Black liquor gasification with calcium looping for carbon-negative pulp and paper industry
Introduction
To meet the ambitious targets set by the Paris Agreement, the net greenhouse gas (GHG) emissions must be zero or even negative between 2055 and 2080 (Fuss et al., 2014; Millar et al., 2017; Rogelj et al., 2018). As a result, the energy-intensive industries (EIIs) need to improve material and energy efficiencies, implement carbon capture and storage (CCS) technologies and reduce their dependence on fossil fuels (Gerres et al., 2019; McGrail et al., 2012). The development of clean energy sources and vectors, such as hydrogen, is of paramount importance (Spallina et al., 2019; Vogl et al., 2018). However, nearly all hydrogen is still produced from fossil fuel sources (International Energy Agency, 2020). Due to its abundance and carbon neutrality, biomass is considered to be an alternative feedstock in the energy sector (Sanna, 2014). The pulp and paper industry is one of the major consumers of biomass. Yet, this industry is still the fourth EII, contributing 4% of the total EU industry emissions in 2017 (Gerres et al., 2019). However, in contrast to other EIIs, the pulp and paper industry has the potential to become a carbon-negative industry on the integration of CCS (Santos et al., 2021).
Black liquor formed during the Kraft process is normally combusted in a recovery boiler to recover inorganic chemicals and produce process utilities (steam and electricity). Although the recovery boiler has been used for decades, this technology presents some drawbacks, such as low energy efficiency and health and safety issues such as smelt-water explosions and reduced-sulphur gas emissions (Larson et al., 2006; Whitty, 2005). Thus, technologies with improved efficiency and safety have been considered. Pressurised entrained flow gasification of black liquor, a technology developed by Chemrec AB, has been proven as a valid alternative to the recovery boiler. This technology has been demonstrated at the 3 MWth LTU Green Fuels pilot plant in Piteå, Sweden (Jafri et al., 2016). As the alkali metals are present in black liquor, which have a catalytic effect, the produced syngas is characterised by low content of CH4 and tars, even at short residence times and temperatures around 1000–1050 °C (Öhrman et al., 2012). As the smelt is dissolved in a quench bath at the bottom of the gasifier, the inorganic chemicals can be recovered as green liquor and recycled to the Kraft process. The techno-economic feasibility of black liquor gasification (BLG) for the production of biofuels (Andersson et al., 2014; Carvalho et al., 2018) and bulk chemicals, such as ammonia (Akbari et al., 2018), were reported. Andersson et al. (2014) assessed methanol production via black liquor gasification (BLG), Carvalho et al. (2018) studied methanol production via co-gasification with renewable feedstocks, such as pyrolysis liquid, crude glycerol and fermentation residues. The production of ammonia via BLG and co-gasification with pulp sludge or with waste sludge was evaluated by Akbari et al. (2018). However, the economic assessment of the integration of BLG with CO2 capture (BLG-CCS) is still limited.
Zhang et al. (2011) have carried out a preliminary feasibility assessment of the integration of a BLG polygeneration system for the production of methanol, steam and electricity to a pulp and paper plant. This study also compared the BLG combined cycle with (BLG-CC-CCS) and without CO2 capture (BLG-CC-NCC). The former considered both pre-combustion physical absorption (Selexol) from syngas and oxy-combustion of the unreacted gas from methanol synthesis. Their study concluded that the BLG polygeneration system achieved a higher first law efficiency (34.1%) than the reference pulp and paper plant (15.7%). Moreover, BLG-CC-CCS based on oxy-combustion showed a lower net energy penalty (4.0%) and CO2 capture cost (26.3 €/tCO2) than that of the Selexol process (a net energy penalty of 6.5% and 40.9 €/tCO2). Pettersson and Harvey (2012) have assessed the techno-economic performance of the BLG polygeneration system for the production of dimethylether (DME) and electricity. This study compared the implementation of different BLG concepts and evaluated the economic performance based on the net annual profit and potential of CO2 emission reduction under different energy market scenarios. The BLG with DME production was shown to be the most attractive option among the considered scenarios. The BLG with electricity generation in pulp plant was still attractive under policies that promoted biofuels and high-penetration of low-carbon electricity. The integration of CO2 capture with BLG in pulp plants and pulp and paper plants was also studied. It was shown to have a high potential for CO2 emissions reduction, but the process profitability was strongly dependent on the CO2 emission allowance price. Ferreira and Balestieri (2015) have compared the techno-economic performance of BLG-CC-CCS, which considered a conventional CO2 absorption technology, and BLG-CC-NCC integrated to a pulp and paper plant. This study showed that the energy efficiency of the BLG-CC-NCC (34%) was higher than that for the BLG-CC-CCS (28%). The CO2 avoided cost was shown to be around 20.5 €/tCO2, which was estimated without considering the costs associated with CO2 transport and storage. Moreover, Ferreira and Balestieri (2015) considered a carbon credit of 5.3 €/tCO2 captured which implies that the economic feasibility of BLG-CC-CCS depends on incentives.
