The present invention is related to the removal of soot from sulfurous process gas streams by carbon oxidation with the aid of a catalyst of the VK type, prepared and sold by the applicant.

Soot is the common word for impure carbon particles result- ing from the incomplete combustion of hydrocarbons. It is a powder-like form of amorphous carbon. The gas-phase soots contain polycyclic aromatic hydrocarbons (PAHs) . Most properly it is restricted to the product of the gas-phase combustion process, but it is commonly extended to include the residual pyrolysed fuel particles, such as coal, charred wood, petroleum coke, etc., which may become air ^¬ borne during pyrolysis and which are more properly identi ^¬ fied as cokes or chars. Soot as an airborne contaminant in the environment has many different sources, all of which are results of some form of pyrolysis. They include soot from coal burning, internal combustion engines, power plant boilers, ship boilers, cen ^¬ tral steam heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, furnaces, etc. The formation of soot depends strongly on the fuel composition. The rank ordering of the tendency of fuel components to produce soot is: naphthalenes → benzenes → ali- phatics. This phenomenon is also known as cracking. How- ever, the order of sooting tendencies of the aliphatics

(alkanes, alkenes, alkynes) varies dramatically depending on the flame type. The difference between the sooting tendencies of aliphatics and aromatics is thought to result mainly from the different routes of formation. Aliphatics appear to first form acetylene and polyacetylenes, which is a slow process; aromatics can form soot both by this route and also by a more direct pathway involving ring condensa ^¬ tion or polymerization reactions building on the existing aromatic structure.

Carbon black is a specific variant of soot produced by the incomplete combustion of heavy petroleum products like tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of paracrystalline carbon that has a high surface area-to-volume ratio, albeit lower than that of activated carbon. It is dissimilar to ordinary soot in its much higher surface area-to-volume ra ^¬ tio and significantly lower (negligible as well as non- bioavailable) PAH content. However, carbon black is widely used as a model compound for diesel soot for diesel oxida ^¬ tion experiments. Carbon black is mainly used as a reinforcing filler in tires and other rubber products. In plastics, paints and inks, carbon black is used as a color pig ^¬ ment .

The catalytic combustion of soot is well-known within the automotive field, especially diesel engines. Thus, in US 6,764,664 a catalyst composition is disclosed for the re ^¬ duction of soot and undesirable gaseous emissions from en ^¬ gine exhaust, particularly exhaust from diesel engines. The catalyst contains a catalytic alkali metal oxide, prefera- bly lithium platinum oxide, in which the catalytic metal is atomically isolated. For improved performance in a diesel particulate filter, the alkali catalytic metal oxide is uniformly dispersed on an alkali metal aluminate such as lithium aluminate.

WO 1990/012646 also deals with catalytic combustion of soot from diesel engines. In order to obtain said catalytic com ^¬ bustion, the exhaust gases are conveyed over a catalyst ma ^¬ terial comprising vanadium and copper oxides in a molar ratio V:Cu of 85:15 to 95:5, preferably 90:10. In order to increase the activity at low temperatures, the catalyst preferably also contains elementary platinum, palladium or rhodium.

Applicant's US 2011/0283680 relates to a method for purifi ^¬ cation of exhaust gas from a diesel engine in a system, which comprises a device for selective catalytic reduction and a diesel particulate filter, preferably at least par ^¬ tially covered by a catalytic layer installed downstream of the device for selective catalytic reduction. A device for catalytic oxidation is installed upstream of the device for selective catalytic reduction and/or between said device and the diesel particulate filter. The selective catalytic reduction preferably takes place in the presence of a vana ^¬ dium-based catalyst, which is vanadium oxide on titanium oxide with possible addition of tungsten or molybdenum ox- ides.

WO 2014/169967, also belonging to the applicant, relates to a method and a system for removal of soot, ash and heavy metals, and optionally additionally NOx and SOx being pre- sent in the exhaust gas from an engine operated on heavy fuel oil with a sulfur content of 0.1% - 4.0 wt% and a heavy metal element content of 5 mg/kg to 1000 mg/kg. The catalyst used in the system preferably comprises titanium dioxide, oxides of vanadium and tungsten and metallic pal ^¬ ladium .

Finally, US 6,013,599 describes a low temperature diesel exhaust soot oxidation catalyst comprising a porous refrac ^¬ tory metal or metal oxidic support and a wash coat. The wash coat is formed by the steps of mixing an acidic iron- containing compound and a copper-containing compound, add- ing an aqueous alkali metal solution, adding an acidic va ^¬ nadium compound containing solution and adding an alkaline earth metal compound solution. The copper-containing compound can be copper sulfate, and the vanadium-containing compound can be vanadium pentoxide. The exhaust may com- prise sulfur compounds, but there is no indication of the exhaust comprising more than 500 ppm SO ₂ and/or SO ₃ .

While the catalytic combustion of soot on vanadium cata ^¬ lysts is well described in patent documents within the au- tomotive field, this is not the case for VK type catalysts for the combustion of soot or carbon black. So far, catalytic oxidation of carbon black seems to be described only in a few scientific articles, viz. Carbon black oxidation in the presence of AI ₂ O ₃ , Ce0 ₂ and Mn oxide catalysts: An EPR study, in Catalysis Today 119, 286-290 (2007), Catalyt ^¬ ic oxidation of carbon black - I . Activity of catalysts and classification of oxidation profiles, in Fuel 97 (3) , 111- 119 (1998), Catalytic oxidation of an amorphous carbon black, in Combustion and Flame 99, 413-421 (1994), and Ef- fects of ozone on the catalytic combustion of carbon black, in Applied Catalysis B: Environmental 5_4, 9-17 (2004) .

