A NEW PRODUCT FOR EXHAUST AIR PURIFICATION

Transcription

1 A NEW PRODUCT FOR EXHAUST AIR PURIFICATION Frédéric MILCENT, engineer, Research and Development Division, GAZ DE FRANCE Philippe BOUGIT, engineer, Research and Development Division, GAZ DE FRANCE 1. INTRODUCTION The legislation relating to the emissions of volatile organic compounds (COV) was considerably reinforced with the adoption of the European Directive " Solvents " and its transcription in French legislation. Many industrial sectors must from now on satisfy limiting values of emissions (VLE) for the channeled rejections (coming from fixed sources). If the reduction with the source of the emissions (by substitution or re-use) is not possible, it is necessary to treat the effluents before their rejection outside. 2. THERMAL OXIDATION Thermal oxidation is one way to reach this goal. It converts the organic molecules into carbon dioxide and water under the effect of the high temperatures. VOC are oxidated thanks to thermal equipment called incinerator or oxidiser. They consist of a combustion chamber where the conversion of the COV is carried out and about a heat recovery system. Heat is produced by the oxidation reaction and is used for preheating the effluent or for generating a thermal fluid. Heat recovery is carried out by an heat exchanger (recuperative oxidation) or by a ceramics bed (regenerative oxidation). The level of temperature necessary for COV oxidation is function of the type of treated molecules (730 C at 850 C in general case, higher than 1100 C in case of halogenous hydrocarbons) and can be reduced by the addition of a catalyst. An addition of energy is necessary for the rise and the stabilization of the temperature in the combustion chamber. This extra energy can be carried out thanks to the use of natural gas. These systems have a very high pollutants reduction efficiency (> 99,9 %), and are very largely used in industry. These oxidisers are dimensioned for a flow of effluents ranging between m3(n)/h and m3(n)/h and for a concentration in VOC ranging between 1 and 15 g/m3(n). Although this range of use is broad, new needs appear for equipment adapted to low flows (500 to m3(n)/h) or to weak concentrations (0,5 g/m3(n)), and with reduced operating costs. In the present situation GAZ DE FRANCE developed a new oxidiser for exhaust air purification, in partnership with european company SAACKE. The following paragraphs present this oxidiser as well as the results of the tests carried out by the Research and Development Division of GAZ DE FRANCE. 1

2 3. PRESENTATION OF THE WHOLE INSTALLATION 3.1. A new product for treatment for weak VOC concentrations For weak VOC concentrations (concentration < 1 g/m3(n)), it is necessary to use a consequent additional energy to ensure a good oxidation of pollutants, that increases strongly operating costs. A system of preconcentration for effluents makes it possible to approach or reach the threshold of no energy use, and thus to reduce considerably operating costs. There are some equipment in industry that uses a mobile concentrating wheel to concentrate effluents from process. Car industry (paintings) was one of the first users of these systems, for treating exhaust air from painting lines. Such a system is efficient, but it is generally designed for very strong flow rate, higher than m3(n)/h. 3.1 Presentation of the SAACKE Concentrating-Oxidiser Company SAACKE developed with GAZ DE FRANCE an original concentrating-oxidiser, made up of a fixed preconcentration system (a fixed activated carbon bed) and a thermal recuperative oxidiser (see figures 1,2 & 3). In a first stage (adsorption stage), the exhaust air polluted by VOC crosses the activated carbon bed, where polluting molecules will set by adsorption. In a second stage (desorption and oxidation stage) there is a desorption with a reduced flow rate ; so air from fixed activated carbon bed, which is highly VOC concentrated, penetrates the thermal recuperative oxidiser, in which pollutants are oxidized. Purified exhaust air Adsorption phase Desorption phase VA2 TICR4 VA3 Activated carbon bed Fresh air Heat exchanger burner Oxidation chamber VA1 VA4 Fan Gas Exhaust air containing VOC substances Fresh air for oxidiser starting up Figure 1 : Principle diagram of the SAACKE concentrating-oxidiser 2

3 Figure 2 : Overall view of the concentrating-oxidiser and the activated carbon filter Figure 3 : Overall view of the concentratingoxidiser by chamber room side 3.2 Interest Desorption is carried out by the thermal contribution provided by part from oxidiser flue gases. A monitoring system controls flue gases temperature which go right through the coal bed, by opening or closing an adding external air valve. Thanks to this innovative system, both operating and investment costs are cut compared with a classical system, respectively because auxiliary gas consumption and flow rate to treat in oxidiser are reduced. It could be finally mentionned that the tested system was made up of only one activated carbon bed, but it can be equipped with two beds so that the VOC desorption/oxidation is carried out at the same time as adsorption. Moreover, activated carbon can be replaced by zeolite for special cases. 3

