SNOX flue gas treatment for boilers burning high-sulphur fuels

Transcription

1 SNOX flue gas treatment for boilers burning high-sulphur fuels

2 SNOX flue gas treatment for boilers burning high-sulphur fuels 1 / 18 Summary The processing of more and more high-sulphur crude oil makes it necessary for the refineries to find ways of getting rid of the high-sulphur residuals produced. One obvious way is to burn them to produce steam and power for the refineries' own use. This, however, creates a flue gas with high sulphur dioxide (SO 2 ) content that has to be cleaned up. The SNOX TM process is a catalytic flue gas cleaning process, which removes up to 99% of sulphur dioxide (SO 2 ) and sulphur trioxide (SO 3 ), up to 96% of the nitrogen oxides (NOx) and essentially all particulates from flue gases. The sulphur is recovered as commercial grade concentrated sulphuric acid while NOx is reduced to free nitrogen (N 2 ). The process does not consume water or materials, except for ammonia for the catalytic NOx reduction, and it does not generate any secondary sources of pollution, such as waste water, slurries or solids. The operation cost of a SNOX plant decreases with increasing sulphur content in the flue gas. The process is in particular suited for treatment of flue gases with high contents of SO 2 and SO 3 and heavy metals from combustion of petroleum coke and other refinery residues with none of the environmental and operational problems encountered with other flue gas desulphurisation (FGD) processes. Today, the SNOX process is used in large scale on a coal fired power plant in Denmark (Fig. 2), a petcoke fired power plant in Italy (Fig. 1) and a heavy residual fuel oil fired steam and power plant in Austria. The paper describes design and operating experience of the 1,200,000 Nm 3 /h SNOX plant at the petcoke-fuelled power plant of the Raffineria di Gela, Sicily, Italy, belonging to AgipPetroli, and compares SNOX with alternative technologies for combustion of high-sulphur residues in power stations. The SNOX plant in Gela has operated with 99% availability and practically unchanged performance since it was commissioned in September The Austrian SNOX plant is installed in the OMV refinery at Schwechat, near Vienna, and has been in operation since late Two SNOX plants in Brazil are in the final stages of design.

3 SNOX flue gas treatment for boilers burning high-sulphur fuels 2 / 18 Fig. 1. 1,200,000 Nm 3 /h SNOX TM plant at the power plant of Raffineria di Gela, Italy Fig MW SNOX TM equipped power plant of Nordjyllandsværket, Denmark Introduction Sweet crude oil with low sulphur content becomes more and more scarce, and the price gap between sour and sweet crude increases. Therefore more and more heavy, sulphurous crude oil must be processed, and more and more process units, such as thermal cracking, visbreaking and coking, are introduced in the oil refineries in order to squeeze out of the crude oil as much valuable distillate product as possible. This gives the refineries a better economy, since heavy crude is generally cheaper than sweet crude, but the refiners are left with the problem of disposing of the high-sulphur residues in the form of petroleum coke (petcoke) and residual oil fractions. Today's fuel specifications do not allow high sulphur contents in, practically speaking, any fuel type. An increasing fraction of the petcoke and the heavy residual oil fractions must be used as fuel in the production of heat and power. This can most advantageously take place directly in the refineries where considerable amounts of steam and power are required. Often refineries can even export power and heat to their neighbouring plants or to the civil communities nearby. All traditional flue gas desulphurisation (FGD) technologies have the disadvantage of being increasingly more expensive to operate, the higher the SO 2 content in the flue gas. In addition, the SO 3 and the metal oxide dust created by burning of petcoke and residual oil give severe environmental and operational problems in traditional FGD units.

