GB1572117A - Method of introducing an organic compound having an oxygen-containing functional group and/or a parent hydrocarbon compound thereof into combustion exhaust gases - Google Patents

Description

(54) METHOD OF INTRODUCING AN ORGANIC COMPOUND HAVING AN OXYGEN-CONTAINING FUNCTIONAL GROUP AND/OR A PARENT HYDROCARBON COMPOUND THEREOF INTO COMBUSTION EXHAUST GASES (71) MITSUBISHI JUKOGYO KABUSHIKI KAISHA, a Japanese Body Corporate, of 5-1 Marunouchi 2-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-- This invention relates to a method for controlling oxides of nitrogen (NOx) in combustion exhaust gases, and more specifically to improvements in oxidation of nitric oxide (NO) to nitrogen dioxide (NO2) in wetprocess denitrification of combustion exhaust gases whereby NO in the exhaust is oxidized to NO2 and then the latter is removed by absorption in an absorbing solution.

As is well-known, NOx are notorious sources of air pollution (photochemical smog) and are poisonous compounds early control of which has been called for in all quarters. In an effort to meet this requirement, we previously proposed, in the copending British Patent Application No.

3818/77 (Serial No. 1,572,211) a process for controlling NOx in combustion exhaust gases, such as from a boiler, which comprises adding an organic compound having an oxygen-containing functional group and/ or its parent hydrocarbon compound to the gases and thereby oxidizing NO in the exhaust to NO2 in the presence of oxygen.

The proposed process utilizes methanol, ethanol, formaldehyde, formic acid or the like as the organic compound having an oxygen-containing functional group, and / or methane, ethane, propane or the like as the parent hydrocarbon compound. Of those compounds, methanol appeared the most promising because of low cost, ease of handling, and other merits. The oxidation reaction with methanol is presumed to consist of the following elementary reactions: CH,OH+O 9 HCHO+H202 (1) HCHO+O2 9 HCOOH+O* (2) HCOOiH+O2 9 Hss02+C 32 (3) H,O,+NO e NO2,+H20 (4) 0*I+NO 9 NO2 (5) (*activated atom) It is presumed that, in the elementary reactions (1) through (5), H202 and 08 are formed as intermediates, which then oxidize NO.

Since the reactions take place where the organic compound having an oxygen-containing functional group is an aliphatic alcohol or an aliphatic aldehyde, such as methanol or formaldehyde, and oxygen coexist, it is possible to use, as a parent hydrocarbon compound of an organic compound having an oxygen-containing functional group an alkane, such as methane, ethane, propane, or other such parent hydrocarbon compounds which produces the organic compound having an oxygen-containing functional group in the presence of oxygen.

In the course of our further study on application of the process on an industrial scale, we have found a variety of problems involved in the introduction of an organic compound having an oxygen-containing functional group and/or its parent hydrocarbon compound into exhaust gases.

Methanol, which appeared most advantageous from the industrial viewpoint for example, is desirably admitted to an exhaust gas stream in a temperature range between 5000 and 7000C, but its introduction presents the following problems. First, if the injection temperature is too high (over 6000C), there will be the possibility of liquid methanol or 100% methanol vapor being wastefully consumed by combustion.

In some cases there will be even the danger of explosion, and methanol may fail to mix thoroughly with exhaust gases in the combustion equipment owing to a short mixing time. Second, the methanol vapor that passes through the injection pipes may be decomposed by the catalytic action of the pipe material at the elevated temperature and will eventually lose its NO-oxidizing action.

The first problem can be solved by utilizing the combustion exhaust gases as carrier gases for methanol, as described and claimed in our copending Patent Application No.

179161/77 (Serial No. 1,572,118). The present invention is directed to the solution of the aforesaid second problem.

When liquid methanol, 100% vaporized methanol, or methanol vapor being conveyed by one of many different carrier gases is to be introduced into a high-temperature region (at 500 to 7000C) where it can best react with NO to effect oxidation to NO2, it may seem appropriate to use methanol injectors of heat-resisting materials usually considered suitable for such high temperatures, namely, the stainless steels of the SUS grades (conforming to Japanese Industrial Standards G3445, 3446, 3459, and 3463), the STBA grade (alloy steel for boilers and heat exchangers, conforming to JIS G3462), and the STB grade (carbon steel for boilers and heat exchangers, conforming to JIS 3461).

However, our experiments have indicated, contrary to our expectation, that from 60 to 100%. of methanol injected through pipes of these materials is decomposed before reaching the proper temperature region in the combustor or its flue, decomposed by the catalytic action of the interior surface of the injection pipes.

After extensive tests we have found that aluminium (actually in the form of aluminium oxide due to oxidation) has only a slight catalytic action for methanol decomposition.

