Biogas to biomethane
Chemical absorption is both technically and economically feasible for biogas upgrading under atmospheric conditions, thereby avoiding the need for compression.
Diego Di Domenico Pinto, Hovyu B.V.
Ralph H. Weiland Optimised Gas Treating
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In its final processed form, biogas is essentially methane, sometimes referred to as biomethane, produced by the anaerobic digestion of organic waste. As produced by digestion of biomass, however, the raw gas contains substantial concentrations of carbon dioxide (CO₂) as well as other gases such as hydrogen sulphide, mercaptans, and sundry other species. It is also water saturated.
One of the most common and accepted ways to upgrade biogas is by absorption of the CO₂ and sulphur compounds into alkaline solvents. As shown in Figure 1, with more than 100 upgrading plants, chemical absorption ranked third in IEA Bioenergy Task 37 member countries in 2019 behind water scrubbing and membranes.
However, new and better solvents are still sought in an effort to reduce the cost of CO₂ removal. The industry standard or benchmark is high strength (~30 wt%) MEA (monoethanolamine), but by no means does this produce the most advantageous economics. Given the biogas characteristics (such as high CO2 concentration and low pressure), solvents with higher capacities are necessary. A whole host of solvents has been proposed with superior characteristics. For example, blends of promoted tertiary amine (Mitsubishi MH-1, Eastman AdapT 200-series) are commercially used cases in-point.
There are two main concerning factors with chemically reactive alkaline solvents in CO₂ removal:
Œ CO₂ reacts only slowly if the solvent lacks sufficient alkalinity
If the solvent is sufficiently alkaline to achieve good absorption rates, it tends to require a lot of energy to reverse the CO₂-solvent reaction and to release the absorbed CO₂, to allow the solvent to be repeatedly reused in a closed-circuit process.
These can be viewed as competing characteristics of any alkaline solvent in this kind of application.
This article shows that chemical absorption is both technically and economically feasible for biogas upgrading. It also shows that:
• The process can operate under atmospheric conditions, thereby avoiding the need for compression
• If post-purification compression of the biomethane is necessary, it can be achieved with about 40% less gas volume than pre-purification, saving compression costs
• Chemical absorption is highly selective, and methane slip is below 0.1%. If absorption is performed under atmospheric conditions, methane slip is below 0.04%
• Methane losses appear to be directly proportional to the pre-flash pressure (flash following the absorber)
• For cases in which excess thermal energy is available, such as ethanol production from sugar cane, a chemical absorption process is very advantageous.
There are a host of processing schemes using chemical solvents, some in actual use but many more proposed, for removing CO₂ and generally upgrading biogas to make it suitable for use as a fuel. But at the heart of them all is a packed absorber, a thermal regenerator, and assorted pots and pans, such as flash and level control drums, pumps, and flow control valves.
Figure 2 is a simplified schematic of the generic process. In general terms, the water-saturated biogas needs a certain amount of compression to overcome the pressure drop generated by the gas flowing through the packed column where CO₂ is absorbed. For this simulation study, on a dry basis, the raw gas is assumed to be 10 kNm3/h of 60 mol% methane and 40 mol% water.
Following compression and cooling of the gas, a small amount of water condenses out and is removed via Stream 20. The block labelled “S” is a Solver that controls the calculations so that the solvent flow (Stream 24) is just enough to upgrade the biogas to 0.5% ± 0.01% CO₂. Both absorber and regenerator contain Mellapak M250.X structured packing with bed heights of 8 m and 5 m, respectively.
In all simulations, the reboiler’s molar boil-up ratio was held constant at 0.1 (ratio of molar vapour flow produced by the reboiler to molar flow of liquid withdrawn from it). The solvent is 35 wt% MDEA with 5 wt% piperazine, and its flow rate is whatever the Solver calculates is necessary to produce biomethane with 0.5 mol% CO₂ (water-saturated basis).
A CO₂ content of 0.5 mol% is not an especially stringent requirement for the performance of a piperazine-promoted solvent, so a very low lean solvent CO₂ loading is not needed to reach it.
In this discussion, we show the influence of pressure in the upgrading unit. With the fixed plant design as already described, the inlet gas and the flash pressures were varied, and the solvent flow rate was optimised to reach 0.5 vol% CO₂ in stream 3. All other parameters were kept constant. The simulations were performed in ProTreat using the flowsheet shown in Figure 2.
A total of 16 cases were run where the sulphur-free biogas pressure ranged from 1.05 to 6 bar(a). After the absorber, the solvent was fed to a flash unit so soluble CO₂/CH₄ could be flashed out prior to the regeneration section. The pressure of the flash unit was also varied to see the effect on process performance. Table 1 shows the varied operating parameters (such as gas and flash pressures) and the main results of the simulations for each case.
The solvent circulation rate, indicated by the liquid over gas ratio, decreases with increasing gas pressure independently of the flash pressure. This indicates that the size of the equipment could be reduced for plants operating pressurised. As shown in Figure 3 Case 16 (about ambient pressure), the absorber is used at its full height, whereas in Case 1, about half of the absorber is not performing any separation (pinched in the upper half of the packing).
The specific reboiler duty (SRD), the ratio between the reboiler duty (MW) and the total mass flow of CO₂ removed (flash and stripper, kg/s), also decreases with the increase in the gas pressure, and it is almost independent of the flash pressure. However, the CH₄ loss increases almost linearly with the increase in pressure, as shown in Figure 4.
Nevertheless, methane loss is still negligible compared to other processes such as water scrubbing and membranes, which can show methane losses up to 20%.² If the upgrading plant is operated under atmospheric conditions, CH₄ loss is lower than 0.02%. This benefit is also enhanced by the fact that gas compression is avoided in the upgrading phase, and the final biomethane volume to be further compressed for transportation is reduced by about 40%, significantly saving compression costs.
Chemical absorption with amine-based solvents can be used for biogas upgrading. The process can take advantage of working at nearly ambient pressure and avoid compression costs. However, operations at elevated pressures can also be performed. The operating pressure depends on the chosen pretreatment (such as H₂S removal technology). When operating under pressure, the size of the upgrading unit can be reduced. In our example, for a process operating at 6 bar, the absorber and stripper columns could be reduced by almost half compared to operations under atmospheric conditions.
The design of biogas upgrading plants should be performed on a case-by-case basis. Plants where biogas pretreatment is done under atmospheric conditions require larger equipment while equipment size reduces as the operating pressure increases. Larger equipment, most likely, increases the Capex, while operating at elevated pressures increases the Opex. This trade-off should be evaluated, and the optimal plant should be designed to avoid oversized or undersized equipment.
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