PROCESS FOR THE REMOVAL OF HYDROGEN CHLORIDE AND SULFUR OXIDES FROM A GAS STREAM BY ABSORPTION

Abstract

In a process for the removal of hydrogen chloride and/or sulfur oxides from a landfill gas stream, which contains impurities such as siloxanes, H.sub.2S, organic and inorganic sulfides and volatile organic compounds (VOCs), the heated gas is passed through a siloxane removal bed, where siloxanes are absorbed and then through one or more sulfur removal beds, where hydrogen sulfide and/or organic sulfides are absorbed. The effluent is passed through a reactor containing an oxidation catalyst enabling catalytic oxidation of VOCs, organic and inorganic CI- and/or S-containing compounds, COS and CS.sub.2 to their respective combustion products, and finally the effluent from the reactor is passed through one or more beds, where hydrogen chloride and/or sulfur oxides are absorbed.

Claims

1. A process for the removal of hydrogen chloride and/or sulfur oxides from a gas stream which contains primarily some or all of the following compounds: methane, carbon dioxide, nitrogen, oxygen and water, and which also contains impurities such as siloxanes, hydrogen sulfide, organic and inorganic sulfides and volatile organic compounds (VOCs), said gas stream originating from a landfill or an anaerobic digester or another industrial operation producing a similar gas stream, and said process comprising the steps of heating the gas, optionally passing the hot gas through a siloxane removal bed, where siloxanes are absorbed, optionally passing the hot gas through one or more sulfur removal beds, where hydrogen sulfide and/or organic sulfides are absorbed, passing the effluent from the said optional absorption beds through a reactor containing an oxidation catalyst, said catalyst enabling catalytic oxidation of VOCs, organic and inorganic chlorine- and/or sulfur-containing compounds, COS and CS.sub.2 to their respective combustion products, and passing the effluent from the reactor through one or more beds, where hydrogen chloride and/or sulfur oxides are absorbed.

2. Process according to claim 1, wherein the oxidation catalyst is a catalyst with increased oxidation activity as well as a negligible SO.sub.2 selectivity.

3. Process according to claim 1, wherein the oxidation catalyst is selected from vanadium/titania catalysts with or without palladium and catalysts containing precious metals, such as platinum, supported on silica or alumina.

4. Process according to claim 1, wherein the materials for the beds, where hydrogen chloride and sulfur oxides are absorbed, are selected from oxides, hydroxides, carbonates, hydrogen carbonates and hydroxy carbonates of alkali metals or alkaline earth metals dispersed on carriers selected from alumina, silica and titania or mixtures thereof.

5. Process according to claim 4, wherein the alkali metals and alkaline earth metals are selected from potassium, sodium, magnesium and zinc.

6. Process according to claim 4, wherein the bed for hydrogen chloride and sulfur oxides absorption is K.sub.2CO.sub.3 or Na.sub.2CO.sub.3.

Description

EXAMPLE

[0020] A raw landfill gas, which is predominantly composed of CH.sub.4, CO.sub.2, N.sub.2, H.sub.2O and O.sub.2, also contains impurities such as H.sub.2S and organic chlorine- and sulfur-containing compounds as well as other impurities, e.g. siloxanes and VOCs.

[0021] After the gas heat-up step, the hot gas is passed through a siloxane removal bed, where any siloxanes are absorbed. Then the gas is passed through a hydrogen sulfide removal bed, where H.sub.2S is absorbed, and from there it is passed through a reactor containing an oxidation catalyst. The oxidation catalyst is selected so as to facilitate catalytic oxidation of VOCs, organic chlorine- and sulfur-containing compounds, COS or CS.sub.2 with O.sub.2 to their respective combustion products, which means that the compounds are converted to a mixture of CO.sub.2, H.sub.2O, HCl, SO.sub.2 and SO.sub.3 still entrained in the process gas (predominantly consisting of CH.sub.4, CO.sub.2, N.sub.2, H.sub.2O and O.sub.2).

[0022] According to the present invention, the process gas is passed through one or more beds where HCl, SO.sub.2 and SO.sub.3 are absorbed, optionally with concurrent release of one or more compounds from the sorbent material (in case of K.sub.2CO.sub.3 as sorbent: K.sub.2CO.sub.3+2 HCl.fwdarw.2 KCl+CO.sub.2+H.sub.2O). The invention is particularly useful since an application as described above, where the bulk of the sulfur H.sub.2S is removed by absorption rather than by conversion to SO.sub.2, allows for utilization of oxidation catalysts having an increased oxidation activity but at the same time being characterized by a low negligible SO.sub.2 selectivity. This in turn entails that O.sub.2 can be removed to even lower levels (and potentially also with less overdosing) than those which are attainable with SMC. Thus, exploiting the invention will also present benefits to various downstream CO.sub.2 removal devices.

[0023] Furthermore, HCl, SO.sub.2 and SO.sub.2 are all corrosive in the presence of water and also poisonous to certain catalysts. Therefore, it is desirable to remove these compounds in order to protect piping and downstream equipment and catalysts.