The present invention resides in a method of introducing an organic compound having an oxygen-containing functional group and/ or a parent hydrocarbon compound thereof into combustion exhaust gases containing free or molecular oxygen so as to oxidise nitric oxide in the gases to nitrogen dioxide, characterised by the use of injectors of which the surfaces that contact said organic compound having an oxygen-containing functional group and/or said parent hydrocarbon compound are made of aluminium or aluminium oxide.

The organic compound having an oxygencontaining functional group is preferably an aliphatic compound.

The injectors may be in the form of aluminium pipes or heat-resisting pipes lined with aluminium plating or calorized on the interior surface.

The present invention is based on the fact that, as will be illustrated by an example thereof later, methanol in the injection pipes is only slightly decomposed as long as it is in contact with an aluminium (or aluminium oxide) surface, and therefore remains able to act effectively as an NO-oxidizing agent for the combustion exhaust gases. Aluminium pipes will, of course, give satisfactory result. In practice, however, preference is given to pipes of mild steel or stainless steel (SUS) lined with aluminium plating or calorized on the interior surface in the usual manner, rather than solid aluminium pipes, in view of the heat resistance, economy, strength, ease of fabrication, and service life.

Using an experimental arrangement as shown schematically in the accompanying drawing, tests for decomposition of methanol vapor through contact with various materials were conducted. The experimental apparatus comprised feed lines, 101, 102, and 103 respectively for 02, NO and N2, and N2, all provided with means capable of controlling the gas flow rates. The O2 and NO were supplied from standard gas cylinders (not shown), and methanol was supplied in the form of an aqueous methanol solution (25 wt% at room temperature) in a Muenke scrubbing bottle 104 through which was passed the N2 from the line 103 so that an amount corresponding to the vapor pressure was obtained. The gases were fed through a line 112 to a mixer 105 and thence via an inlet gas sampling point 106, where a line is branched out, leading to a gas chromatographic apparatus ("GC-3BT" made by Shimadzu) 107 and an infrared spectrophotometer ("IR-430" made by Shimadzu, with a cell length of 20 cm), 108 for analysing the methanol concentration at the inlet, to a replaceable test piece 109 in the form of a pipe 2 cm in inside diameter. A transparent electric furnace 110 was placed under strict temperature control by a PID controller. An outlet gas sampling point 111 was branched off, leading to the gas chromatographic apparatus 107 and the infrared spectrophotometer 108, whereby the methanol concentration at the outlet could be analyzed.

The results of experiments using test pieces 109 of different materials are tabulated below. The methanol decomposition rate (%) is expressed as (Inlet methanol conc.) - (Outlet methanol conc.) x 100.

(Inlet methanol conc.) As shown in the table, the rate of methanol decomposition inside injection pipes of SUS 321, STBA 24 and STB 42 grades were more htan 60% at 5000C and a retention time of 1.6 NTP (in terms of conditions at OOC and 1 atm.) sec., more than 88% for a retention time of 3.2 NTP sec., more than 83% at 7000C and a retention time of 1.6 NTP sec., and 100% for a retention time of 3.2 NTP sec. On the other hand, with aluminium pipes or SUS pipes lined with aluminium plating, the rates were only 4% at 5000C and a retention time of 1.6 NTP sec., 10% for a retention time of 3.2 NTP sec., 9% at 7000C and a retention time of 3.6 NTP sec., and 19% for a retention time of 3.2 NTP sec. Throughout the test runs, the test gas composition was 2% 02, 110 ppm NO, 0.95% CH2OH and balance N2, and the gas feed rate was 2 Nl/min.

Table Decompostn. Decompostn.

rate (%) rate (%) Pipe Temp. Retention time Pipe Temp. Retention time Material (OC) (NTP sec) Material (OC) (NTP sec) 1.6 3.2 1.6 3.2 250 1 4 250 2 16 300 4 23 300 8 31 SUS 400 37 77 STRA 400 44 60 321 500 61 90 24 500 62 88 600 70 100 600 & 1 100 700 83 100 700 97 100 250 2 19 250 1 1 300 10 38 300 1 2 STB 400 49 66 Al 400 2 5 42 500 68 91 pipe 500 3 10 600 83 100 600 5 14 700 98 100 700 9 17 SUS 250 0 0 SUS 250 2 4 321 300 0 1 321 300 3 5 with 400 2 4 with 400 5 9 Al. 500 4 10 Al 500 8 15 plating 600 7 16 calorizing 600 14 25 700 9 19 700 30 40 It will be appreciated from the experimental results that the effects achievable by using pipes of or lined with aluminium are of remarkable industrial value. The method of the invention, when combined with the method of utilizing combustion exhaust gases themselves as the carrier gases for methanol, will attain a further increase in the industrial utility.

