PRES22_0231.docx DOI: 10.3303/CET2294126 Paper Received: 17 March 2022; Revised: 12 April 2022; Accepted: 15 April 2022 Please cite this article as: Khoury J.A., Nemer M., Bouallou C., 2022, A Comparative Study of Biomethane Liquefaction Processes, Chemical Engineering Transactions, 94, 757-762 DOI:10.3303/CET2294126 CHEMICAL ENGINEERING TRANSACTIONS VOL. 94, 2022 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš, Sandro Nižetić Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-93-8; ISSN 2283-9216 A Comparative Study of Biomethane Liquefaction Processes Joseph Al Khoury*, Maroun Nemer, Chakib Bouallou MINES ParisTech, PSL Research University, Centre for Energy Efficiency of Systems (CES), Paris, France. Joseph.al_khoury@mines-paristech.fr Made from organic waste, biomethane is a source of renewable energy with reduced emission of greenhouse gases, that could replace fossil fuels in transportation, heating, and electricity production. Liquefaction of biomethane is necessary to reduce its volume and facilitate its transport from the production site to the site of use. The liquefaction processes suffer from low efficiency operating with less than a third of the Carnot coefficient of performance. This paper study and compare the performance of different potential refrigeration processes, to select the best candidate for the liquefaction of biomethane. All possible liquefaction processes were examined and two potential processes have been selected and modelled. The non-integrated cascade operating with pure refrigerants and the integrated cascade operating with mixed refrigerants were thermodynamically investigated and optimised. The results show that the non-integrated pure refrigerant cascade has a COP of 0.411, approximately twice as high as the integrated mixed refrigerant cascade which had a COP of 0.191. Consequently, it was selected as the best candidate for micro-scale biomethane liquefaction. This study highlights a new refrigeration cycle for the liquefaction of biomethane. 1. Introduction To fight climate change, the world is trying to reduce the usage of fossil fuels and to rely more on renewable energy. Made from organic waste such as animal, agricultural and industrial waste, biomethane is a source of renewable energy that reduces emissions and the greenhouse effect. It can replace fuels for heating, propulsion, or electricity production. The carbon content of biomethane is approximately 10 times lower than that of natural gas (NG). Using biomethane instead of diesel or gasoline decreases greenhouse gas (GHG) emissions by 95 % and the NOx emissions drastically, and eliminates the fine particles emissions. What makes biomethane carbon neutral is that it releases the same amount of carbon dioxide that the organic matter used to produce it, has absorbed as it grows, so it doesn't break the carbon balance. The use of biomethane presents additional advantages like the reduction of dependence on imports, lower water, soil, and air pollution, development of the local economy, environmental sustainability, a perfect example of circular economy, and maximum flexibility since it can be used in many places. The liquefaction of the biomethane is essential to facilitate its transportation from the site of production to the site of use when the grid is not available near the biomethane factory. The liquefaction reduces the biomethane volume 600 times which leads to a reduced number of shipments and thus reduced transportation costs. The liquefaction is helping the development of this renewable energy sector because it allowed the construction of sites of production in places far from the grid. To be liquified, the biomethane is cooled to extremely low temperatures below -120 ˚C, and this cooling is achieved by a refrigeration cycle. There are many refrigeration cycles for liquefaction and they are classified into three large categories: pure refrigerants cycles, mixed refrigerants cycles, and gas expansion cycles. Nowadays, liquefaction processes suffer from low-efficiency operating with less than a third of the Carnot coefficient of performance (COP). Most of the previous research involves modelling a specific liquefaction process and optimising its operating parameters using a numerical optimisation algorithm (Yoon et al., 2012). Some studies have compared the performances of the different natural gas liquefaction processes (Vatani et al., 2014). However, there is a lack of research on biomethane liquefaction and no clear comparison between the performance of its liquefaction processes. Moreover, the non-integrated pure refrigerant cascade cycle is not previously investigated for biomethane liquefaction. 