Gas cleaning and scrubber technologies
The key issue for any commercial application of SNG-Systems is the gas cleaning. Raw syngas contains undesired impurities such as particles, tars, sulphur, halogens, nitrogen species and metal traces inherent in coal [1]. Especially for the strict desulfurization levels scrubber technologies with chemical absorption in alkaline-water solution or in alkanolamines or physical solvents (RectisolTM, SelexolTM) are used. More sulphur tolerant applications allow solid sorption of H2S with metal oxides like zinc oxide [2]. Amine systems e.g. MEA, DEA and MDEA normally operate in the low to medium pressure range (5-25 bar) but the economy of an amine system declines in comparison to physical solvents (high partial pressure of acidic gases) like RectisolTM (chilled methanol) or SelexolTM (dimethyl ether of polyethylene glycol). In particular the RectisolTM process found application in various IGCC plants [3]. It uses chilled methanol to remove acid gases from gas streams by physical absorption without chemical reaction in the temperature range of -60 to -75 °C. The Rectisol process removes also most efficiently higher hydrocarbons (tars) due to the low temperatures.
Pressurized hot alkaline salt solutions (sodium or potassium carbonate) meet all of the prerequisites, combine sulphur (H2S and organic sulphur) as well as CO2 removal (reaches up to 99% removal efficiencies [4]). The “Benfield Process” (invented in 1954) [5] licensed by Honeywell UOP, uses an activated, inhibited hot potassium carbonate (AHPC) solution to remove CO2 , H2S and other acid gas components. Other companies operate similar processes like Eickmeyer & Associates (“Catacarb process”), Exxon Research (“Flexsorb HP Process”) and Giammarco-Vetrocoke [4]. The CO2CRC’s patented process is similar to the Benfield Process but operates at temperatures higher than 120 °C [6]. The key challenges for carbonate based scrubbing technologies are material issues. However recent developments provided several solutions to solve this problem by means of inhibitors, materials and operational issues. Currently more than 700 commercial Benfield units are in operation worldwide [7].
These carbonate based processes are used to remove CO2 , H2S and other acid gas components. Carbonate based processes are not applied for the cleaning of tar-loaded gases since the elevated temperatures do not allow a substantial removal of light tars and BTX (benzene-toluene-xylene) components.
[1] Rezaiyan, J., Cheremisinoff, N.P., Gasification Technologies-A Primer for Engineers and Scientists, 2005, Taylor & Francis Group
[2] European Commission, Project CO2free-SNG: Substitute Natural Gas from Coal with internal sequestration of CO2, (2009-2012) - Available at: <http://www.co2freesng.eu> [accessed 22.04.2012]
[3] Stevens, D.J., Hot Gas Conditioning: Recent Progress With Larger-Scale Biomass Gasification, Systems Update and Summary of Recent Progress, NREL/SR-510-29952
[4] Kohl, A. and Nielsen, R., Gas Purification, Fifth Edition, Gulf Publishing Company
[5] H.E. Benson, J.H.. Field, R.M., Jimson, CO2 absorption employing hot potassium carbonate solution, Chem. Eng. Prog. 50 (1954) 356– 364. (in order to remove CO2 from syngas from FT-synthesis – at higher pressures)
[6] Hooper, B., Stevens, G., Kentish, S., 2006. Plant and process for removing carbon dioxide from gas streams. CO2CRC Technologies Pty. Ltd., International Patent No. WO/2007/019632, PCT/AU2006/001177
[7]http://www.uop.com/wp-content/uploads/2011/02/UOP-Overview-of-Gas-Processing-Technologies-and-Applications-tech-presentation.pdf (24.08.12)