Экология/2 Экологические и метеорологические проблемы больших городов и промышленных зон

Dolidovich A.F., Akhremkova G.S.

A.V.Luikov Heat and Mass Transfer Institute (HMTI)

National Academy of Sciences of the Republic of Belarus

Municipal gas cleaning technologies preventing the environmental contamination by heat and CO₂

The greatest environmental challenge of the new century is global warming. Therefore, sustainable development of world society is impossible without elaboration of both wasteless industrial production technologies and up-to-date gas cleaning technologies, reducing emission of greenhouse gases.

It is well known that different types of hazardous VOCs (Volatile Organic Compounds) contaminate the air basin of large cities and industrial centers in many countries. Medicine has proven the failure of hygienic adaptation of individuals to these harmful VOCs penetrating into living organisms from the environment.

VOCs cover thousands of chemical species, some of which are precursors of photochemical pollution, greenhouse gases or stratospheric ozone depletion. VOC emissions consist of a mixture of many organic substances, each having its own reactivity and time-scale in ozone formation. According to the data of [1], an European emissions of VOC are 25 megatons per year. The VOC Protocol under LRTAP Convention adopted in 1991 introduces a stepped approach to controlling VOC emissions. The first step is to take effective measures to reduce national emissions of VOC by at least 30% by year 1999 using 1988 as the baseline.

The largest contribution to total contamination of the atmosphere by hydrocarbons (about 44%) is made by small-capacity industrial emission sources containing less than 1.0-2.0g of VOC per 1m³ of exhaust gases available at the majority of enterprises.

Therefore, many countries face now an urgent problem to elaborate gas cleaning technologies acceptable on environment protective and economic efficiencies for these numerous emission sources, which will be based on application of both the traditional and new advanced and low-cost methods.

It is known, that different approaches to control of emissions into air, water or soil separately may encourage the shifting of pollution between various environmental media rather than protecting the environment as a whole.

According to Integrated Pollution Prevention and Control (IPPC) Directive, emission limit values, parameters or equivalent technical measures should be based on the Best Available Techniques, without prescribing the use of one specific technique or a technology and taking into consideration the technical characteristics of an installation concerned, its geographical location and local environmental conditions; whereas in all cases the authorization conditions will lay down provisions on minimizing long-distance or transfrontier pollution and ensure a high level of protection for the environment.

The best available technologies (BAT) signify the latest, most effective and state-of-the-art techniques and technologies in the development of activities, processes and their methods of operation which minimize emissions and the impact on the environment [2].

In present paper the existing traditional "wet", "dry" and “combined” technologies of gas cleaning from VOCs are considered in the context of the Best Available Technologies (BAT). It was shown that these technologies do not completely satisfy to the BAT approach and led, as a rule, to the secondary or tertiary environmental contamination by aqueous wastes polluted with organic substances or greenhouse gases like carbon dioxide. So, even gas-cleaning systems based on adsorption and catalytic principles being most applicable for decontamination of multicomponent VOCs do not fit BAT approach because carbon dioxide and water vapor as the final products of hydrocarbon oxidation in these technologies are the main contributors to the enhanced greenhouse effect (about 55%) and global warming. Therefore, it is necessary to research and develop technologies for control, capture and disposal of dozen tons CO₂ emitted during catalytic oxidation of VOCs from small-capacity sources.

With this aim, a broad spectrum of theoretical and experimental research studies has been carried out at the Luikov Heat and Mass Transfer Institute of the National Academy of Sciences of the Republic of Belarus for development innovative gas-cleaning technologies preventing the environmental contamination by heat and CO₂.

Materials tested for mechanical, structural, adsorptive, catalytic, regenerative, heat transfer, hydrodynamic and etc. properties represented different types of spherical granular alumina-based metal-oxide catalyst-adsorbents (CuCr₂O₄/g-Al₂O_3, MgCr₂O₄/g-Al₂O_3, Cu_xMg_1-xCr₂O₄/g-Al₂O_3, Fe₂O₃/g-Al₂O₃) sized 0.4 £ d_p £ 0.6mm, 0.6 £ d_p £ 1.0mm, 1.0 £ d_p £ 1.6mm, 1.6 £ d_p £ 2.0mm, 2.0 £ d_p £ 2.5mm, 2.5 £ d_p £ 3.0mm. Crushed activated carbon with an average particle size varied from 1.0 to 2.0mm, from 2.0 to 3.0mm, from 3.0 up to 5.0mm, activated fibrous carbon fabrics with a surface density ranged from 240 up to 290g/m² were also used as bed materials. Some results of these research studies are partially described in [3, 4, 5, 6].