757 Therefore, this paper investigates the liquefaction processes of the biomethane to find the highest efficiency refrigeration cycle. The first step is selecting the potential refrigeration cycles from all possible liquefaction processes. The second step is a modelization and thermodynamic optimisation (Refrigerants composition, temperatures, pressures, and mass flow rates) of the selected candidates, using Excel, Refprop, and the Generalized Reduced Gradient (GRG) solver. 2. Identifying the potential liquefaction cycles The liquefaction processes could be divided into three main categories. First, the Pure refrigerants cycles, which include, the Non-integrated pure refrigerant cascade cycle (NIPRC) with or without progressive cooling (Kamalinejad et al., 2015). The NIPRC can consist of several Multi-stage vapor compression. Second, the mixed refrigerants cycles, which include the single mixed refrigerant (SMR) (Kohler et al., 2014), the dual mixed refrigerant (DMR) (Vikse et al., 2018), the propane pre-cooled mixed refrigerant (C3/MR) (Sanavandi and Ziabasharhagh, 2016), the integrated mixed refrigerant cascade cycle (IMRC) (Yousfi et al., 2018), and the Multiple Mixed fluid cascade (MMFC) (Vatani et al., 2014). Third are the gas expansion cycles, which include the Single or Double N2 expander (Kohler et al., 2014), and the Reverse Brayton close-loop (Cryostar). Classifying the liquefaction processes from the lowest efficient to the highest efficient process gives the following order: The Single N2 expander, Double N2 expander, SMR, C3/MR, DMR, MMFC (Vatani et al., 2014). This is the same classification from the lowest to the highest complexity and fixed process cost. The N2 expander and the SMR dominate small-scale liquefaction because of their low price. The DMR and the C3/MR are also widely used. The gas expansion cycles are very simple with few components and available at a moderate price. The mixed refrigerant (MR) fluids at a specific composition are capable to imitate the very steep Composite Curve of the NG and thus reduce the Exergy destruction. Using a gas expansion cycle for biomethane liquefaction results in a very low process COP due to high Exergy destruction in the heat exchanger which provides the cooling and the liquefaction. From one side the refrigerant gas stays in a gas state and from the other side, the biomethane passes from vapor to two phases then to liquid which leads to the big spacing between the temperatures Composites Curves of the biomethane and the operating refrigerant gas (Venkatarathnam, 2008). Moreover, using gas to accomplish the liquefaction duty requires a high expansion and consequently a high compression ratio to be able to reach a very low refrigerant temperature, because the gas temperature increase very fast since it does not exhibit a phase change. The NG composite curve is much steeper than that of biomethane, due to 3 main reasons, which make us reconsider the investigation of the liquefaction processes for the biomethane. These reasons are as follows: First, the NG is present in pockets underground at a certain depth and high pressure, so the NG when extracted from pipes has a high pressure (90 bar). This pressure is higher than the critical pressure (46 bar), which means that the NG is liquified at supercritical pressure without passing inside the two-phase zone. This is not the case with the biomethane available at around 8 bar. Second, the presence of other substances in the NG composition (Ethane, Propane, Azote, CO2, …) so it does not exhibit phase change at a constant temperature, meanwhile the biomethane is around 99 % methane. Third, the liquefied NG is transported at 1 bar in ships, so it requires to be subcooled at a very low temperature to avoid its vaporization after expansion from high pressure to this low pressure. All these factors give the NG a steep Composite Curve in the heat exchanger, unlike the biomethane which has an almost constant temperature in a two-phase zone and is barely subcooled. One or many mixed refrigerants in series are capable to approach the NG Composite Curve but not the biomethane curve. So the SMR, the DMR, the C3/MR, and the MMFC will result in high Exergy destruction in the heat exchangers and thus a low process efficiency. The cycles that may have high COPs are the ones that will have close temperatures in their heat exchangers. The IMRC is a good candidate because the mixed refrigerant composition in this type of cycle changes, and the methane constitute a very high percentage of the mixed refrigerant that is used to liquefy the biomethane. The methane is used to liquefy the biomethane at the lowest stage in the NIPRC cycle and consequently, it will be selected for the study as a competitor of the IMRC. The two-cycle architectures are described in detail later. 