PTQ Q4 2022 Issue

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, Inc.

I n 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 sub - stantial 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 char - acteristics (such as high CO 2 concentration and low pres - sure), 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:  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 tech - nically 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 nec - essary, 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 atmo - spheric conditions, methane slip is below 0.04%

Other + unknown, 42

Cryogenic upgrading, 8

Organic physical scrubber, 18

Water scrubber, 181

PSA, 81

Chemical scrubber, 103

Membrane, 173

Figure 1 Upgrading plants in IEA Bioenergy Task 37 member countries in 2019¹

• 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. The process 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 cer - tain 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 kNm 3 /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 calcu - lations so that the solvent flow (Stream 24) is just enough

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PTQ Q4 2022

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