conventional gasification is a reduction in the carbon intensity associated with hydrogen production. The conversion of MSW using conventional technology would result in specific CO₂ emissions of about 181 gCO₂/MJH₂. This is due to the char and tail gas combustion to supply heat for the gasification process and produce electricity for the auxiliary equipment. The carbon intensity would drop by more than 90% (11.67 gCO₂/MJH₂) for the sorption-enhanced process¹⁵. SEG achieves such a reduction in the carbon intensity because the char and tail gas combustion takes place in the oxy-fired calciner (Figure 2). A small fraction of tail gas is used in the gas turbine to supply electricity to drive the auxiliary equipment, hence the carbon intensity is still positive¹⁵. However, considering the UK’s Low-Carbon Hydrogen Standard²³, which specified that hydrogen can be considered low-carbon when the specific CO₂ emissions of its production is less than 20 gCO₂/MJH₂, the SEG of MSW will produce LCH that meets this standard. Therefore, such processes can substantially contribute to improving waste management and reduce our reliance on fossil fuels to increase the LCH supply.

Economic viability

Based on 20 years of past research and development activities, we know that carbon capture is expected to increase the cost of hydrogen production compared to conventional production technologies due to increased capital and operational expenditures²⁴. This also applies to SEG processes, although the cost estimations for this technology are scarce due to its low technology maturity.

Considering the layout of the SEG process, it benefits from a simplified syngas processing train because the WGS reaction takes place simultaneously with the gasification reaction in the sorption-enhanced gasifier. In addition, the syngas clean-up unit, which includes a liquid oxidation catalyst process and zinc oxide bed for hydrogen sulphide (H₂S) removal, is not required. This is because of the strong affinity of CaO to sulphur compounds, these will be removed in the sorption-enhanced gasifier and then removed with the purged spent sorbent. However, the resulting CaSO₄ would reduce the quality of the purged sorbent and reduce it market appeal. Moreover, the SEG plant needs an air separation unit (ASU) and CO₂ compression unit, and a calciner for sorbent regeneration. Consequently, its capital cost is 20–50% higher than that of conventional biomass gasification. Moreover, the operating cost of the SEG will be higher due to the need to factor in the cost of fresh CO₂ capture sorbent and the cost of CO₂ transport and storage.

Consequently, the initial economic assessments of the SEG of MSW (Figure 6) showed that the levelised cost of hydrogen produced via this route is expected to be nearly double (3.9–6.1 €/kgH₂) than that produced from the conventional MSW gasification (2.0–3.4 €/kgH₂)¹⁵. This corresponds to the cost of CO₂ avoided, which stands for the minimum CO₂ price to offset the difference between the low-carbon and conventional hydrogen production, of about 115 €/tCO₂¹⁵. As of September 2022, such a figure is still above the CO₂ emission allowance price on the EU Emission Trading Scheme, which stands at about 90–100 €/tCO₂²⁵. Consequently, the carbon tax or price alone there may not provide sufficient financial incentives yet for the technology developers to pursue deploying this technology. However, the amount of allowed CO₂ emissions in the EU Emission Trading System is expected to decrease gradually over time, in line with the EU’s “Fit For 55” plan to achieve at least 55% greenhouse gas reductions by 2030²⁶. In turn, this is expected to push the prices of CO2 emission allowances above 140 €/tCO₂²⁷. Furthermore, the recent survey by the International Emission Trading Association (IETA) found that to limit global warming to 2°C, the global average carbon price needs to be about €124.35/tCO₂ and €200.50/tCO₂ by 2030 and 2050, respectively²⁸. Such a level of the carbon price (or tax) is significantly higher than the tax credits introduced recently in the US through the Inflation Reduction Tax that assume €85/tCO₂ and €60/tCO₂ when CO₂ is stored and utilised, respectively²⁹.

Nevertheless, there is an alternative option to the ETS and carbon tax credits that can make CCUS technologies financially viable. Industrial use of CO₂ can provide a valid business opportunity for developers of the MSW SEG process. Taking the UK food industry as an example, the CO₂ price has surged from €235/tCO₂ (£200/tCO₂) in September 2021³⁰ to €1,183/tCO₂–€5,324/tCO₂ (£1,000/tCO₂–£4,500/tCO₂) in September 2022³¹. Even at the lowest end of this scale, the CO₂ price is still substantially above the carbon price necessary for MSW SEG to be financially viable, and products which tie up the CO₂ captured such as polyols for polyurethanes, and olefins for polyolefins would go further than simple fossil fuel displacement. As the majority of CO₂ supply in the UK comes from the fertilizer industry, such a drastic increase in CO₂ market price is associated with a sharp increase in the natural gas price. Although the latter figure comes from an uncertain source, using CO₂ from SEG of MSW can provide a financially viable supply for industrial utilisation. Moreover, the waste treatment facilities can attract a gate fee, which can vary between 40–60 € per tonne of waste processed15. Such a carbon price and gate fee would promote investment in MSW utilisation technologies via the sorption-enhanced route.