4 SNOX flue gas treatment for boilers burning high-sulphur fuels 3 / 18 Topsøe has developed a FGD technology, SNOX TM that has the opposite economical dependency on the SO 2 content in the flue gas: the more SO 2, the more economical the SNOX plant will be. The SNOX technology has no problems with SO 3 and metal oxide dust. In a SNOX plant, the SO 2 in the flue gas is converted to concentrated sulphuric acid of high purity with the help of oxygen and water vapour which are already present in the flue gas. This means that in a SNOX plant there is no consumption of absorbents and no production of waste materials that have to be disposed of. Furthermore, the SNOX process yields a product with a significant positive sales value and increases the overall steam and power production by producing hot combustion air for the boiler plant. The metals contained in the fuel are removed as a dry powder along with possible other dust and soot in the flue gas. Fig. 3. The logic behind the requirement for SNOX TM flue gas cleaning A SNOX plant cleaning flue gas can also accept other sulphur-containing gases from a refinery, such as amine regenerator H 2 S off-gas, Claus plant tail gas, sour water stripper off-gas and other sulphurous flue gases. The SNOXTM process The SNOX process is a catalytic flue gas cleaning process which removes 95-99% of SO 2 and SO 3 and 90-96% of the NOx in flue gases. The sulphur is recovered as typically 95% concentrated sulphuric acid of high purity. NOx is catalytically reduced to N 2 by ammonia that is added to the flue gas. Essentially all dust and particulates are removed from the flue gas. The heat produced in the process and by cooling of the flue gas to 100 C is recovered and used in the form of p reheated combustion air for the boiler, thus increasing boiler thermal efficiency and gross power production.

5 SNOX flue gas treatment for boilers burning high-sulphur fuels 4 / 18 The process generates no secondary sources of pollution such as waste water, slurries or solids. It consumes no water or materials, except for ammonia for the catalytic NOx reduction. The process is in particular suited for purification of flue gas from combustion of high sulphur petroleum coke (petcoke) and other petroleum residues such as heavy fuel oil, tars and sour gases. In principle, there is no upper limit to the content of SO 2 and SO 3 in the flue gas. The principal SNOX process steps are (see Fig. 4): Dust removal in electrostatic precipitators or bag filters at about 200 C Heating of the flue gas to about 400 C in a feed/e ffluent heat exchanger Catalytic reduction of NOx by NH 3 Catalytic oxidation of SO 2 to SO 3 in the subsequent SO 2 converter Cooling of the SO 3 gas in the feed/effluent heat exchanger Further cooling of the gas to about 100 C in the W SA condenser, whereby the sulphuric acid vapour condenses Fig. 4. Simplified flow diagram of boiler equipped with SNOX TM

6 SNOX flue gas treatment for boilers burning high-sulphur fuels 5 / 18 Since SO 2 is converted to SO 3 in the process anyway, and essentially all heavy metal oxides are removed from the flue gas in the filter, neither SO 3 nor the metal oxides give corrosion problems. An added advantage is that, even before credit is taken for sales of the produced sulphuric acid, the operating cost of SNOX units slightly decreases with increasing SOx content in the flue gas. This is due to the recovery of the heat of formation of H 2 SO 4 from SO 2 amounting to 8 MJ per kg S in the fuel. The first full scale SNOX plant treating 1,000,000 Nm 3 /h flue gas from a 300 MW bituminous coal fired power plant was started up in 1991 at Nordjyllandsværket in Denmark. In USA, SNOX was demonstrated in the scale of 35 MW on a coal fired power plant in Niles, Ohio, in under the DOE Clean Coal II program. Today, five SNOX plants and eighty WSA plants treating a wide range of sulphur containing off-gases have been or are being implemented worldwide by Topsøe. (WSA plants are smaller SNOX plants with or without the DeNOx step, handling other types of waste gases.) The largest SNOX plant treats 1,200,000 Nm 3 /h flue gas from four petcoke fired downshot boilers, built in the 1960'ies at an AgipPetroli refinery, Raffineria di Gela in Sicily, Italy. The SNOX plant was supplied on turn-key basis by Snamprogetti SpA and went into operation in September Topsøe supplied process engineering, catalysts and proprietary equipment. Originally the plant was designed for and operated with only three boilers, but in 2006, a 4th boiler was connected to the SNOX plant without any change of the plant. Experience with a SNOX TM plant in Raffineria di Gela, Italy The Gela plant lay-out is shown in Fig. 5.

