PRODUCTION OF FERTILIZERS FROM LANDFILL GAS OR DIGESTER GAS

Abstract

In a method for the production of a fertilizer from the sulfur and ammonia content in a feed gas, including steps of (a) combustion of the H.sub.2S-rich gas in air to convert H.sub.2S to SO.sub.2, (b) formation of ammonium hydrogen sulfite (AHS) by absorption of SO.sub.2 and NH.sub.3 in water, and (c) reaction of the AHS from step (b) with H.sub.2S and NH.sub.3 to form an aqueous solution of ammonium thiosulfate (ATS), reaction (a) is carried out in a catalytic reactor as a selective oxidation of the H.sub.2S content to SO.sub.2 over a selective catalyst having one or more metal oxides, in which the metal is selected from the group of V, W, Ce, Mo, Fe, Ca and Mg, and one or more supports taken from the group of Al.sub.2O.sub.3, SiO.sub.2, SiC and TiO.sub.2, optionally in the presence of other elements in a concentration below 1 wt %.

Claims

1. A method for the production of a fertilizer from the sulfur and ammonia content in a feed gas such as landfill gas, digester gas, off-gas from geothermal power production or coke oven gas, said method comprising the steps of: (a) combustion of the H.sub.2S-rich gas with oxygen to convert H.sub.2S to SO.sub.2, (b) formation of ammonium hydrogen sulfite (AHS) by absorption of SO.sub.2 and NH.sub.3 in water, and (c) reaction of the AHS from step (b) with H.sub.2S and NH.sub.3 to form an aqueous solution of ammonium thiosulfate (ATS), wherein reaction (a) is carried out in a catalytic reactor as a selective oxidation of the H.sub.2S content to SO.sub.2 over a selective catalyst consisting of one or more metal oxides, in which the metal is selected from the group consisting of V, W, Ce, Mo, Fe, Ca and Mg, and one or more supports taken from the group consisting of Al.sub.2O.sub.3, SiO.sub.2, SiC and TiO.sub.2, optionally in the presence of other elements in a concentration below 1 wt %.

2. Method according to claim 1, wherein the inlet temperature to reaction (a) is restricted to levels of less than 350 C.

3. Method according to claim 1, wherein NH.sub.3 is added by decomposition of an ammonia precursor.

4. Method according to claim 1, wherein the source of ammonia is urea decomposed by thermal or catalytic decomposition in a mixture with air.

5. Method according to claim 4, wherein the hot gas from step (a) is used as a heat source.

6. Method according to claim 1, wherein the source of ammonia is urea decomposed by thermal or catalytic decomposition in a mixture with a gas, in which CO.sub.2 is the main gaseous component to avoid excessive amounts of oxygen and nitrogen in the product gas.

7. Method according to claim 1, wherein wet SO.sub.2 absorption is carried out in an absorption section comprising at least two absorbers in series connection.

8. Method according to claim 1, wherein reaction (c) is carried out in a reactor with a structured packing material.

9. Method according to claim 1, wherein the final ATS product is concentrated through reverse osmosis.

10. Method according to claim 1, wherein the SO.sub.2 absorbers are operated at pH values in the range 4.5 to 7.5.

11. Method according to claim 1, wherein the ATS reactor is operated at pH values in the range 6.5 to 9.

12. Method according to claim 1, wherein the small amounts of SO.sub.3 formed in step (a) react with water to form sulfuric acid vapor, of which a part condenses as small droplets and wherein an aerosol filter is installed to treat the product gas downstream from step (b) in order to reduce or eliminate emission of sulfuric acid mist in the product gas.

13. Method according to claim 12, wherein the filter is a low velocity candle filter or a wet electrostatic precipitator, and wherein the liquid drain from the filter is optionally returned to the liquid of the second absorber.

