The present invention relates to a process for performing selective catalytic reduction (SCR) of a coke oven flue gas at low temperatures.

SCR systems use catalysts to promote a reaction between flue gas NOx and a reducing agent, typically ammonia, that is injected into the flue gas stream. The catalysts for this purpose selectively convert NOx into nitrogen and wa ^¬ ter, thereby reducing NOx emissions by up to 97%.

The SCR is a catalytic reaction of nitrogen oxides (NO and NO ₂ ) with ammonia to form elemental nitrogen and water in accordance with the reaction schemes

4 NO + 4 NH ₃ + 0 ₂ -> 4 N ₂ + 6 H ₂ 0

NO + N0 ₂ + 2 NH ₃ -> 2 N ₂ + 3 H ₂ 0

6 N0 ₃ + 8 NH ₃ -> 7 N ₂ + 12 H ₂ 0

2 N0 ₂ + 4 NH ₃ + 0 ₂ -> 3 N ₂ + 6 H ₂ 0

The first two reactions are the predominant ones, with one mole of ammonia consumed for each mole of NOx converted. The last two reactions occur in gases, in which large frac ^¬ tions of NOx are present as N0 ₂ . To make the reactions oc ^¬ cur at temperatures in the range 250-450°C, a catalyst is used. The most common SCR catalyst type is based on V ₂ Os catalyst on a Ti0 ₂ carrier.

Coke oven plants are facing increasingly stricter legislation with respect to NOx emissions, especially in China which is by far the largest market for coke production.

In a coking plant, coke is produced from coal in a coke oven. Here, the volatile constituents in the coal are pyro- lyzed by heating to a temperature of 900 to 1400°C and then liberated and extracted. This forms the coke, which con ^¬ sists essentially of carbon and an off-gas that contains the volatile constituents and is referred to as coking plant gas. The pyrolysis in the coke oven takes place in the absence of oxygen. This is in principle a batch pro ^¬ cess, and the composition of the liberated coking plant off-gas fluctuates. However, since a plurality of coke col ^¬ lectors are always operated, the average gas composition is subjected to only small fluctuations. So the coking plant gas formed contains H ₂ (about 55%) , N ₂ , CO, CO ₂ , sulfur and higher hydrocarbons .

Coke oven gas (COG) is a byproduct of the coke making pro ^¬ cess. COG consists of a complex mixture of various gases. Its composition typically consists of 55 % H ₂ , 6 % CO, 25 % CH ₄ (methane) , plus small percentages of CO ₂ (carbon diox ^¬ ide) , H ₂ O (moisture) , heavy tars, volatile hydrocarbons and sulfur impurities. It also contains some ₂ (nitrogen) . The higher hydrocarbons, tars and aromatic compounds in the coke oven gas are often extracted from the gas stream through a series of cooling, condensation and separation steps. Such liquid by-products are marketed and known as coke oven tars which are traded on a worldwide basis. COG typically denotes the remaining gases from such a series of such tar extraction processing steps and is typically used as fuel gas for various heating applications within the steel plant, and surplus COG is used to produce either steam or electrical power, or it is flared. The use of COG for direct reduced iron (DRI) production has always been of interest, but the challenge has been converting the methane to CO and ¾ and cleanup of the tars and volatile hydrocar- bons .

The heat for the coke oven furnace is provided through re ^¬ generative burners that combust the COG.

Coke oven gas contains hydrocarbons such as BTX (mixtures of benzene, toluene and the three isomers (o, m and p) of xylene), methane, ¾ and CO. Processing and utilization of COG not only boosts the energy efficiency of the steel in ^¬ dustry, but also prevents the emission of harmful green ^¬ house gases including CO ₂ and methane. Coke oven gases with high hydrogen contents are of huge industrial and commer- cial value considering the future hydrogen economy. At present, many steel enterprises are aiming to minimize their COG surplus while utilizing the gas in various on-site pro ^¬ cesses during steel manufacturing. Although extensive research and development has been carried out to utilize the COG surplus, substantial amounts of COG are still being wasted, resulting in poor production efficiencies and serious greenhouse gas emissions.

In the prior art, coke oven gas has been used for various purposes. Thus, US 4.270.739 describes a process and an ap- paratus for the direct reduction of iron oxide utilizing sulfur-containing gas, such as coke oven gas, as process gas. The process is particularly well adapted for the use of gaseous process fuels which contain organic sulfur. WO 2006/013455 describes a method and an apparatus for pro ^¬ ducing clean reducing gases from coke oven gas, where volatile components derived from coal are transformed into re ^¬ ducing gases suitable for utilization as synthesis gas, as a reducing agent for the direct reduction of iron ores and/or as a clean fuel.