7 SNOX flue gas treatment for boilers burning high-sulphur fuels 6 / 18 Fig. 5. The SNOX TM plant at the Gela Refinery treating flue gas from three utility boilers The SNOX plant, as seen to the right of the dotted line, treats the flue gas from three (later four) utility downshot boilers (1) fuelled with 1900 t/d of high-sulphur petcoke and minor amounts of fuel oil and refinery sour gases. Each boiler emits up to 370,000 Nm 3 /h flue gas and comprises an electrostatic precipitator (3) and a flue gas fan (4) which were retained when the boilers were retrofitted with the SNOX plant. The flue gas from the boilers is collected in the manifold (m) and passed to the new additional ESP (5). Dust removal The content of dust (mainly unburned petcoke) in the flue gas from the "old" ESP's is further reduced in the additional ESP (5) to less than 1 mg/nm 3 of which 70% is carbon. The small amount of dust that is not captured in the ESP's is mainly ashes of metal oxides (mainly vanadium oxide) most of which accumulate in the catalyst panels of the SO 2 converter as oxy-sulphates.

8 SNOX flue gas treatment for boilers burning high-sulphur fuels 7 / 18 Gas heating After passing through the flue gas fan (6), the flue gas is heated in the rotating gas-gas heat exchanger (7) to 385 C and further to C in the heater (8) using fuel gas (natural gas, refinery gases and/or H 2 S gas from the refinery). The amount of fuel gas required depends on the content of SO 2 in the flue gas. NOx reduction NH 3 (pre-mixed with hot air) is then injected and mixed with the flue gas through a grid of nozzles (9) placed in the flue gas duct upstream of the DeNOx reactor (10). In the DeNOx reactor, the gas flows horizontally through a vertical bed of monolithic DeNOx catalyst. Almost all of the NOx is present as NO which is reduced by the selective catalytic reduction (SCR) reaction: NO + NH O 2 N H 2 O kj/mol The degree of NOx removal obtained depends on the NH 3 /NOx ratio and on how well the NH 3 is mixed with the flue gas. With typically 320 ppm NOx in the flue gas and an NH 3 /NOx ratio of 1.00, the flue gas exiting the DeNOx reactor contains about 10 ppm NOx and 20 ppm NH 3. The NH 3 slip is destroyed in the SO 2 converter. SNOX plants are always designed with NOx reduction, even if the legislative emission values can be could fulfilled without NOx reduction. This is in order to avoid a visibly coloured stack gas and NOx in the product acid. SO 2 oxidation In the subsequent SO 2 converter (11), the gas passes in parallel through 24 catalyst panels loaded with SO 2 oxidation catalyst in the form of "daisy"-shaped rings which give low pressure drop and high capacity for dust uptake. The DeNOx reactor and the SO 2 converter are integrated in one single vessel.

9 SNOX flue gas treatment for boilers burning high-sulphur fuels 8 / 18 In the SO 2 converter, about 98% of the SO 2 is oxidised to SO 3 : SO O 2 SO kj/mol At the same time, the ammonia slip from the DeNOx reactor is completely oxidised to N 2 and NOx whereby 95% overall NOx removal measured after the SO 2 reactor is achieved. Higher hydrocarbons in the flue gas are also oxidised completely. After 10 years of operation at C, neither the DeNOx catalyst nor the SO 2 oxidation catalyst have deactivated significantly. At operating temperature, the sulphuric acid catalyst is slightly sticky and acts as an effective dust filter removing practically all of the remaining dust content. At the same time, the catalyst will make most soot and hydrocarbons burn when they come in contact with its sticky surfaces. The dust accumulates in the voids of the catalyst bed so that the catalyst in the panels must be cleaned at intervals that are inversely proportional to the dust content in the gas. The cleaning can be performed by circulating and screening the catalyst in a closed system, without interrupting the operation of the SNOX plant. The ESP performs better than expected and <1 mg/nm 3 dust is found in the gas entering the SO 2 converter. About 70% of the dust is carbon (unburned petcoke), which is oxidised to CO 2 when it gets in contact with the hot catalyst, so that <0.3 mg/nm 3 of ash (mostly V 2 O 5 ) from the gas phase accumulates in the catalyst beds. Hence, the catalyst is expected to operate 50,000 hours or more on full load between catalyst screenings. Actually, after 7 years of operation at average 90% load, less than 10-20% increase in pressure drop across the SO 2 converter was observed without the catalyst having been touched. The catalyst still looked clean with no visible dust. Any trace of dust that could theoretically pass through the SO 2 converter would be removed by the condensing sulphuric acid in the WSA condenser and would end up in the product acid. The extremely low concentration of vanadium and the water-clear appearance of the acid confirm that no dust passes through the SO 2 converter. SO 3 gas cooling After the SO 2 converter, the gas is cooled to about 260 C in the rotating heat exchanger (7). The heat exchanger is equipped with recycle of sealing gas reducing the gas leakage to 2-2.5%. The leakage means that the % SO 2 conversion obtained in the SO 2 converter decreases to % when measured downstream of the heat exchanger.