14. Method according to claim 1, wherein step (a) also converts sulfur compounds other than H.sub.2S.

15. Method according to claim 1, wherein the oxygen content in the gas leaving the selective catalytic step is below 1%.

16. Method according to claim 1, wherein conventional technology for CO.sub.2 and N.sub.2 removal is installed downstream of the absorption steps, thereby upgrading the gas to natural gas pipeline quality.

17. Method according to claim 1, wherein the inlet temperature to reaction (a) is restricted to levels of less than 350 C., but more than 170 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1 shows a process wherein H.sub.2S and NH.sub.3 contained in an off-gas from a digester are converted to an aqueous solution of ammonium thiosulfate.

DETAILED DESCRIPTION

[0044] The invention is illustrated in more detail in the example which follows. In the example, reference is made to the appended FIGURE.

EXAMPLE

[0045] In this example, the H.sub.2S and NH.sub.3 contained in an off-gas from a digester are converted to an aqueous solution of ammonium thiosulfate in the process illustrated in the FIGURE. The feed gas (1) in an amount of 2800 Nm.sup.3/h contains 58 vol % CH.sub.4, 39 vol % CO.sub.2, 2.4% H.sub.2O, 0.5 vol % H.sub.2S and 0.1 vol % NH.sub.3. The feed gas is split into two streams, where the main part (2) is mixed with the effluent (3) from the ATS reactor (4). Air (6) is added to the mixed stream (5), and the combined stream is sent to the catalytic reactor (7), in which H.sub.2S is oxidized selectively to SO.sub.2 over an SMC-type catalyst, which does not convert CH.sub.4.

[0046] The SO.sub.2-containing stream (8) is contacted with an aqueous solution of AHS and DAS in the first absorber (9) at 30 C. and a pH of 5.8 to produce a partially cleaned gas (10) and a rich AHS solution (11) containing 44 wt % AHS and 2 wt % DAS. The temperature of the first absorber is controlled by means of heat exchange with cooling water. The effluent gas (10) is further cleaned in a second absorber (12) by contact with an aqueous solution of AHS and DAS at 28 C. and a pH of 5.8 to produce a cleaned gas (13) and a lean AHS solution (14) containing 9.6 wt % AHS and 0.4 wt % DAS. A mist filter (15) can be installed downstream the second absorber to capture aerosol droplets formed from small amounts of SO.sub.3 and H.sub.2SO.sub.4 in the effluent (8) from the catalytic reactor.

[0047] The cleaned gas (16) is sent to the stack (17) or to further processing, and the mist filter drain liquid (18) is returned to the second absorber (12). The rich AHS solution (11) is contacted with a fraction of the feed gas (18) in the ATS reactor (4) at 37 C. and a pH of 7.5 to produce the ATS product (19), which is an aqueous solution of 55 wt % ATS with small amounts of AHS and DAS. The pH values in the ATS reactor (4), the first absorber (9) and the second absorber (12) are controlled by addition of small amounts of NH.sub.3 via streams (20), (21) and (22). The ATS concentration is controlled by addition of water (23) to the second absorber.

[0048] An overview of the main streams is given in Tables 1 and 2 below.

TABLE-US-00001 TABLE 1 Stream 1 5 8 16 Mol % Mol % Mol % Mol % H.sub.2S 0.5 0.33 0 0 H.sub.2O 2.4 3.1 3.4 3.6 O.sub.2 0 0 0.1 0.1 NH.sub.3 0.1 0.30 0.29 0.0005 SO.sub.2 0 0 0.32 0.0050 CO.sub.2 39 38.7 37.7 37.8 CH.sub.4 58 57.5 56.0 56.3 N.sub.2 0 0 2.2 2.2 Total (Nm.sup.3/h) 2800 2822 2901 2886

TABLE-US-00002 TABLE 2 Stream 11 14 19 wt % wt % wt % ATS 0 0 55 DAS 2 0.5 0.4 AHS 44 9.6 0.2 H.sub.2O 54 89.9 44.4 Total(kg/h) 89 53 83