4 4. MAIN RESULTS 4.1. Experimental results Various solvents were tested (cyclohexane, MEC, ethyl acetate, isopropanol...) alone or in mixture. Adsorption was carried out with a flow of m3(n)/h and several concentration values. Desorption tests were carried out with a flow of 500 m3(n)/h and a temperature of 730 C in oxidation chamber. For the adsorption stage, tests showed a good activated carbon adsorption ability and weak pressure losses. All the polluted flow is purified and the VOC concentration is less than 10 mg/m3(n) at the exit of the activated carbon bed (for a value of 110 mg/m3(n) laid down by regulation). For the desorption phase, the principal results are the following (values at chimney exit): - CH 4 concentration values are lower than 1 mg/m3(n) and those for CO concentration lower than 10 mg/m3(n); - NOx concentration values are in conformity with regulation 1 and are all the weaker that the VOC concentration at bed exit (in phase of desorption) was raised and that the treshold of no energy use is close; - VOC concentration at bed exit before oxidiser chamber rises to 4,5 g/m3(n). So high preconcentrating factors can be obtained. For example, if the initial concentration is about 0,5 g/m3(n), the preconcentrating factor rises to 9! - VOC concentrations after oxidiser chamber are in all cases lower than 38 mg/m3(n), which is lower than Maximum Emission Level of 50 mg/m3(n) (purification efficiency is about 99 %). However, this result is higher than values generally recorded on this type of oxidiser. Thanks to an analysis of the results, a bad sealing within the oxidiser chamber has been detected, but this defect has not been repaired immediately not to stop the experimental campaign in progress. So VOC purification efficiency should be higher under real conditions (that means no leak) An example of test The previous paragraph indicates main results of the whole experimental campaign. Two next pages focus on an example of one test results, thanks to table 1 and figure 4. It deals with one test realized with cyclohexane. 4

5 Qgaz (m 3 (n)/h) Chimney température ( C) Temperature at the top of the bed ( C) Temperature at the bottom of the bed ( C) VOC concentration at the exit of the bed (ppm C3H8) VOC concentration at the exit of the bed (mgcov/m 3 (n)) Cmahber oxidiser temperature ( C) Average 3, Minimum 1, Maximum 8, Standard deviation 1,34 9,3 26,5 27,7 700, ,94 CO2 (%) CO (mg/m 3 (n)) O2 (%) NOX (mg/m 3 (n)) HCT concentration at chimney (mgc/m 3 (n)) CH4 (mg/m 3 (n)) HCnm (mgc/m 3 (n)) Average 1, , Minimum 0,63 8, Maximum 2, , Standard 0,23 4,8 0,73 9,1 18,0 18,0 deviation Desorption flow rate (m 3 (n)/h) Desorption length (min) Desorbed mass (g) Gas consumption (m 3 (n)) Average thermal efficiency (%) Treated exhaust air per cubic meter of gas ,7 98,71% 414,2 Table 1 : Experimental results for cyclohexane desorption phase 5

6 Figure 4 : Evolution of setting temperature and VOC concentration Essai du 27/06/01- Désorption de cyclohexane à 5 g/m 3 at bed exit (n) et 600 m 3 (n)/h Température ( C) TICR4SS COV entrée (mgcov/m3(n)) COV sortie lit (mgcov/m 3 (n)) :00:00 0:30:00 1:00:00 1:30:00 2:00:00 2:30:00 3:00:00 Temps Economical approach From these results, it is possible to compare overall costs between a regenerative oxidiser with high thermal efficiency (RG) and a recuperative one with preconcentration (PREC). An economical example of assessment is indicated in table 2. This comparison shows a recuperative oxidiser with preconcentration (PREC) is less expensive than a a regenerative one with high thermal efficiency (RG). It is true for both investment cost and running cost. The difference is all the higher as you can set up a heat recuperation. 6

7 Characteristics Unit RG PREC - Pressure losses Pa Heat exchanger efficiency % Preconcentrating factor Treshold of no energy use - No Yes - Oxidiser room temperature C Desorption temperature C Temperature rising time h 4 0,75 - Chimney exit temperature C 55 20/270 (ads./des.) - Gas consumption - for stable running kw for temperature rising stage kwh Electrical consumption kw Investment cost (installation put up) k Recoverable thermal power (water at 70/90 C) kw Annual cost - gas k /an 77,5 11,7 - electricity k /an maintenance k /an 15 22,80 TOTAL COST without investment cost k /an 120,5 45,5 TOTAL COST with investment cost (with depreciation) TOTAL COST with heat recuperation (water at 70/90 C) k /an 175,5 83,5 k /an 175,5 47 Table 2 : Comparative table for economical costs between a regenerative oxidiser with high thermal efficiency (RG) and a recuperative one with preconcentration (PREC). 7

8 Calculations of this table were carried out with the following assumptions : - concentration of exhaust air to purify is equal to 0,5 g VOC /m3(n), the flow rate is m3(n)/h and the inlet temperature is 20 C; - VOC effluent has a Net Calorific Value of 11,6 kwh/kg and concentration for desorption (for the concentrating oxidiser) is 4,5 g VOC /m3(n); - flow rates penetrating in oxidation room are in opposite ratio of the concentrating factor, that is to say m3(n)/h for the regenerative oxidation, and m3(n)/h for the recuperative one with preconcentration; - the annual running time is around hours for 48 weeks; - depreciation is calculated over a period of 10 years; - energy rates : 2,1 c /kwh for gas and 5,2 c /kwh for electricity. 5. CONCLUSION This new product has been tested and developed jointly by the Research and Development Division of GAZ DE France, in alliance with the European manufacturer SAACKE. It totally answers needs from small and medium-sized industries for exhaust air purification, because it reduces investment and operating costs compared to classical current systems ones as it purifies exhaust air with respect to lawful regulations into force in Europe. 8