WO1994016806A1

WO1994016806A1 - METHOD FOR NOx REMOVAL BY CATALYTIC CARBON

Info

Publication number: WO1994016806A1
Application number: PCT/US1994/000881
Authority: WO
Inventors: Richard A. Hayden; Thomas M. Matviya
Original assignee: Calgon Carbon Corporation
Priority date: 1993-01-21
Filing date: 1994-01-21
Publication date: 1994-08-04
Also published as: CA2131996A1; JPH07505089A; JP2667059B2

Abstract

An improved process is provided for the selective removal of NOx from gas streams containing oxygen and ammonia by contacting said stream with a catalytically-active carbonaceous char. The improvement is provided by the use of a catalytically-active carbonaceous char prepared by low-temperature carbonization and oxidation of a bituminous coal or bituminous coal-like material followed by exposure to a nitrogen-containing compound during the initial high-temperature exposure of the low-temperature oxidized char. Following this initial high-temperature treatment the material can be further calcined and/or activated as desired.

Description

TITLE METHOD FOR NOx REMOVAL BY CATALYTIC CARBON

FIELD OF THE INVENTION

The present invention relates to the use of a highly catalytic carbonaceous material for the removal of nitrogen oxides from gaseous streams containing oxygen and ammonia.

BACKGROUND OF THE INVENTION

As restrictions on the emission of acid rain precursors such as SOχ and NO_x have grown in recent years, a corresponding need has arisen for technologies capable of removing such compounds from gaseous streams in which they are present. Examples of such gaseous streams include flue gases from incinerators and fossil-fueled power plants. The use of activated carbons and cokes in the presence of oxygen and ammonia as catalysts for the removal of nitric oxide from such gaseous streams is known.

The use of carbonaceous catalysts treated with nitrogen-containing compounds during their manufacture is a known method for producing catalytic carbonaceous materials. One such process for making a catalytic carbonaceous material involves a high-temperature nitrogen-poor activated carbon or coke oxidized by sulfate and exposed to a nitrogen-containing ammonium salt at temperatures above 350 C until the evolution of sulfur dioxide ceases. A similar carbonaceous char has been described in which a high-temperature nitrogen-rich carbon derived from polyacrlyonitrile is oxidized with sulfuric acid to provide a nitric oxide removal carbonaceous char with enhanced utility. Another NO* removal carbonaceous catalyst has been prepared by oxidizing high-temperature, nitrogen-poor chars with oxygen in the presence of ammonia. Other NOx removal carbonaceous catalysts have been prepared by calcining high-temperature, nitrogen-poor activated carbons in the presence of a nitrogen-containinc compound such as glucosaminc. in all cases, high-temperature carbonaceous chars are those produced by thermal treatment at temperatures greater than 700 C. Low temperature chars have not experienced temperatures greater than 700 C.

Each of the prior art methods for preparing NO_x removal carbonaceous chars has certain disadvantages which limit their utility. For example, many use high-temperature chars as starting materials. Since such materials are fairly inert chemically, the use of aggressive chemical post-treatment is usually required to effect significant changes in their catalytic activity. Additionally, such starting materials are inevitably more expensive than the raw materials from which they are made. In some cases, the generation of large quantities of toxic process byproducts such as sulfur dioxide and cyanide are unavoidable, while in others the use of highly hazardous treatment agents such as sulfuric acid is required.

Accordingly, it is the object of the present invention to provide an improved process for the removal of nitrogen oxides from gaseous streams containing oxygen and ammonia using a carbonaceous catalyst prepared directly from a nitrogen-poor, naturally-occurring starting material such as a bituminous coal or bituminous coal-like material. It is further the object of the present invention to limit the use of agents responsible for imparting catalytic activity to the carbon by performing the essential treatments during the transition of the starting material to the final product.

SUMMARY OF THE INVENTION

Generally, the present invention comprises a method of removing nitric oxide from gaseous streams containing oxygen and ammonia by the catalytic action of carbonaceous materials prepared by the low- temperature (less than 700 C) carbonization and oxidation of a nitrogen-poor feedstock. Carbonization oxidation is followed by exposure of the low-temperature char to a nitrogen-containing compound at high temperatures (greater than 700 C) during the initial calcination or calcination/activation. Following the initial exposure, the nitrogen-treated material can be further calcined and/or activated as desired. This material is then contacted with a gaseous stream containing Ni l x. O; and NO, to effect the removal of the NO,. The gaseous stream preferably contains a molar ratio of NH . to NO, of 0.5 to 1.5 and more preferably 0.85 to 0.95. The preferred nitrogen-poor feedstock is a bituminous coal or a bituminous coal-like material such as those deπved from higher or lower rank bitumens, coals, or lignocellulose materials by various chemical treatments. Examples of higher rank coals include anthracite or semi-anthracite coals, while examples of lower rank coals include peat, lignite, and sub-bituminous coals. Examples of the chemical treatment of these feedstocks include alkali metal treatment of the high rank materials and zinc chloride or phosphoric acid treatment of the low rank materials. These types of treatments can also be applied to lignocellulose materials.

In a preferred embodiment of the invention, the feedstock material is pulverized, mixed if necessary with small amounts of a suitable binder such as pitch, briquetted or otherwise formed, and sized. The sized material is then extensively oxidized at temperatures less than 700 C, preferably less than 400 C. The oxidation is continued until additional gains in the catalytic activity of the final product are no longer evident. The oxidation is well beyond that typically required to remove the coking properties of bituminous coals and produces an optimally oxidized char. Other convenient means of oxidation can also be used to effect the low-temperature oxidation and carbonization of the starting material.