3. Modelization and Optimisation The thermodynamic modelling of the refrigeration cycles was done in an excel sheet linked to refprop and optimised with GRG non-linear solver. The biomethane is available at 8 bar and ambient temperature (25 °C) after the biogas purification, and it’s stored at its saturated liquid state for transportation. Then, it is injected at 8 bar in the grid. Referring to the methane phase diagram, the biomethane could be liquefied in a range of temperature between -90 ⁰C and -170 ⁰C. At 8 bar the biomethane should be cooled to -129 °C (saturation temperature). The biomethane mass flow rate considered for the study is 100 Nm3/h which corresponds to the micro-scale application. The water temperature is considered 15 ˚C, the minimum cycle pressure is 1.2 bar, the 758 minimum heat exchanger Pinch is 3 degrees, compressor isentropic efficiency is 70 %, and 2 degrees of subcooling before valves. The COP Carnot is equal to 1.4 and calculated with Eq(1) or Eq(2). Where TC is the average temperature of the fluid that is cooled (168 K), and the TH is the average temperature of the environment where the heat is rejected, in this case, it’s water at 288 K. Both temperatures could be obtained using Eq(3). 𝐶𝐶𝐶𝐶𝐶𝐶𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝑇𝑇𝐶𝐶 𝑇𝑇𝐻𝐻−𝑇𝑇𝐶𝐶 (1) 𝐶𝐶𝐶𝐶𝐶𝐶𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝑄𝑄 𝑊𝑊𝑚𝑚𝑚𝑚𝑚𝑚 (2) 𝑇𝑇𝐻𝐻/𝐶𝐶 = ℎ𝑚𝑚𝑚𝑚−ℎ𝑜𝑜𝑜𝑜𝑜𝑜 𝑆𝑆𝑚𝑚𝑚𝑚−𝑆𝑆𝑜𝑜𝑜𝑜𝑜𝑜 (3) Q is the total heat to be removed from the fluid (refrigeration capacity). Wmin is the minimum work required to cool the fluid, in other meaning to bring the fluid from one state to another. The Wmin is equal to the Exergy difference between the inlet and the outlet of the fluid. The minimum work required for a specific refrigeration duty depends only on the fluid (to be cooled) inlet and outlet states and the temperature at which the heat is rejected, calculated with Eq(4). Where m (kg/s) is the fluid mass flow rate. hin (Kj/kg) and Sin (kJ/kg.K) are the enthalpy and entropy of the feed biomethane. hout and Sout are the enthalpy and entropy of the outlet biomethane. 𝑊𝑊𝑚𝑚𝑚𝑚𝑐𝑐 = 𝑚𝑚. (ℎ𝑚𝑚𝑐𝑐 − ℎ𝑐𝑐𝑜𝑜𝑐𝑐 − 𝑇𝑇𝐻𝐻. (𝑆𝑆𝑚𝑚𝑐𝑐 − 𝑆𝑆𝑐𝑐𝑜𝑜𝑐𝑐)) (4) The Carnot COP decreases when the biomethane exit temperature decreases, and increases when the biomethane inlet pressure increases. Thus, the COP Carnot varies in an opposite way to the variation of the minimum required work for the refrigeration. The Refrigeration Capacity Q equals 15.24 kW calculated with Eq(5), where hin and hout are the biomethane inlet and outlet enthalpies. 𝑄𝑄 = 𝑚𝑚. (ℎ𝑚𝑚𝑐𝑐 − ℎ𝑐𝑐𝑜𝑜𝑐𝑐) (5) The refrigeration cycle COP is calculated with Eq(6), where Wcomp is the sum of all compressors' work. 𝐶𝐶𝐶𝐶𝐶𝐶 = 𝑄𝑄 𝑊𝑊𝑐𝑐𝑜𝑜𝑚𝑚𝑐𝑐 (6) Exergy destruction occurs when there is entropy generation in an irreversible process. The entropy is generated because of two main reasons. First, the friction (case of compressors, valves, turbines). Second, the heat transfer across a finite temperature difference (Heat exchangers). Table 1 shows the calculations of the Exergy destruction ∆Ex for each component. Where Wc is the compressor work (positive) and WT is the turbine work (positive). exin and exout are the specific Exergy flow (kJ/kg) at the inlets and the outlets of the component respectively calculated with Eq(7). The Exergy rate Ex (kW) is calculated with Eq(8). The index 0 refers to the dead state or the reference state. 𝑒𝑒𝑒𝑒 = ℎ − ℎ0 − 𝑇𝑇0. (𝑆𝑆 − 𝑆𝑆0) (7) 𝐸𝐸𝑒𝑒 = 𝑚𝑚. 𝑒𝑒𝑒𝑒 (8) Table 1: Exergy destruction of mechanical components Component Exergy destruction Compressor Turbine Valve ∆Ex = m(exin − exout) + WC ∆Ex = m(exin − exout) − WT ∆Ex = m(exin − exout) Mixer and Separator ∆Ex = ∑ mi,inexi,in − ∑ mi,outexi,outi=1i=1 Heat Exchanger ∆Ex = ∑ mi(exi,in − exi,out)i=1 The process Exergy efficiency Ƞeff is calculated with Eq(9). Ƞ𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑜𝑜𝑢𝑢𝑒𝑒𝑒𝑒𝑜𝑜𝑢𝑢 𝐸𝐸𝐸𝐸𝑒𝑒𝑐𝑐𝐸𝐸𝐸𝐸 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑢𝑢 𝑢𝑢𝑠𝑠𝑒𝑒𝑐𝑐𝑐𝑐 𝐸𝐸𝐸𝐸𝑒𝑒𝑐𝑐𝐸𝐸𝐸𝐸 = 1 − 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑢𝑢 𝐸𝐸𝐸𝐸𝑒𝑒𝑐𝑐𝐸𝐸𝐸𝐸 𝑢𝑢𝑐𝑐𝑢𝑢𝑢𝑢 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑢𝑢 𝑢𝑢𝑠𝑠𝑒𝑒𝑐𝑐𝑐𝑐 𝐸𝐸𝐸𝐸𝑒𝑒𝑐𝑐𝐸𝐸𝐸𝐸 = 𝑊𝑊𝑚𝑚𝑚𝑚𝑚𝑚 𝑊𝑊𝑐𝑐𝑜𝑜𝑚𝑚𝑐𝑐 = 𝐶𝐶𝐶𝐶𝐶𝐶 𝐶𝐶𝐶𝐶𝐶𝐶𝑐𝑐𝑐𝑐𝑐𝑐𝑚𝑚𝑜𝑜𝑜𝑜 (9) 3.1 Integrated Mixed Refrigerant Cascade cycle The IMRC cycle is a three-stage refrigeration cycle (Figure 1) operating with a mixed refrigerant which is a combination of 4 pure refrigerants methane, ethylene, propylene, and isobutane. For the same pressure, the saturation temperatures of these fluids are as follows: Tmethane