Method and apparatus for controlled reaction in a reaction matrix

#The present invention is directed to a method and apparatus for the controlled reaction, and in particular for combustion, of gases or vapors within a bed matrix whereby the stability and movement of the reaction wave is controlled to minimize or eliminate undesirable emission products such as NO.sub.x and products of incomplete reaction such as CO and hydrocarbons.

#Many process streams of vapors, such as effluents from chemical processing plants, refineries, etc., utilize combustors to destroy the gases or vapors prior to release to the atmosphere. However, with increasing demands on environmental control of emissions, the use of free flames to combust such effluents is in many cases unsatisfactory. A free flame also results, in some instances, in incomplete combustion and uncontrollable production of undesirable side products. The present process and apparatus provide a method for controlling and stabilizing the reaction wave, which is flameless, in which the gases are reacted within a controlled area of the matrix at substantially uniform and relatively low temperatures. The uniformity of the reaction wave provided by the present invention, and the increased mixing and heat treatment afforded by the matrix according to the present invention, provide for a high conversion of reactants to products. Moreover, this conversion may be obtained at lower temperatures and residence times than those required in a conventional incinerator. There is also inherent safety in the use of a process in which there are no open flames, and in which the mixture of gases to be introduced into the matrix is relatively cool, outside the flammability limits of the constituents, and therefore, not explosive under ambient conditions.

#It is therefore an object of the present invention to provide a method and apparatus for flameless oxidative reaction of gases or vapors to minimize or reduce NO.sub.x emission and products of incomplete combustion.

#FIG. 1 is a schematic view of a preferred apparatus for use in accordance with the present invention.

#FIG. 2 shows the hot and cool zones in the apparatus of FIG. 1 during operation.

#FIG. 3 is a second preferred apparatus for use in accordance with the present invention.

#The apparatus according to the present invention contains a bed of heat-resistant material and means for monitoring the temperature along the flowpath of gases through the heat-resistant material. By monitoring the temperature of the reaction wave within the bed matrix and controlling the flow and/or volume of gases entering and leaving the matrix as well as the temperature of the matrix, the reaction wave may be maintained at a relatively uniform temperature radially and at a constant location within the bed. The reaction matrix bed may be sized for any desired flow stream by altering the matrix flow cross-section, height, material, void fraction, outlet temperature, and supplemental heat or fuel value addition, if desired. Preferred matrix materials are ceramic balls, but other bed materials and configurations may be used, including, but not limited to, other random ceramic packings such as saddles or pall rings, structured ceramic packing, ceramic or metal foam, metal or ceramic wool and the like. By maintaining the stability and uniformity of temperature of the reaction wave within the matrix, and as a result of the fundamentally enhanced reaction wave propagation mechanism of inner body surface radiation coupled with forced convection and inner matrix solids thermal conduction, it is believed that the matrix burning velocity of the mixture in the matrix may be independent of, or more independent of the system pressure than the burning velocity of an identical mixture by conventional incineration. This is an improvement over conventional combustors wherein the laminar flame speed of fuel mixtures in air decreases significantly with increasing system pressure. See "Laminar Flame Speeds of Methane-Air Mixtures Under Reduced and Elevated Temperatures", Egolfopoulos, F. N., et al., Combustion and Flame, Vol. 76, 375-391 (1989). According to the present invention, the combustion intensity of reacting gases through a unit cross-sectional area would increase directly with pressure, since the mass flow of gases would increase directly with pressure, and the matrix burning velocity is not expected to decrease with pressure. The non-negative dependence of matrix burning velocity on pressure is expected to be a result of one or both of the phenomena of in-matrix, backward-propagating, inner-body surface radiation of heat, and forced-convection of heat from the solid matrix to the incoming gases.

#The radiative heat transfer within the bed, the walls of the combustor containing the bed, and the gas molecules within the reactor themselves are important features of the operation of the apparatus and the method of the present invention. Therefore, the types of materials in the bed may be varied so that the inner body heat transfer characteristics; the radiative characteristics, the forced convective characteristics, and the inner matrix solids thermally conductive characteristics, may be controlled within the bed. This may be done by varying the radiative heat transfer characteristics of the matrix by using different sizes of bed materials to change the mean free radiative path or varying the emissivity of the bed materials, varying the forced convection heat transfer characteristics of the matrix by varying its surface area per unit volume, or geometry, varying the inner matrix solids thermally conductive heat transfer characteristics of the matrix by using bed materials with different thermal conductivities, or changing the point to point surface contact area of the materials in the bed. These properties may be varied either concurrently or discretely to achieve a desired effect. In addition to changing the properties of the reaction matrix itself, an interface or several interfaces can be introduced into the bed where one or more of the heat transfer properties of the bed are discretely or concurrently changed on either side of the interface and wherein this variation serves to help stabilize the wave in that location and acts as a "reaction wave anchor." This may be done, for example, by introducing an interface where void fractions change across the interface within the bed. The interface may change the mean free radiative path across the interface independent of the void fraction. By changing materials, the emissivity may change across the interface within the bed. Changing the area per unit volume of the bed media across an interface, the forced convective heat transfer characteristics may change as the gas is passed across the interface.

#The matrix cross-section perpendicular to the flow axis may be configured in a circular, square, rectangular, or other geometry. The area of the cross-section may be intentionally varied (i.e. as a truncated cone or truncated pyramid) to achieve a wide stable range of reactant volumetric flowrates at each given matrix burning velocity.