POROUS HEAT AND MASS EXCHANGER FOR RECOVE RY FROM FLUE-GASES

The increase in the energy efficiency of processes is now a constant p reoccupation that is included in all procedures aiming for the technic o-economic optimization of industrial production. This trend is leadin g to the designing of systems integrating several simultaneous functio ns: chemical reaction, separation and heat transfer. In addition to th e energy increase there is often a reduction in the bulk of installati ons, an improvement in reliability and better control of how the proce ss operates. The porous-wall heat and mass exchanger (ECMP) proposed b y Institut Francais du Petrole (IFP) for this purpose, which is covere d by a basic patent, aims to separate gaseous mixtures containing at l east one condensable product. The first application under the responsi bility of Chaudieres Seccacier SA involved industrial or district heat ing. The aim was to transfer part of the heat and water from the flue gases of a condensation boiler to the air of combustion. The ECMP was substituted for a heat exchange system based on patents held by Chaudi eres Seccacier SA and Gaz de France. Its job was to perform heat and m ass transferts between the two fluids in two successive phases in two direct combustion air/water and flue-gas/water contact columns (Fig. 1 a). The simplification made possible by the ECMP (Fig. 1b) stems from the performing of both operations in a single module. The two gases be tween which the heat and mass exchanges occur circulate on either side of a porous membrane, which, because of its hydrophilic nature and po rous structure (pore diameters and porosity in particular), separates the two gases by a film of condensed water held in place by capillarit y in its pores. In addition to the advantages of compactness and relia bility thus obtained, this design defines the circulation spaces of th e fluids (distance between two porous membranes, with both fluids bein g acted upon alternately) according to the pressure drops requested by the user. This technique is thus based mainly on the choice of the po rous medium (nature of the constituent material, structure) so as to g ive the membrane the right properties for it to fulfill its heat and m ass transfer and physical separation functions of the two fluids. In a ddition to these purely operational aspects, the membrane must also me et the technological constraints required by the manufacturing and the physical operating conditions. The analysis of the functional roles ( heat and mass transfer, separation) leads to a definition of the range of pore diameters capable of ensuring both mass transfer according to a kinetics compatible with the flow rates required by the evaporation and condensation phases and the capillary retention of water to creat e a film resisting the pressure variations that may occur on either si de of the wall. The values to which this analysis leads obviously depe nd on the specific conditions chosen by the user. However, it can be a ssumed that the pore diameters must not exceed 150 mum for application s of the type considered. The choice of the nature of the material mak ing up the porous medium is very large: metals, polymers, glass or com posites. The way they are implemented also covers a wide range: sinter ing, felting, weaving. The obvious simplification resulting from the u se of the ECMP for the applications developed by Chaudieres Seccacier SA (Figs. 4 and 5) gives access to this means of recovery for the mark et for medium-powered boilers of around 500 kW, which, to date, has be en inaccessible for systems implementing two coupled heat-exchange col umns. The increase in the energy efficiency of boilers equipped with t he ECMP and technological improvements making for optimum boiler opera ting (now fed with saturated air at a temperature close to that of the flue gases) is between 5 and 10% when referring to conventional conde nsation boilers and between 1 and 5% compared to boilers equipped with the device including the two heat-exchange columns (Fig. 6). Likewise , the impact on the environment resulting from the increase in efficie ncy and the use of water-saturated oxidizer air results during operati ng in a reduction of emissions, which, for CO2, is between 20 and 50% depending on the installations and the energy displaced, and about 65% for NO(x) (Fig. 7). After probatory tests performed on a 200 kW boile r, thus confirming the proper choice of the porous medium, development is continuing on 500 kW boilers (Fig. 8) designed for district heatin g. This achievement displayed at EXPOTHERM 92 was the reason why Chaud ieres Seccacier SA received the Super Oscar for Innovation.