The present invention relates to a method for removal of soot from varying sulfurous process gas streams by carbon oxidation using a catalyst of the VK type. Generally, VK catalysts can be used to oxidize S O2 to SO ₃ in sulfuric ac ^¬ id plants, and they consist of vanadium (as V ₂ O ₅ ) , sulfur (as sulfate, pyrosulfate, tri- or tetrasulfate) , S 1 O2 and alkali metals, such as Li, Na, K, Rb and Cs and mixtures thereof, as promoters. VK catalysts can be used in conjunc ^¬ tion with other catalyst types, such as SCR (selective cat ^¬ alytic removal), for removal of NOx .

More specifically, the invention concerns a process for the removal of soot from a sulfurous gas stream, wherein a pro ^¬ cess gas containing O2 and more than 500 ppm S O2 and/or SO ₃ together with soot is brought into contact with a VK type catalyst in a reactor, said catalyst comprising vanadium pentoxide (V ₂ O ₅ ) , sulfur in the form of sulfate, pyrosul ^¬ fate, tri- or tetrasulfate and one or more alkali metals on a porous carrier.

The alkali metals are preferably selected from Na, K, Rb and Cs .

The porous carrier of the catalyst is preferably silicon dioxide ( S 1 O2 ) , optionally containing a minor amount of alumina (up to 10 wt%, preferably up to 5 wt% and most preferably up to 2 wt% or 1 wt%) .

An overview of applicant's VK catalysts is given in Table 1 below. The ignition temperature of the catalyst, i.e. the temperature at which the melt becomes active, can be up to 100 °C lower than the operating temperature.

Table 1

According to the invention, a process gas containing O ₂ and SO ₂ and/or SO ₃ together with soot is brought into contact with a VK type catalyst, as defined above, in a reactor. The soot in the process gas does not combust upon exposure to the normal operating temperature conditions of a VK cat ^¬ alyst, but it has turned out that it does combust under these temperature conditions when it is brought into con ^¬ tact with said catalyst type. The combustion occurs because of the presence of an alkali metal pyrosulfate melt which is formed on the surface of the catalyst in the presence of S0 ₂ and/or S0 ₃ . This leads to a number of benefits: First of all there is not any pressure drop, which could be caused by accumula ^¬ tion of soot in the catalyst bed, and therefore extended periods between screening of the catalyst are possible. An added benefit comes from the fact that the combustion of soot reduces the required inlet temperature to the catalyst bed, e.g. by 3°C for a soot load of 100 mg/Nm ³ /h, which in turn reduces the amount of energy needed to heat the pro ^¬ cess gas upstream the reactor. Furthermore, since the soot is combusted on the VK catalyst, less removal of soot is required upstream the reactor, where electrostatic filters, bag filters or ceramic filters are typically used.

The alkali metal content in the catalyst used in the pro- cess of the invention is 2-25 wt%, preferably 4-20 wt% and most preferably 8-16 wt%. The catalyst contains 1-15 wt% V ₂ O ₅ , preferably 2-12 wt% and most preferably 4-10 wt% V ₂ O ₅ .

Furthermore, the catalyst used in the process of the inven- tion contains 1-25 wt% sulfur, preferably 2-20 wt% or 3-18 wt% and most preferably 4-16 wt% or 4-10 wt% sulfur, in the form of sulfate, pyrosulfate, tri- or tetrasulfate .

In the process of the invention, the catalyst temperature is between 250 and 700°C, preferably between 300 and 650°C.

The process gas has a soot concentration of > 0.1 mg/Nm ³ /h, and the soot concentration can be more than 50 mg/Nm ³ /h with the preferred concentrations of > 0.5 mg/Nm /h, > 1 mg/Nm ³ /h, > 2 mg/Nm ³ /h, > 5 mg/Nm ³ /h, > 10 mg/Nm ³ /h and > 20 mg/Nm ³ /h in between.

The process gas preferably contains more than 1000 ppm S O2 and/or S O ₃ , preferably more than 2500 ppm S O2 and/or S O ₃ and most preferably more than 5000 ppm S O2 and/or S O ₃ .

Moreover, the process gas has a soot composition comprising > 20 wt% C. Preferably the process gas has a soot composi ^¬ tion comprising > 50 wt% C, > 75 wt% C, > 90 wt% C or even > 95 wt% C.

In a preferred embodiment of the process according to the invention, the soot is carbon black. Carbon black can be differentiated from most other types of soot, such as the soot originating from combustion engines, in the elemental composition and the surface area-to-volume ratio.

The invention is illustrated in more detail in the follow ^¬ ing example without being limited thereto.

Example

The ability of the VK-WSA catalyst to remove soot at 400°C was tested by covering the surface of the catalyst with carbon black of the type Printex U and exposing the soot- covered catalyst to a gas in a reactor. The gas contained 10 vol% S0 ₂ and 10 vol% 0 ₂ .

A comparative study of the thermal oxidation of carbon black in air at 400°C showed a considerably lower removal of soot. The results are shown in Table 2 below.

Table 2

Thermal oxidation

Treatment VK-WSA at 400°C at 400°C

Soot removal, wt% 47 > 99