10 SNOX flue gas treatment for boilers burning high-sulphur fuels 9 / 18 During the cooling, most of the SO 3 reacts with H 2 O in the flue gas, forming H 2 SO 4 vapour: SO 3 + H 2 O H 2 SO 4 (gas) kj/mol The WSA condenser The gas has a temperature well above the acid dew point when it enters the WSA condenser (12) which, in principle, is a falling film condenser in which the gas is further cooled to about 100 C in air cooled glass tubes. In the glass tubes, the remaining SO 3 is hydrated, and the sulphuric acid vapour condensed and concentrated to 95% strength. The mass and heat balance of the condensation is: H 2 SO 4 (gas) H 2 O H 2 SO 4 (liquid, 95% conc.) + 80 kj/mol The acid is collected in the bottom part of the condenser, from where it flows to the acid cooling system (16) where the acid is cooled to about 30 C in a water cooled plate heat exchanger. The WSA condenser contains a large number of vertical glass tubes divided into modules. The cooling air is delivered by the air fan (13) and heated to about 190 C in the condenser. It is cooled in the boiler/trim cooler (17) to C before it is heated in the air preheaters (2) and used as combustion air in the boilers. Excess air is passed to the stack. The boiler (17) is used to control the temperature of the flue gas entering the SNOX plant at about 200 C. Any acid droplets in the stack gas are removed in a guard demister (14) installed in the duct upstream the stack. Formation of acid mist (aerosol) in the condenser is suppressed by heterogeneous nucleation control. The mist control is patented by Topsøe and is essential for the operation of SNOX and WSA plants. The SNOX-treated stack gas is invisible even against a blue sky, as seen in Fig 4. Operation and maintenance On the average, the SNOX plant in Gela has operated with around 99% availability, and no decrease in performance from the first start-up in September 1999 has been observed when measured conversions and pressure drops are corrected for changes in flow rate. The results of the performance test run in February 2000 are summarised in Table 1.

11 SNOX flue gas treatment for boilers burning high-sulphur fuels 10 / 18 Inlet flue gas flow, Nm 3 /h 971,000 SO 2 inlet, ppm (vol.) 2885 NOx inlet, ppm (vol.) 337 SOx removal efficiency, % 96.5 NOx removal efficiency, % 90.5 % 1) Acid mist in stack gas (prior to air dilution), vol.ppm < 3 2) (as SO 3 ) Product sulphuric acid concentration, wt% 95 Ammonia consumption, kg/h 238 Support fuel consumption (natural gas), kg/h 328 Total power consumption (blowers, pumps, etc), MW 10.4 Flue gas pressure drop across the SNOX plant, mbar 45 Stack gas opacity, % < 5 (stack gas is invisible) 1) NOx removal was later increased to 93-95% after adjustments. 2) More accurate measurements later showed 1-2 ppm SO 3. Table 1. Results of performance test run at the Gela SNOX plant The quality of the sulphuric acid product is measured regularly. The acid concentration is generally between 94 and 95 wt%, and the level of impurities is low. Table 2 gives a survey. Fe 2 Hg < Ni V Cr < 0.03 As 0.09 SO 2 < 10 HCl < 10 Table 2. Impurities in the sulphuric acid product from the Gela SNOX plant (ppm by wt) The SNOX plant has been shut down for about a week every year for inspection and maintenance. The catalysts have not been touched, apart from topping up the panels with a small volume of catalyst to compensate for catalyst settling in the panels. In the summer of 2006, half of the catalyst volume was replaced with new catalyst, not because its performance was bad, but in order to secure several more years of troublefree operation. The plant was always found to be in excellent shape and the catalysts with no signs of dust accumulation. No deformation and corrosion have been found in the reactors and the connecting ducts. The expansion joints were all in good shape.