Still, the levelised cost of hydrogen (LCOH) production for the SEG of MSW does not appear to be economically competitive when compared to more established hydrogen production technologies (Table 2), such as steam methane reforming (SMR; 1.0–1.5 €/kgH₂ w/o capture; 1.2–2.7 €/kgH₂ w/ capture), coal gasification (0.9–1.8 €/kgH₂ w/o capture; 1.2–3.4 €/kgH₂ w/ capture) or biomass gasification (1.4–3.2 €/kgH₂ w/o capture; 2.1–3.3 €/kgH₂ w/ capture). It is, however, competitive with the SEG of biomass, for which the low fidelity estimate of the LCOH falls between 6.1–10.1 €/kgH₂¹⁵. This can be attributed to the fact that, in contrast to biomass, there is a potential gate fee that the plant operator can receive in return for processing MSW. SEG of MSW process will also result in a competitive LCOH to variable renewables integrated with electrolysis, such as wind and solar, which currently is about 4-9 €/kgH₂, but is expected to reach less than 2 €/kgH₂ after 2030 and to below 1.5 €/kgH₂ by 2050¹.

Current development status

Although the SEG of MSW has resulted in a higher cost of hydrogen production, this technology is still in a relatively early stage of development. Much R&D activity is focused on designing and optimising sorption materials and reducing sorbent deactivation during cyclic operation. However, the pilot and demonstration scale experience are scarce. At the pilot plant scale, the SEG was performed in a dual fluidised bed unit at the Technical University Wien. The design fuel input of this pilot plant is 100 kW. Moreover, 20 kW and 200 kW pilot plants were developed and tested at the Institute of Combustion and Power plant Technology of the University of Stuttgart. Finally, a 71 kW pilot plant was developed by the Gas Technology Institute (GTI) in the US. The pilot-scale experiments showed that about 70–80% volume fraction of H2 was achieved³³. This was in line with the figures reported by Santos and Hanak¹⁵. It is important to emphasise that, to date, no demonstration scale units have been operated for the SEG of solid fuels. The first 1.5 MW demonstration plant is currently being developed at Cranfield University in collaboration with GTI, Doosan Babcock and the UK Department for Business, Energy and International Strategy (BEIS)³⁴. However, some of the existing dual-fluidised-bed gasifiers were adopted to operate under the SEG, including the 8 MW gasification plant in Güssing, Austria and the 8.5 MW gasification plant in Oberwart, Austria. As these units were not designed specifically for the sorption-enhanced process, the maximum hydrogen purity reached 52–57%³³.

Future development needs and perspective

There is no simple answer to whether the SEG of MSW is a viable technology to supply LCH to support the decarbonisation of our economy. On the one hand, such a process would allow us to efficiently utilise MSW and significantly reduce the amount of MSW ending up in landfills. It also has the capability of supplying LCH that meets the requirements of the UK’s Low-Carbon Hydrogen Strategy. Furthermore, SEG of MSW could reduce the reliance on fossil fuels of, and the cost of CO₂ supply to, various sectors of our economy. This is especially critical considering the current supply crisis associated with soaring prices of natural gas. Notably, if all MSW generated in the UK (7.2 million tpa) was processed in SEG, it could supply about 35% (0.25 million tpa) of current UK H2 production (0.7 million tpa)³⁵. Therefore, SEG of MSW can play a crucial role in the decarbonisation of large cities and industrial clusters, reducing the need for MSW and H₂ transport. However, as with other carbon capture technologies, the challenge is a high economic penalty. A higher energy requirement and capital cost of the SEG process than that of the conventional gasification process results in an uncompetitive LCOH.

Notably, SEG is currently at a relatively early stage of development, with the technology readiness level assessed to be about 3–5. The mature hydrogen production technologies, such as steam-methane reforming, coal gasification or electrolysis, have already reached commercialisation deployment and are well established. This is equivalent to the technology readiness level of 9. Therefore, further development of the sorption-enhanced processes and new sorbents with better cycling performance can bring further cost reductions. This, combined with policy and subsidy support from the government and the development of carbon markets, can make the SEG of MSW competitive with well-established processes.

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