While the invention has been herein described as using methanol as an example of the organic compound having an oxygencontaining functional group, other organic compounds having an oxygen-containing functional group, such as ethanol, formaldehyde, and formic acid, or parent hydrocarbon compounds of such organic compounds, e.g.

methane, ethane, and propane, may be employed as well to achieve similar effects.

WHAT WE CLAIM IS:- 1. A method of introducing an organic compound having an oxygen-containing functional group and/or a parent hydrocarbon compound thereof into combustion exhaust gases containing free or molecular oxygen so as to oxidize nitric oxide in the gases to nitrogen dioxide, characterized by the use of injectors of which the surfaces that contact said organic compound having an oxygen-containing functional group and/or said parent hydrocarbon compound are made of aluminium or aluminium oxide.

2. A method according to claim 1 characterised in that said surfaces are those of aluminium pipes.

3. A method according to claim 1, characterised in that said surfaces are those of heat-resisting pipes lined with aluminium plating or calorized.

4. A method according to any preceding claim wherein the organic compound having an oxygen-containing functional group is an aliphatic alcohol, an aliphatic aldehyde or an aliphatic carboxylic acid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. As shown in the table, the rate of methanol decomposition inside injection pipes of SUS 321, STBA 24 and STB 42 grades were more htan 60% at 5000C and a retention time of 1.6 NTP (in terms of conditions at OOC and 1 atm.) sec., more than 88% for a retention time of 3.2 NTP sec., more than 83% at 7000C and a retention time of 1.6 NTP sec., and 100% for a retention time of 3.2 NTP sec. On the other hand, with aluminium pipes or SUS pipes lined with aluminium plating, the rates were only 4% at 5000C and a retention time of 1.6 NTP sec., 10% for a retention time of 3.2 NTP sec., 9% at 7000C and a retention time of 3.6 NTP sec., and 19% for a retention time of 3.2 NTP sec. Throughout the test runs, the test gas composition was 2% 02, 110 ppm NO, 0.95% CH2OH and balance N2, and the gas feed rate was 2 Nl/min. Table Decompostn. Decompostn. rate (%) rate (%) Pipe Temp. Retention time Pipe Temp. Retention time Material (OC) (NTP sec) Material (OC) (NTP sec) 1.6 3.2 1.6 3.2 250 1 4 250 2 16 300 4 23 300 8 31 SUS 400 37 77 STRA 400 44 60 321 500 61 90 24 500 62 88 600 70 100 600 & 1 100 700 83 100 700 97 100 250 2 19 250 1 1 300 10 38 300 1 2 STB 400 49 66 Al 400 2 5 42 500 68 91 pipe 500 3 10 600 83 100 600 5 14 700 98 100 700 9 17 SUS 250 0 0 SUS 250 2 4 321 300 0 1 321 300 3 5 with 400 2 4 with 400 5 9 Al. 500 4 10 Al 500 8 15 plating 600 7 16 calorizing 600 14 25 700 9 19 700 30 40 It will be appreciated from the experimental results that the effects achievable by using pipes of or lined with aluminium are of remarkable industrial value. The method of the invention, when combined with the method of utilizing combustion exhaust gases themselves as the carrier gases for methanol, will attain a further increase in the industrial utility. While the invention has been herein described as using methanol as an example of the organic compound having an oxygencontaining functional group, other organic compounds having an oxygen-containing functional group, such as ethanol, formaldehyde, and formic acid, or parent hydrocarbon compounds of such organic compounds, e.g. methane, ethane, and propane, may be employed as well to achieve similar effects. WHAT WE CLAIM IS:-

1. A method of introducing an organic compound having an oxygen-containing functional group and/or a parent hydrocarbon compound thereof into combustion exhaust gases containing free or molecular oxygen so as to oxidize nitric oxide in the gases to nitrogen dioxide, characterized by the use of injectors of which the surfaces that contact said organic compound having an oxygen-containing functional group and/or said parent hydrocarbon compound are made of aluminium or aluminium oxide.

2. A method according to claim 1 characterised in that said surfaces are those of aluminium pipes.

3. A method according to claim 1, characterised in that said surfaces are those of heat-resisting pipes lined with aluminium plating or calorized.

4. A method according to any preceding claim wherein the organic compound having an oxygen-containing functional group is an aliphatic alcohol, an aliphatic aldehyde or an aliphatic carboxylic acid.

5. A method according to claim 1, 2 or

3 wherein the organic compound is methanol, ethanol, formaldehyde, or formic acid.

6. A method according to claim 1, 2, or 3, wherein the parent hydrocarbon compound is an alkane which yields an organic compound having an oxygen-containing functional group in the presence of oxygen.

7. A method according to claim 6, wherein the alkane is methane, ethane, or propane.

8. A method as claimed in any of the preceding claims substantially as herein described.