12 SNOX flue gas treatment for boilers burning high-sulphur fuels 11 / 18 Only a few glass tubes in the WSA condenser broke during initial operation. A small number of broken glass tubes are of no consequence for the performance of the plant because the WSA condenser operates with higher pressure on the air side than on the flue gas side. Only a few signs of corrosion have been found and repaired, and later inspections have shown that the repairs are in good condition. The internal stack brick lining has remained dry and in perfect shape, confirming that essentially all acid droplets are removed before the flue gas enters the stack. A weak spot in the design is the rotating gas-gas heat exchanger (No. 7 in Fig. 3). Its sealing air system is often out of order, and then unconverted SO 2 gas is leaking into the converted gas, meaning a significant drop in overall SO 2 conversion efficiency. However, despite this internal leakage, the emission limits are still observed. It is planned to make a revamp of the sealing air system to make sure that the sulphur capture is always high. Newer SNOX plants are designed with leakage-free heat exchangers. Fig. 4.The SNOX TM plant treating the flue gas from 3 boilers burning high-sulphur petroleum coke at the AgipPetroli refinery in Gela, Sicily, Italy. The stack to the left emits 1,000,000 Nm 3 /h of SNOX TM treated flue gas. The picture was taken before the flue gas from the forth boiler was connected to the SNOX TM plant. This flue gas was emitted through the stack to the right. The difference in stack plumes is apparent.

13 SNOX flue gas treatment for boilers burning high-sulphur fuels 12 / 18 New SNOX projects A SNOX plant treating 820,000 Nm 3 /h flue gas from combustion of heavy oil residue in the power plant of the OMV refinery in Schwechat, Austria started up in the forth quarter of The OMV refinery burns high-sulphur visbreaker residue in its steam and power production. The SNOX plant replaced a Wellman-Lord FGD plant that was installed more than 20 years ago. Based on the good experience with the dust removal in the ESP's in the Gela SNOX plant, the OMV SNOX plant is designed without possibility of dedusting the catalyst during operation. This gives a simpler and more compact SNOX plant. The OMV SNOX plant is designed with by-pass free gas-gas heat exchanger to achieve 98% SOx removal. Two SNOX units for the RNEST refinery project of Petrobras in Brazil are in the design phase. These units are going to clean flue gas from combustion of petcoke and heavy residual oil. At the same time they shall also treat H 2 S waste gases, sour water stripper gas and Claus plant tail gases. Start-up is scheduled for Economy of the SNOX technology The economy of producing own power and steam in a refinery by burning the high sulphur residues and cleaning the flue gases in a SNOX plant of course depends on a long array of parameters. In order to illustrate the economy and its dependence of the various factors, four different scenarios have been established: 1. A refinery that imports all of its power requirements and sells the high sulphur residues. 2. A refinery that sells the high sulphur residues and imports natural gas for power generation. 3. A refinery that produces all of its power requirements by burning high sulphur residues and cleans the flue gases in a SNOX plant. 4. A refinery that produces all of its power requirements by burning high sulphur residues and cleans the flue gases in a limestone scrubber. Some of the factors influencing the economy are mentioned and discussed below.

14 SNOX flue gas treatment for boilers burning high-sulphur fuels 13 / 18 Does the amount of residues fit with the power consumption? Every refinery must consider how much high sulphur residue can be made available for power and steam production, and whether this fits with the power and steam requirements. If too much high sulphur residue is available, there may be the opportunity of producing excess power or steam for export to neighbouring industries or the local community. If too little is available, it may still be a good idea to produce what power can be produced and import the balance. Sulphur content of heavy residue Usually the sulphur content of petcoke and residual oil is high. The content of sulphur has little influence on the economy of producing power, but the choice of FGD technology depends on the sulphur content of the fuel. If it is higher than 3-4 per cent, the SNOX technology is very competitive against traditional limestone-based FGD units. The installation of a flue gas purification plant that can economically handle high SO 2 flue gas gives the refinery certain flexibility to import high sulphur crude at a lower price. Prices of fuel and power The prices of imported power and alternative fuels, e.g. natural gas that can be used for power generation are of course very important. However, they vary tremendously with time and location, so no general rules can be presented, except that the prices of power and clean fuels have a general tendency to increase. The value of high sulphur residues also varies much. Some residue may be sold to clients who can make use of it, e.g. by mixing it into other fuels used for utility power generation. If the utility power plants are already equipped with FGD, they may be able to accept a certain increased sulphur load. A little residue may be blended into other commercial fuel products without jeopardising the specification with regard to sulphur content. However, the general tendency is that the value of high sulphur residues declines relative to sulphur-lean fuels. Selling price of sulphuric acid The price a refinery can obtain for sulphuric acid will depend on the distance to the market and cost of transportation. Although some sulphuric acid is shipped over great distances, most is consumed close to where it is produced. Many refineries are situated at the sea, and then transportation of acid to the market can take place by ship. The acid from a SNOX plant easily fulfils the requirements of the vast majority of acid consumers as to purity and concentration. Fertiliser production consumes most of the sulphuric acid.

