Échangeur de chaleur et de matière à paroi poreuse. Application en récupération sur fumées Porous Heat and Mass Exchanger for Recovery from Flue Gases

Abstract

L'optimisation technico-économique des procédés industriels requiert de nouvelles techniques assurant un meilleur rendement énergétique et une protection accrue de l'environnement. Parmi celles-ci, l'échangeur de chaleur et de matière à parois poreuses proposé par l'Institut Français du Pétrole (IFP) est destiné aux opérations de séparation de mélanges gazeux contenant au moins un produit condensable. La première application assurée par Chaudières Seccacier SA concerne le chauffage au gaz industriel ou collectif. Le procédé consiste à effectuer le transfert de chaleur et d'eau entre fumées et air de combustion à travers une paroi poreuse qui assure la séparation des deux fluides par le film d'eau retenu par capillarité dans ses pores. Après la phase probatoire qui a concerné une chaudière de 200 kW, le développement en cours, avant la diffusion commerciale, s'effectue sur des chaudières de 500 kW. Le rendement énergétique est accru de 5 à 10 % si l'on se réfère aux chaudières à condensation, ce qui induit une réduction des émissions de CO2 de 20 à 50 % selon les installations et l'énergie déplacée. La réduction des émissions de NOx imputable à l'utilisation d'un air comburant saturé d'eau est de 65 % environ. The increase in the energy efficiency of processes is now a constant preoccupation that is included in all procedures aiming for the technico-economic optimization of industrial production. This trend is leading to the designing of systems integrating several simultaneous functions : chemical reaction, separation and heat transfer. In addition to the energy increase there is often a reduction in the bulk of installations, an improvement in reliability and better control of how the process operates. The porous-wall heat and mass exchanger (ECMP) proposed by Institut Français du Pétrole (IFP) for this purpose, which is covered by a basic patent, aims to separate gaseous mixtures containing at least one condensable product. The first application under the responsibility of Chaudières Seccacier SA involved industrial or district heating. 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 Chaudières Seccacier SA and Gaz de France. Its job was to perform heat and mass transferts between the two fluids in two successive phases in two direct combustion air/water and flue-gas/water contact columns (Fig. 1a). The simplification made possible by the ECMP (Fig. 1b) stems from the performing of both operations in a single module. The two gases between which the heat and mass exchanges occur circulate on either side of a porous membrane, which, because of its hydrophilic nature and porous structure (pore diameters and porosity in particular), separates the two gases by a film of condensed water held in place by capillarity in its pores. In addition to the advantages of compactness and reliability thus obtained, this design defines the circulation spaces of the fluids (distance between two porous membranes, with both fluids being acted upon alternately) according to the pressure drops requested by the user. This technique is thus based mainly on the choice of the porous medium (nature of the constituent material, structure) so as to give the membrane the right properties for it to fulfill its heat and mass transfer and physical separation functions of the two fluids. In addition to these purely operational aspects, the membrane must also meet 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 create a film resisting the pressure variations that may occur on either side of the wall. The values to which this analysis leads obviously depend on the specific conditions chosen by the user. However, it can be assumed that the pore diameters must not exceed 150 µm for applications of the type considered. The choice of the nature of the material making up the porous medium is very large : metals, polymers, glass or composites. The way they are implemented also covers a wide range : sintering, felting, weaving. The obvious simplification resulting from the use of the ECMP for the applications developed by Chaudières Seccacier SA (Figs. 4 and 5) gives access to this means of recovery for the market for medium-powered boilers of around 500 kW, which, to date, has been inaccessible for systems implementing two coupled heat-exchange columns. The increase in the energy efficiency of boilers equipped with the ECMP and technological improvements making for optimum boiler operating (now fed With saturated air at a temperature close to that of the flue gases) is between 5 and 10% when referring to conventional condensation 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 efficiency and the use of water-saturated oxidizer air results during operating 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 NOx (Fig. 7). After probatory tests performed on a 200 kW boiler, thus confirming the proper choice of the porous medium, development is continuing on 500 kW boilers (Fig. 8) designed for district heating. This achievement displayed at EXPOTHERM 92 was the reason why Chaudières Seccacier SA received the Super Oscar for Innovation.