Up-to-date technologies in Russia: integrated gas fuel-generating membrane bioreactor systems

Professor Teplyakov Vladimir Vasilyevitch, Doctor of Chemistry

Due to depletion of nonrenewable energy sources (oil, gas, peat and pyroschist), the new low-cost and available fuel resources based on renewable raw materials (food, pulp-and-paper, textile and other industry wastes) must be searched for substitution. The most perspective ones are represented by biomass in the form of timber, agricultural remains and micro-algae.

Kioto agreements of 1997 concluded for limitation of ÑÎ₂ emission anthropogenic share, for the purpose of reduction of greenhouse effect, regulate the means of ÑÎ₂ total utilization by means of both current and developed technologies. If to take all the known types of anthropogenic-origin organic waste treating and elimination technologies, biological method of utilization is considered as the one to be able to totally recover any processed material in the form of applicable agents. Any organic biomass used as raw material is principally advantaged by potential of energy generating biotechnologies, i.e. application of the technologies that do not impact environment ecological condition. The new technology wastes and by-products to be the components of biosphere cycles may also be used as raw materials, thereby resulting in application of totally waste-free future technologies.

Quite a high cost of such methods as feedstock pretreatment and extraction of combustible components from gas mixtures is the very factor that holds on wide-range application of biomass fuel gas processing technologies. A number of countries that possess large resources of green biomass (India, Brazil) or arranged mechanism of food waste collection and sorting (Norway, Germany, Netherlands) produce commercial biogas for subsequent generation of electric power. The rate of carbon dioxide contained in biogas may achieve to 40-60% resulting in decrease of its thermal value. With the classic technologies of gas treatment and separation applied, the process is not profitable even with available low-cost biomass and, therefore, biogas is often used as energy-generated low-thermal-value fuel.

For the purpose of studying this problem, we simulated the carbon natural cycle in lab conditions: “ÑÎ₂ + light + water à organic substances à CÎ₂ + ÑÍ₄ + Í₂ (energy)”. In terms of this, the scheme of continuous gas fuel generation by means of microorganisms processed in the three-unit solar energy operating bioreactor integrated with active membrane systems (membrane contactors), to be used without preliminary gas mixture compression, was developed.

Membrane bioreactor system comprises three tandem bioreactors: photosynthesis phototrophic (oxygenic), anaerobic digester (biogasious) and anaerobic phototrophic (hydrogenous), each of which is connected to separate gas phase membrane contactors.

 The first (oxygenic) unit is used for build-up of green bacteria biomass at the expense of solar energy (photosynthesis) to be subsequently applied as methanogenic community substrate with oxygen released and carbon dioxide absorbed. The initial gas mixture to be fed to the bioreactor should contain maximum 10% of ÑÎ₂. For the purpose of this, contaminated air (combustion gas) may be used.

  Continuous flow of selected phototrophic biomass is directed for deoxygenating (oxygen release) and, then, is fed to the anaerobic digester. Oxygen-enriched air mixture with residual carbon dioxide is delivered from the bioreactor to post-treatment unit and, then, totally purified air (up to 23% of Î₂) is exhausted to atmosphere.  The released amount of carbon dioxide is delivered back and absorbed by micro-algae.

The built-up biomass is continuously fed to anaerobic digester, where methanogenic community processes it in biogas consisting of (depending on initial substrate) ÑÎ₂ ~ 20-40%, ÑÍ₄ ~ 80-20%, Í₂ ~ 1 %, admixtures Í₂S, NH₃, N₂ etc. Biogas mixture was fed in the membrane contactors, where the main components were separated into technically pure methane and ÑÎ₂.

With the above membrane reactor scheme applied, the anaerobic digester residual biomass is filtered, solid residue is taken as agricultural fertilizer and culture broth (low-molecular organic substance water liquid) is delivered to the phototrophic anaerobic hydrogen bioreactor with bacteria Rhodobacter capsulata Â10. For the hydrogen to be totally extracted out of the reactor it is necessary to continuously bubble the cells by methane or argon for the purpose of three-component gas mixtures (Í₂+ÑÎ₂+ÑÍ₄ or Í₂+ÑÎ₂+Ar) gained at the bioreactor output. The above mixtures are subject to dividing into three components in the selective membrane gate and carbon dioxide is delivered from three dividing systems to the first (oxygenic) bioreactor. Thus, with the above process applied, ÑÎ₂ is not emitted in atmosphere due to its continuous absorption from any source.

The report covers detailed description and design of the active membrane systems. As per the experiment data, this kind of systems equipped with high-permeability nonporous membranes are able to make quite an efficient selection in terms of the parameters exceeding the ones of standard polymeric membranes. The membrane contractor and selective membrane gate can operate either in the flow mode and recycling mode.

The amount of carbon dioxide extracted out of three gas mixtures may be utilized both for reactor photosynthesis (ensuring totally closed ÑÎ2 circuit) and for direct hothouse plant photosynthesis (in rural area).