15 SNOX flue gas treatment for boilers burning high-sulphur fuels 14 / 18 Cost of limestone and deposition cost of gypsum When SNOX is compared to limestone FGD technologies, the cost of limestone and the cost of deposition of gypsum have to be considered. Also these costs differ from one region to another. Gypsum produced in a FGD plant may have a small positive value, if it is clean enough to be used as construction material. Gypsum produced in a CFB boiler burning petcoke is dirty and contains a lot of unused limestone, so it will definitely have to be deposited. The costs of deposition vary depending on local regulations and infrastructure. Depreciation time and interest rate Investments in infrastructure like a power plant necessarily must have a long depreciation time. It is designed to last for many years, and it does not easily become obsolete. As an example, the boilers in the Gela power plant described above have been used for about 50 years, and nothing hints that they should be taken out of operation soon. Naturally, interest rates are different from one case to another and furthermore change over time. Calculation Topsøe has established a calculator that can calculate the economy of the four scenarios described above. The calculator can be accessed from the SNOX TM website. The calculator is based on a number of assumptions. If you want a calculation that takes more factors into consideration, we shall be pleased to perform a dedicated calculation based on your specific conditions. Example In an example, we have set the following criteria: Residue Type Heavy residual oil Heating value, MJ/kg 39 Sulphur content, wt% 5 Amount, tons/h 75 Sales value, USD/ton 70 Power consumption, MW 300 Price of electric power, USD/MWh 60 Price of natural gas, USD/ton 200 Sales value of sulphuric acid, USD/ton 50 Limestone cost, USD/ton 20 Deposition cost of gypsum, USD/ton 20 Depreciation time, years 12 Interest rate, % p.a. 4

16 SNOX flue gas treatment for boilers burning high-sulphur fuels 15 / 18 The resulting total costs have been calculated and are shown in Table 3. Million USD per year Scenario 1 A refinery that sells the high sulphur residues and imports all of its power requirements Scenario 2 A refinery that sells the high sulphur residues and imports natural gas for power generation Scenario 3 A refinery that produces all of its power requirements by burning high sulphur residues and cleans the flue gases in a SNOX plant Scenario 4 A refinery that produces all of its power requirements by burning high sulphur residues and cleans the flue gases in a limestone scrubber Net operating costs (costs minus income) Table 3. Comparison of operating and capital costs in million USD per year Capital costs Net operating costs + capital costs Manpower for operation and costs of maintenance and catalyst replacements have been considered. It is seen that with the chosen set of criteria, the scenario No. 3 with the SNOX FGD is the overall most attractive. Burning petcoke and cleaning the flue gas The main attraction of the SNOX process is that it makes it possible to burn cheap high sulphur petcoke and heavy residue for generation of steam and power and clean the flue gases without formation of waste products and with no more emissions and operating problems than when burning any other fuel. In practice petcoke cannot be used in ordinary powdered coal (PC) boilers equipped with FGD scrubbers unless diluted with normal coal in the ratio petcoke/coal = 1:4 based on heating value. Use of up to 100% high sulphur petcoke as fuel is only possible either in circulating fluid bed (CFB) boilers with injection of large excess of limestone in the combustion zone, or in downshot PC boilers. Hitherto CFB boilers have been the prevailing technology for combustion of petcoke for power production in spite of its disadvantages: Very high consumption of limestone and production of a correspondingly large amount of waste contaminated with all the vanadium and nickel from the petcoke, and the necessity of supplementary desulphurisation of the flue gas to achieve 98% SO 2 removal. To put it in perspective:

