Process for the combined removal of siloxanes and sulfur-containing compounds from biogas streams

Abstract

A process for combined removal of siloxanes and sulfur-containing compounds from biogas streams, such as streams from landfills or anaerobic digesters, comprises heating the biogas stream and optionally mixing it with air, feeding the gas to a first filter unit with high temperature resistance, injecting a dry sorbent into the first filter unit to capture siloxanes present in the gas, recycling part of the exit gas from the first filter unit to the inlet thereof for the sulfur-containing compounds to be captured by the dry sorbent or optionally to a second filter unit inlet for the sulfur-containing compounds to be captured by a sulfur-specific sorbent and recovering clean gas from the first or optionally from the second filter unit.

Claims

1. A process for the combined removal of siloxanes and sulfur-containing compounds from biogas streams, said process comprising the steps of: heating the biogas stream and, if necessary to obtain an oxygen level of at least 0.1%, mixing it with air, feeding the gas mixture to a first filter unit provided with catalytic filter candles or filter bags with high temperature resistance, injecting a dry sorbent into said first filter unit to capture siloxanes present in the gas mixture, recycling part of the exit gas from the first filter unit to the inlet thereof for the sulfur-containing compounds to be captured by the dry sorbent or optionally feeding the exit gas from the first filter unit to a second filter unit inlet for the sulfur-containing compounds to be captured by a sulfur-specific sorbent and recovering the clean gas from the first or optionally from the second filter unit.

2. Process according to claim 1, wherein the mixed gas is heated to at least 180 C. before entering the first filter unit.

3. Process according to claim 1, wherein the catalytic filter candles or filter bags in the first filter unit are impregnated with a TiO.sub.2-based catalyst supported by vanadia or a combination of vanadia and palladium.

4. Process according to claim 1, wherein only the first filter unit is used, while the second filter unit is omitted.

5. Process according to claim 4, wherein the catalyst is situated inside the filter candles or filter bags, so that it is protected from the siloxanes present in the gas mixture.

6. Process according to claim 1, wherein both the first filter unit and the second filter unit are used, the siloxane removal being accomplished in the first filter unit with catalytic filters and sulfur removal being accomplished in the second filter unit with non-catalytic filters.

7. Process according to claim 6, wherein the first filter unit and the second filter unit are combined into one filter unit divided into two compartments, so that the siloxane removal is accomplished in the first compartment with catalytic filters and the sulfur removal is accomplished in the second compartment with non-catalytic filters.

8. Process according to claim 7, wherein the siloxane specific sorbent is injected into the first compartment, while sulfur-specific sorbents are injected into the second compartment.

Description

(1) The present invention represents the combined knowledge of the Applicant within SMC H.sub.2S oxidation technology and catalytic filtration.

(2) More specifically, the present invention relates to a process for the combined removal of siloxanes and sulfur-containing compounds from biogas streams, in particular biogas streams from landfills and anaerobic digesters, said process comprising the steps of:

(3) heating the biogas stream and if necessary, to provide a sufficient oxygen level, mixing it with air,

(4) feeding the gas mixture to a filter unit provided with catalytic filter candles or filter bags with high temperature resistance,

(5) injecting a dry sorbent into the filter unit to capture siloxanes present in the gas mixture,

(6) recycling part of the exit gas from the filter unit to the filter house inlet for the sulfur-containing compounds to be captured by the dry sorbent and

(7) recovering the clean gas from the filter unit.

(8) The gas from the landfill is heated and mixed with air, so that there is at least 0.1% residual air when the oxygen is consumed in the reaction
2H.sub.2S+3O.sub.2.fwdarw.2SO.sub.2+2H.sub.2O

(9) The mixed gas is heated to at least 180 C. before entering a filter house unit with catalytic filter candles or filter bags with high temperature resistance. The catalytic filters are impregnated with a TiO.sub.2-based catalyst supported by vanadia or a combination of vanadia and palladium. In the filter, H.sub.2S is converted to SO.sub.2 and low levels of SO.sub.3.

(10) The purpose of recirculating part of the exit gas from the first filter unit to the inlet thereof is to minimize the investment costs of the process by utilizing the first filter for removal of both siloxanes and SO.sub.2/SO.sub.3.

(11) The gas stream to be treated is mixed with the recirculation gas, which contains SO.sub.2/SO.sub.3. This creates a larger gas flow than the original gas stream. Dry sorbent(s) is/are injected into this mixed gas to absorb siloxanes from the original gas and SO.sub.2/SO.sub.3 from the recirculated gas. The gas then passes the filters, thereby removing the dry sorbent(s) from the gas. The catalyst located inside the filter is not exposed to the siloxanes, which otherwise may cause poisoning of the catalyst. The catalyst oxidizes the H.sub.2S, present in the original gas stream, to SO.sub.2 and low levels of SO.sub.3. The gas exits the filter unit.

(12) A part of the gas exiting the filter unit is recirculated and mixed into the original gas stream. The purpose of this recirculation is to remove some of the SO.sub.2/SO.sub.3 that was formed over the catalyst by oxidizing the H.sub.2S, thereby reducing the total sulfur content of the gas leaving the system. The amount of recirculated gas depends on the desired level of sulfur content in the gas leaving the system.

(13) The filter house unit is equipped with means for injection of a dry sorbent, where sorbents like limestone, Ca(OH).sub.2, Trona (trisodium hydrogen dicarbonate dehydrate, also called sodium sesquicarbonate dihydrate, Na.sub.2CO.sub.3NaHCO.sub.32H.sub.2O) and others can be used. These sorbents will catch the siloxanes, but specific sorbents targeted to catch siloxanes can be added to the dry sorbent injection step.

(14) The exit gas from the filter unit is recycled back to the filter house inlet duct and led to the filter house, where SO.sub.2 and SO.sub.3 are captured by the dry sorbent. This way, the catalyst is protected from siloxanes since it is situated inside the filter, and one unit can handle both siloxane and sulfur removal.

(15) Another valid alternative is to divide the filter house unit into two compartments, where siloxane removal is accomplished in the first compartment with catalytic filters and sulfur removal is accomplished in the second compartment with non-catalytic filters. In this case, the siloxane specific sorbent can be injected into the first unit, while sulfur-specific sorbents can be injected into the second unit.

(16) The clean gas from the filter house unit(s) is used to preheat the feed gas using a feed/effluent heat exchanger.