SOx Sorbent Behaviour in Oxyfuel Applications Luke Morris, David - - PowerPoint PPT Presentation

SOx Sorbent Behaviour in Oxyfuel Applications

Luke Morris, David Large & Colin Snape EngD Centre Efficient Fossil Energy Technologies University of Nottingham In association with Doosan Power Systems Ltd

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Background » Project Overview » Test facilities » Mineral Analysis » Air-fired vs. Oxyfuel Fired » Sorbent Behaviour » Concluding Remarks

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» SO2 expressed in mg/MJ is lower under oxyfuel firing than air firing and has been attributed to;

˃ Increased retention of sulphur in the fly ash; ˃ Increased conversion of SO2 to SO3.

» SO3 concentrations are reported to be 2 – 4 times higher during oxyfuel combustion. » The H2SO4 dew point temperature will be higher raising concerns regarding low temperature corrosion.

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Trials on DPS’s emission test reduction facility focussing

• n;

˃ Sulphur balance across the system; ˃ The formation of SO3 under oxyfuel conditions; ˃ The SO3 reduction potential by injection of dry pulverised sorbents.

» Analysis of fly ashes using SEM-EDX and mineral liberation analysis software as developed by JKTech. » Mineral analysis focuses on;

˃ Fly ash mineral variations between air & oxyfuel firing conditions; ˃ Variations in the major sulphur bearing species. ˃ Sorbent particle behaviours;

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Water cooled refractory lined furnace » 5m long and 0.5m in diameter » Down fired with scaled down Mark 3 Low NOx burner » Flue-gas recycled after ESP (wet) » CO2/O2 used for primary stream (coal transport) » Flue gas duct temperature controlled by air cooler

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Air Firing & Oxyfuel Firing; » Medium Sulphur Bituminous coal; » Two In furnace sorbents;

˃ Limestone & Dolomite.

» Two Post combustion sorbents;

˃ Calcium Hydroxide & Trona.

» Baseline measurements without sorbent injection;

˃ To establish SO2 & SO3 concentrations.

» In furnace & in duct sorbent injection;

˃ To establish SOx reduction & optimise sorbent feed rate.

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» SO2, O2, CO, CO2 and H2O measurements taken at strategic points. » SO3 measured using the controlled condensation method prior to the ESP inlet. » Solid sampling at the furnace exit and prior to the ESP inlet.

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» SO2 concentrations increased by a factor of 2.3, SO3 concentrations by a factor of ~ 1.4 under oxyfuel conditions. » On a mg/MJ the SO2 concentration decreased by a factor of 1.2 in agreement with past research. » The In-furnace sorbents were more effective at removing SO2 while the post furnace sorbents showed greater SO3 capture. » A 5 % S-balance discrepancy during air firing was found. This was approximately four times higher for the oxyfuel combustion trials.

˃ The ‘missing sulphur’ appears to be lost in the furnace. ˃ Results also indicate that the discrepancy in the sulphur balance increases as the sulphur capture potential of the system increases.

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» 22 samples analysed for their major mineral species. » 4 ashes without sorbent addition for variations in the air/oxyfuel ashes. » Samples extracted prior to the ESP inlet.

Firing Sorbent Actual Molar Ratio In-Furnace SO2 In-Furnace Temperature (Input S:X) ppm

• C

Air

• 840

1080 Air

• 913

1110 OF

• 2630

1230 OF

• 2636

1230 Air Dolomite 0.48 1026 1110 Air 0.31 531 1070 OF 0.36 2349 1130 OF 0.42 1834 1190 Air Limestone 0.43 1933 990 Air 0.43 1837 990 Air 0.20 919 990 OF 0.20 1933 1200 OF 0.45 2017 1200 OF 0.37 2017 1200 OF 0.37 2422 1200 AF Calcium hydroxide 0.34 1027

• AF

0.41 1476

• OF

0.37 2627

• OF

0.43 2476

• Air

Trona 0.67 1232

• OF

0.24 2487

• OF

0.30 2355

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» The mineral liberation analysis utilises signals from backscattered electron and EDAX x-ray detectors on Quanta 600 SEM-EDX. » Mineral x-rays are pattern matched against a database of 200 + mineral x-rays built up around a suite of pf ash samples from various stations and coals. » > 50 000 discrete particles analysed per sample to ensure statistically realistic. » For phase classification, the pattern match with the spectra must have a reliability factor of > 0.1.