17 SNOX flue gas treatment for boilers burning high-sulphur fuels 16 / 18 Burning 1 ton of petcoke with 6% sulphur requires injection of kg limestone (CaCO 3 ) and produces kg heavy metal contaminated solid waste. Use of SCR is necessary in order to achieve a low NOx concentration in the stack gas. Downshot boilers are excellent for combustion of petcoke. However, the use of wet scrubbing technologies for treatment of the flue gas from combustion of high sulphur petcoke in PC boilers is hindered by serious problems (caused by the high content of SO 3 and the vanadium rich fly ash) with corrosion, plugging and mist formation. Furthermore, usual high dust SCR NOx reduction is hampered due to rapid fouling of the DeNOx catalyst by the high vanadium content in the fly ash. The high consumption of limestone and other costs of achieving 98% SOx removal is also a significant burden when scrubbing flue gas with high SO 2 concentration. None of these problems are encountered with SNOX flue gas treatment which works in excellent synergy with petcoke firing in downshot boilers. The climate issue Natural gas, coal and refinery residues are all fossil fuels that inevitably generate CO 2 when they are burned to produce heat and power. In order to limit the generation of CO 2 per useful unit of heat and power, it is important to burn the fuels as efficiently as possible. Particularly utilisation of low-temperature energy is difficult when burning high-sulphur fuels, since the generated SO 3 will react with water vapour and condense as strongly corrosive sulphuric acid when cooled below a certain temperature. Most installations where high-sulphur fuels are burned either emit the flue gases to the atmosphere at a rather high temperature to avoid the sulphuric acid problem or quench-cool the flue gases from a high temperature in FGD equipment. Either way leads to poor energy economy and hence high specific CO 2 emission. The SNOX technology, on the contrary, is designed for the purpose of cooling gases with high SO 3 content in a controlled way so as to avoid problems with sulphuric acid corrosion. When SNOX is applied as FGD technology, the low-temperature energy of the flue gas can be utilised to a much higher extent that when applying e.g. limestone scrubbing. Figure 6 gives a comparison of a limestone FGD and SNOX.

18 SNOX flue gas treatment for boilers burning high-sulphur fuels 17 / 18 Boiler with limestone FGD Boiler with SNOX FGD Fig. 6. Comparison of Limestone FGD and SNOX TM It is seen from the figure that in the limestone case, the heat content of the flue gas below 190 C is wasted. In the SNOX case, all the he at content of the flue gas down to 100 C is returned to the boiler. On top of this dif ference comes the heat generated by the formation of the sulphuric acid product. For a boiler burning heavy residue (or petcoke) with 5% sulphur and with a flue gas flow of 1,000,000 Nm 3 /h, the difference in heat utilisation of the flue gas is around 40 MW thermal energy. This means that the boiler with SNOX FGD can produce around 15 MW corresponding to 5% more electric power than the boiler with limestone FGD with the same consumption of fuel. In addition to the fuel savings, SNOX does not generate CO 2 as does a limestone based

19 SNOX flue gas treatment for boilers burning high-sulphur fuels 18 / 18 FGD. For a constant electric output the CO 2 emission will be 7% lower with SNOX than with limestone FGD, corresponding to 19 tons of CO 2 per hour or 160,000 tons of CO 2 per year for a 300 MW electric power boiler. The more sulphur in the fuel, the higher the difference will be. Conclusions After more than 10 years of operation, the SNOX plant treating the flue gas from four petcoke fired downshot boilers at the AgipPetroli refinery in Gela has demonstrated high SOx and NOx removal from the flue gas from combustion of high sulphur petcoke. There are no significant corrosion and fouling problems and there has been no decrease in the performance of the SNOX plant during the 10 years of operation. The treated stack gas is invisible and essentially free of SO 3 and heavy metals. SO 2 and SO 3 are recovered as commercial grade 95% H 2 SO 4 of high purity. The heavy metals (vanadium and nickel) are separated as a dry concentrate of oxides. The availability of the SNOX plant has been around 99% since the start. The economy of SNOX is clearly superior to that of competing FGD technologies, when the content of sulphur in the fuel is high, i.e. more than say 3 4 wt%. Thus, it can be said that SNOX-equipped boilers are constitute technically and environmentally superior solution for power plants burning high sulphur petcoke and residual oil. Formation of waste products is avoided, and no more emissions occur than with burning ordinary coal in usual boilers equipped with FGD. The use of SNOX saves the atmosphere from large amounts of CO 2.