In particular, it was shown, that in conventional gas cleaning systems at catalytic oxidation of one molecule of neutralized hydrocarbons there can be formed from 3 up to 24 molecules of СО₂, depending on their chemical composition i.e. at hydrocarbon concentrations from 0.1 to 1.0g/m³ in a cleaned gas, which are typical for low-capacity emission sources, in an atmosphere is supplied from 0,181 up to 3,32g/m³ of carbon dioxide. It corresponds to bulk emissions of СО₂ in an environment in a range from 4.0 up to 20 tons per year at 8-hour operation of a 3000m³/h gas cleaning installation. In the other words, a «secondary contaminant» is emitted in the atmosphere in amount of 2 - 3 times larger, than the «primary contaminant». Moreover, the same gas cleaning installation for the same operation time emits about 4 Terajoules of heat per year in the atmosphere at the off-gas temperature of 450^oC, if it is not equipped with a special heat recovery system.

CO₂ emissions in the atmosphere can be reduced, as is known, by different methods e.g. those of chemical absorption, electric-gas-discharge dissociation, membranous partitioning, microbiological assimilation, and so on. But these methods are, as a rule, very expensive, require the complicated equipment, high energy consumption and, in a series of cases, can result in «tertiary» environmental contamination. Therefore, the most suitable and cheap way of reducing thermal energy and carbon dioxide emissions from catalytic converters in the atmosphere is suggested by «invented by a nature» method of its photosynthetic assimilation by higher С₃ or С₄-plants in additional to gas cleaning devices, such as hothouses. Because, as it is well known, carbon dioxide feeding of hothouse phytocenoses results in essential magnification of plant growth rates and productivity.

Thus, the data obtained show that the development of adsorptive-catalytic and thermocatalytic gas cleaning systems with a hothouse unit additionally used for photosynthetic purification of off-gases from carbon dioxide will allow creation not only of energy-resources saving and ecologically pure secondary pollutant-free technologies of hydrocarbons neutralization, utilizing the low-potential «waste» heat of off-gases for winter heating of hothouses but also reduction of time required for payback of expenses for its building owing to production of foodstuffs [7].

Moreover, as it is possible to develop gas cleaning systems of any capacity by variation of phytocenoses types, their leaf coefficients, levels of CO₂ feeding to plants, sizes of the hothouses sowing area, and the other parameters these technologies, upgraded due to adding of the sulphur cleaning unit, can be used on large electric power stations and boiler-houses burning low-sulphur fuel, for cleaning and utilizing of low-potential heat of their flue gases.

These research studies have allowed creation of physical-mathematical and physical-biological-chemical models of the processes of adsorptive, adsorptive-catalytic and thermocatalytic purification of the industrial exhaust gases from VOCs, CO₂ and heat and to develop computer programs for their calculation. These models and programs make it possible to simulate these processes under conditions close to actual ones and to optimize operation and design parameters of gas cleaning systems.

Results of these investigations have served as a basis for development of a few innovative lower-cost and energy-saving adsorptive, adsorptive-catalytic and thermocatalytic industrial gas cleaning technologies preventing secondary and tertiary environmental contamination by heat and carbon dioxide, a short description of which are presented in [8]. Each of these gas cleaning systems can be equipped with relevant heat recuperative and photosynthetic CO₂ assimilation units.

It is shown that at the optimal calculated design and operation parameters of the developed gas cleaning systems, their dimensions can be essentially decreased, high efficiencies of gas cleaning from VOCs and CO₂ (up to 98-100%) and thermal energy recuperation (up to 96%) can be attained.

References

1. Simpson D. And Styve H. (1992) The effect of the VOC protocol on ozone concentration in Europe. EMEP/MSC-W Note 4/92. Norwegian Meteorological Institute, Blindern, Norway.

2. CEC (1993b) Proposal for a Council Directive on integrated pollution prevention and control. Commission of the European Communities. OJNoC 311/06, Luxembourg, p.35.

3. Dolidovich A.F., Akhremkova G. S., Efremtsev V.S. (1999) Novel Technologies of VOC Decontamination in Fixed, Moving, and Fluidized Catalyst-Adsorbent Beds. Can.J.Chem.Eng., 77(April), p.342-355.

4. Dolidovich A.F., Akhremkova G. S. (2000) Innovative gas cleaning technologies preventing the environmental contamination by heat and CO₂. Proceeding of 2-nd EUROENVIRONMENT-2000 Conference. Denmark, Aalborg, 18–20 October.

5. Dolidovich A. F., Akhremkova G. S., Lapina V.A., Rubanov A.S. Adsorption of water and organic substances vapors on novel melanin-containing fitoadsorbents. Physic – Chemical Journal (Russia), 2003, V.77, No.1, pp.77-80.

6. Dolidovich A. F., Akhremkova G. S. Theoretical and experimental research studies of pore structure and adsorption properties of carbon fibrous materials. J. of Engineering Physics and Thermophysics, 2010, v.83, No.5, pp. 861-865.

7. Dolidovich A.F. Some economic aspects of the reduction of greenhouse gases emission by gas cleaning equipment. Proc. of 3^rd EURO ENVIRONMENT Conference, Aalborg, Denmark, 18-20 October (2002).

8. Dolidovich A.F. Some Scientific Background for Development of Advanced Energy-Saving Environment Protection Equipment with Fixed, Moving and Fluidized Beds. Journal of Engineering Physics and Thermophysics, 2012, V.85, No.1, pp. 43-58.