US 2016/0083811 describes a method for the reduction of iron oxide to metallic iron using coke oven gas. The method comprises dividing coke oven gas into a plurality of coke oven gas streams, providing a first coke oven gas stream to a hydrogen enrichment unit to form a hydrogen-rich product stream that is delivered to a reduction shaft furnace as part of a reducing gas stream, and providing a tail gas stream from the hydrogen enrichment unit to a reforming re- actor to form a reformed gas stream that is delivered to a reduction shaft furnace as part of the reducing gas stream. Optionally, a spent top gas stream from the reduction shaft furnace is cleansed of CO ₂ and recycled back to the reduc ^¬ ing gas stream. In US 2012/0261244, a method for reducing nitrogen oxides from the exhaust gas of a coke oven is disclosed. The method comprises burning a combustible gas, consisting partly or entirely of coke oven gas, to produce a flue gas containing nitrogen oxides. A reducing agent, preferably ammonia, is fed into the flue gas at 700-1100°C to reduce the nitrogen oxide component of the flue gas by a homogene ^¬ ous gas reaction between the reducing agent and the nitro ^¬ gen oxides, while heat is recovered from the flue gas in a regenerator . CN 107014217 A discloses a system and a method for the uti ^¬ lization of coke oven gas from a coking plant and treatment of the flue gas. Specifically, it describes a process for performing selective catalytic reduction on a coke oven flue gas by mixing the coke oven flue gas with coke oven gas before passing it to the SCR. Although coke oven gas is passed through a boiler before reaching the SCR, a portion of the coke oven gas must remain unreacted for the coke oven gas to be able to retain its reducing capabilities, meaning that the boiler is run at sub-stoichiometric condi ^¬ tions .

The use of COG as a reduction agent instead of natural gas for the direct reduction of iron ore in the process known as the Midrex™ process has been investigated (Kobelco Technology Review 3_3, pages 16-20, Feb. 2015) . That process runs without coke, emits less CO ₂ and thus is gathering at ^¬ tention as an alternative process for blast furnace iron production .

COG can be used for various other purposes, such as a feed- stock for hydrogen separation. At present, some on-site coke plants in the steel industry use pressure swing ad ^¬ sorption (PSA) to obtain ¾ from COG (Bermudez et al . , Fuel Process Technol. 110, pages 150-159, 2013) . The process is carried out in a cyclic adsorption-desorption operation us- ing different adsorbent materials such as alumina oxides or zeolites. COG reforming also provides an attractive alter ^¬ native to a less energy-intensive and more clean syngas production (Li et al . , Chem. Eng. Technol. 3_0, pages 91-98, 2007) . The flue gas from coke oven firing contains NOx (i.e. NO and NO ₂ ) , SO ₂ and particulate matter as the main pollu ^¬ tants. The extent of SO ₂ emissions depends on the degree of desulfurisation of the coke oven gas. The extent of NOx emissions may be reduced by low-NOx-firing techniques. The challenge in connection with gas control of flue gas emissions from coke oven plants is that the flue gas exits the regenerative heating/cooling block at a low tempera ^¬ ture. The heat depleted flue gas contains sulfur dioxide (SO ₂ ) , and thus the formation of ammonia bisulfate and hence a rapid catalyst deactivation becomes a problem when ammonia is used as the reducing agent. Therefore, the idea of the present invention is to replace the normally used ammonia with coke oven gas as the reducing agent.

So the present invention relates to a process for perform- ing selective catalytic reduction (SCR) on a coke oven flue gas at low temperatures, wherein

- coke oven gas is used as the reducing agent, which is mixed into the flue gas, and

- an SCR catalyst is used. Furthermore, the SCR catalyst used can be in the form of a catalytic filter bag or catalytic ceramic filter candles to simultaneously remove particulates and associated catalyst poisons along with NOx and residual hydrogen and hydrocarbons. In addition, methanol and/or DME can be used alone or in combination with the coke oven gas.

By using a catalyst based on vanadium oxide and palladium to oxidize the slip components, the slip of ¾, CO and BTX can be mitigated. This will provide a very efficient means to reduce NOx in coke oven plants.

The low process temperature is preferably a temperature of 100-250°C, more preferably 150-230°C and most preferably 170-210°C.