Carbon Membrane Applications

5__

Carbon Membranes for Biogas Upgrading

Arne Lindbrathen

Department of Chemical Engineering, Norwegian University of Technology and Science (NTNU)

Background

Biogas is a valuable renewable energy source that forms naturally via microbial fermentation under anaerobic conditions such as are found in small lakes or flooded fields, and in the stomachs of ruminants. It can be produced in a more controlled manner by microbial digestion of organic material (agricultural waste, manure, municipal waste, sewage, food waste, etc.) in the absence of oxygen [1,2]. The major components are methane (CH4), carbon dioxide (C02), and humidity at saturation at digestion temperature, with traces of hydrogen sulfide (H2S) and some other gases and vapors [3,4]. The most common applications of biogas are for heating, combined heat and power generation, and as vehicle fuel. Other applications that have been studied or tested are injection into the natural gas grid and H2 production for fuel cells.

A study of different utilizations of biogas reported that biogas upgrading to fuel quality gives the highest exportable energy with a medium range (10%) energy demand [5]. Upgrading was done with a membrane process. Sweden is today the leading nation when it comes to biogas as vehicle fuel, with a projected yearly consumption of 1 TWh in 2020, in comparison to 100 GWh in 2002 [6, 7].

To use biogas as vehicle fuel, the gas must be upgraded to fulfill certain required specifications. The corrosive components (water vapor and sulfur) present in biogas must be removed. CO,, which is one of the major components in the biogas, needs to be separated from the biogas because it dilutes/lowers the heating value of the gas. This results in a reduced burning capacity, which affects the performance of the engine [8].

Carbon Hollow Fiber Production

The precursor for the carbon hollow fibers used in this study was prepared using regenerated cellulose acetate (CA) by the dry/wet phase inversion fiber-spinning process. The fibers were spun using a pilot-scale spinning set up delivered by Philos, Korea. Details of the spinning process are described elsewhere [9]. A dope consisting of CA mixed with N-methylpyrrolidone and polyvinylpyrrolidone was used to spin CA hollow fibers, which were deacetylated batch-wise with a mix solution of NaOH in a short-chain alcohol. Then the deacetylated, dried, and now mainly cellulosic hollow fibers were carbonized at 550 °C under N2 flow (C02 gas was also tried for some production batches) in a tubular three-zone furnace. The carbonization protocol typically consisted of a heating rate of 1 °C/min with several dwells during the heating sequence (in the region of 250 to 300 °C) and a final 2 h dwell at 550 °C. The oven system had no active cooling system so the cooling rate could only be actively controlled at temperatures higher than 100 °C. The oven was tilted ca. 5° in the longitudinal direction of the working tube to assist in the drainage of tar/water produced during the carbonization. Details of development and optimization of carbonization processes can be found in [10] and Chapter 1.

A lot of fibers would necessarily have to be produced in order to serve a decent-sized biogas upgrading facility; some degree of “bad" fibers in the individual carbon bundles produced is unavoidable. The bad fibers may be subdivided into different classes:

  • • Broken fibers.
  • • Self-looping or severely curled fibers.
  • • Fibers with kinks and weak spots.
  • • Collapsed fibers (section of fiber with wall cave-in).
  • • Two or more fibers fused together from trapped tar buildup.

Rectifying the different faults varies from simply shaking out the majority of the broken fiber from the bundle to the laborious scrutiny of individual fibers with barely (if any) visible weak spots. The thoroughness needed in the quality control is to some extent dependent on the frequency of the occurrence of defects. The greatest challenge was in our experience due to weak spots

(which may be very hard to spot) and to too-curly fibers; both of these faults have a tendency to induce fiber breakage during operation.

 
Source
< Prev   CONTENTS   Source   Next >