To date, the economic feasibility of the BLG coupled with CO2 capture has been assessed for amine scrubbing, physical absorption and oxy-fuel combustion. However, the current literature presents limited information on the application of high-temperature solid looping cycles for BLG. Darmawan et al. (2018) considered chemical looping for the co-production of H2 and electricity from BLG. Although their work proved that this approach is feasible from the thermodynamic standpoint, the economic feasibility of such a process needs to be assessed. Moreover, calcium looping (CaL) is an emerging high-temperature solid looping technology considered for thermochemical conversion of biomass. This technology can be either integrated with the thermochemical conversion process, so-called sorption-enhanced gasification (for solid biomass) or sorption-enhanced reforming (for gaseous or liquid biofuels) (Gil et al., 2015; Pfeifer et al., 2007; Wiranarongkorn and Arpornwichanop, 2017), in a single reactor, or can be integrated after the thermochemical conversion process in a secondary reactor (Armbrust et al., 2015; Connell et al., 2013; Müller et al., 2009). Although the pulp and paper process has an inherent CO2 capture capability via CaL, as shown by Santos et al. (2021), no application of CaL for BLG has been considered.
For this reason, this work presents a comparative study between two different routes of CaL retrofit to the pulp and paper plant. The first route assumes that the existing Kraft process in the pulp and paper plant is retrofitted with CaL for CO2 capture (Santos et al., 2021). The second route assumes that the BLG is integrated with CaL for simultaneous CO2 capture and H2 production (BLG-CaL) in the same reference plant. Three co-production routes are evaluated within BLG-CaL, considering BLG with H2 production (BLG-CaL-H2), BLG with gas turbine combined cycle (BLG-CaL-GT) or with solid oxide fuel cell (BLG-CaL-SOFC) for electricity production. The feasibility of the BLG-CaL routes is assessed considering the process thermodynamic and economic performance. Finally, a sensitivity analysis is carried out to evaluate the influence of costs on the economic performance.
Section snippets
Process modelling and design
This study considered a retrofit of the reference pulp and paper plant with CO2 capture and hydrogen production. The process models were developed for the reference pulp and paper plant, CaL for CO2 capture and BLG with CaL for simultaneous CO2 capture and H2 production. Overall, the techno-economic performance of the following cases was evaluated:
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Case 1 – reference pulp and paper plant
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Case 2 – calcium looping retrofit to the reference plant
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Case 3 – integrated black liquor gasification with
Parameters for thermodynamic assessment
The CCS and BLG cases presented above were compared with the reference case without carbon capture (Case 1), considering equivalent efficiency, gross power efficiency and net power efficiency as the key thermodynamic performance indicators.
The equivalent efficiency, Eq. (4), is defined as the ratio of the total useful energy outputs and the total energy inputs. The latter is defined as the total fuel consumed (natural gas, wood and black liquor) and, depending on the cases, the electricity
Thermodynamic assessment
The key outcomes from the thermodynamic analysis are presented in Figure 7. The steam demand of the reference plant, which remained constant for all the cases (132.6 kWth/ADt), was met by a heat recovery steam generator (HRSG) that used the high-grade heat available in CaL. This shows the superior performance of CaL when compared with other CCS technologies, such as physical absorption that require an additional amount of steam to operate, resulting in a steam deficit if BLG is employed (
Conclusions
This study aimed to compare two CCS routes, CaL and BLG-CaL, retrofitted to a pulp and paper plant. Three O2-blown BLG coupled with CaL were assessed: BLG with H2 (BLG-CaL-H2) production, BLG with gas turbine combined cycle (BLG-CaL-GT) or with solid-oxide fuel cell (BLG-CaL-SOFC). The proposed configurations were assessed from the thermodynamic and economic points of view. It was shown that pulp and paper integrated with CaL and integrated with BLG with SOFC showed the best overall performance
Contribution Statement
Monica P. S. Santos: Conceptualization, Methodology, Investigation, Formal analysis, Visualisation, Writing – Original Draft; Vasilije Manovic: Writing – Review & Editing; Dawid P. Hanak: Conceptualization, Resources, Data Curation, Writing – Review & Editing, Supervision, Project administration, Funding acquisition
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This publication is based on research conducted within the “Clean heat, power and hydrogen from biomass and waste” project funded by UK Engineering and Physical Sciences Research Council (EPSRC reference: EP/R513027/1).
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