The oxidized low-temperature carbonaceous char is then exposed to small amounts of an inexpensive, abundant, and relatively non-toxic nitrogen-containing compound such as urea during, not after, the initial calcination and condensation of the carbon structure. The amounts of nitrogen-containing compounds used are typically small, preferably less than 5% by weight of the oxidized low-temperature carbonaceous char or such that additional gains in the catalytic activity of the final product are no longer evident. The treatment is carried out by heating the oxidized low temperature carbonaceous char to high temperatures, preferably between 850 C and 950 C, in the presence of the nitrogen-containing compound. This heating is preferably conducted under an atmosphere that is inert except for the gases and vapors attributable to the carbonaceous char and/or the nitrogen-containing compound. The heating rate and temperatures are preferably selected such that additional gains in the catalytic activity of the final product are no longer evident.

The nitrogen-treated high-temperature carbonaceous char may then be activated to the desired density at temperatures above 700 C in steam and/or carbon dioxide, with or without the addition of other gasifying agents such as air. The calcined or calcined/activated carbonaceous char is then cooled in an oxygen-free or otherwise inert atmosphere to temperatures less than 400 C, preferably less than 200 C. Additional gains in catalytic activity may be realized by repeating the oxidation/exposure to mtroizen- cont.iiπmg compounds calcination or calcinatton/activation/inert cooling as many times as may be desired. Alternatively, any other method known to generate catalytic activity in high temperature char_s may be applied to the resultant product to further enhance its catalytic activity

The catalytically-active carbonaceous char so prepared is then contacted with a NO.-containins: gaseous stream in the presence of oxygen and ammonia. NO, is removed from the stream by catalytic oxidation/reduction primarily to elemental nitrogen.

PRESENTLY PREFERRED EMBODIMENT

The utility of the invention is illustrated by the following two examples. Example 1 demonstrates the NO removal performance of a commercial activated carbon of the prior art. Example 2 demonstrates the NO removal performance of the invention. Comparison of these two examples shows the performance of the invention exceeds that of a typical activated carbon.

EXAMPLE 1 : A carbonaceous char, specifically a commercially available activated carbon, BPL (manufactured by Calgon Carbon Corporation, Pittsburgh, PA) was sized to less than 4 mesh and greater than 6 mesh (U.S. Standard Series Sieves). This sized carbon was loaded to a depth of 16 inches into a stainless steel column having an inside diameter of 1.5 inches. The carbon column was heated to a temperature of 130 +/-5 C after which a gas stream, also at 130 +/-5 C, was introduced into the column at a flow rate of 1 1.5 liters per minute.. The composition of this gas stream was 500 ppmv NO. 450 ppmv NH₃. 7 v/v% O;. 10 v/v% CO₂, and 10 v/v H_:O with the balance of the composition as N_:. The NO content of the effluent gas stream was monitored and the BPL carbon was found to remove 20 to 25 - of the influent NO after steady state performance had been reached.

EXAMPLE 2: Bituminous coal was pulverized, mixed with 4% coal tar pitch, briquetted, crushed and sized to less than 3 mesh and greater than 6 mesh (U.S. Standard Series Sieves). In the presence oi excess air. this sized briquetted coal was heated from 100 C to 350 C over a peπod of 160 minutes, maintained at 350 C for 5 hours, and finally heated from 350 C to 450 C over a period of 60 minutes

SUBSTITUTE SHEET (RULE 26! This oxidized material was then cooled to near ambient temperatures and subsequently impregnated with an aqueous urea solution of sufficient quantity to give a 4 w/w% dry urea loading after drying of the material. The urea impregnated oxidized material was dried and subsequently calcined by rapidly heating the material to 950 C under an inert atmosphere. The sample was maintained at this temperature for 1 hour. Following this calcination step, the calcined material was exposed to steam at 950 C for a sufficient

period of time to activate the material and achieve the desired degree of gasification and activit development. Upon completion of the steam exposure, the catalytically-active carbonaceous char was cooled under nitrogen gas to a temperature of less than 200 C.

A catalytically-active carbonaceous char was prepared as described above. A less than 4 mesh and greater than 6 mesh sized poπion of this sample, when tested in the same manner as described in EXAMPLE 1 , removed 55 to 60% of the influent NO after steady-state performance had been reached.

While a presently preferred embodiment of the invention has been described, the invention may be otherwise embodied within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for the removal of nitrogen oxides from a gaseous stream, which comprises contacting said stream with a carbonaceous material in the presence of oxygen and ammonia at temperatures above 100 degrees C; said carbonaceous material being prepared from a bituminous coal or bituminous coal-like material carbonized and oxidized at temperatures below 700 degrees C, said carbonized and oxidized bituminous coal or bituminous coal-like material thereafter being subjected to nitrogen-containing compounds during initial exposure to temperatures above 700 degrees C .

2. The method in Claim 1 wherein said calcined material is activated at temperatures above 700* c using at least one of steam, carbon dioxide, and oxygen.

3. The method of Claim 1 or 2 wherein said or activated high-temperature carbonaceous char is cooled to a temperature less than 400* C in oxygen-free or inert atmosphere.

4. The method of Claim 1 or 2 wherein said calcined or activated high-temperature carbonaceous char is cooled to temperatures less than 200° C in an oxygen-free or inert atmosphere.