˃ This is good enough to identify the major elemental peaks; Al, Si, Ca, Fe, S, Mg, Na & K.

Mineral Species Criteria Number of species Quartz Al:Si ratio 1:10 5 Si rich Al:Si ratio 1:5-10 5 Rutile

• 4

Iron oxide

• 7

Alumino-silicates (low fluxes) < 5 % wt. fluxes 35 Alumino-silicates (mid fluxes) 5 - 10 wt. % fluxes 38 Alumino-silicates (high fluxes) 10 - 20 wt. % fluxes 10 Ca alumino-silicates Ca > 10 wt. %, fluxes < 10 wt. % 20 Fe alumino-silicates Fe > 10 wt. %, fluxes < 10 wt. % 18 Ca/Fe alumino-silicates Ca, Fe > 10 wt. % 2 Ca/Mg oxides SO3/(CaO + MgO) < 0.1 17 Ca/Mg sulphates SO3/(CaO + MgO) > 0.1 14 Ca/Mg silicates

• 16

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Species grouped based on their Ca, Fe, Mg, K & Na contents. » Covers the major species present in the fly ashes. » Provides a broad compositional range from which the alumino- silicate species can be accurately classified. » Calcium sulphate particles differentiated by their SO3:(CaO+MgO) ratio.

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

10 20 30 40 50 60 70 80 90 100 200 300 400 500 Channel AF Limestone 0964 AF Limestone 0927 OF Limestone 0939 10 20 30 40 50 60 70 80 90 100 200 300 400 500 Channel AF Limestone 0964 AF Limestone 0927 OF Limestone 0939 10 20 30 40 50 60 70 80 90 100 200 300 400 500 Channel AF Limestone 0964 AF Limestone 0927 OF Limestone 0939 10 20 30 40 50 60 70 80 90 100 200 300 400 500 Channel AF Limestone 0964 AF Limestone 0927 OF Limestone 0939

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Acceptable agreement for the major elements, Si, Al, Fe, Ca & S. » Poorer matches for the minor alkali elements Na, K & Mg. » Appears to

• verestimate the iron
• xide content.

10 20 30 40 50 60 70 10 20 30 40 50 60 70 MLA results, wt. % SiO2 Al2O3 Fe2O3 SO3 CaO 1 2 3 4 5 6 1 2 3 4 5 6

ICP results, wt. %

MLA results, wt. % MgO Na2O K2O P2O5

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» The sulphur contents, expressed as SO3, were 1.16 and 2.03 % respectively showing an increase in the uptake of SOx in the fly ash from air to oxyfuel fired conditions. » The weight percentages of calcium and magnesium are comparable between the two firing conditions. » The oxyfuel fly ashes generally Contained slightly less iron.

Species

• Wt. %

Air Oxyfuel SiO2 54.96 55.31 Al2O3 18.94 21.26 Fe2O3 8.97 5.18 CaO 3.09 3.24 MgO 0.54 0.58 TiO2 0.49 0.36 Na2O 0.47 0.50 K2O 2.25 2.54 P2O5 0.18 0.23 SO3 1.16 2.03

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» The particle size distribution showed only minor variations between the air and oxyfuel fired conditions.

˃ The air fired and oxy-fuel fired ashes have a D90 of 7.8 µm and 10.1 µm respectively.

» The oxyfuel ashes generally contained a smaller concentration of quartz and iron oxide minerals but increased concentrations of iron alumino-silicates.

˃ This implies a higher degree of coalescence of the parent ash particle.

Mineral (wt. %) Air Fired Ash Oxyfuel Fired Ash Quartz 9.66 7.15 Si rich 7.87 5.59 Rutile 0.14 0.14 Iron Oxide 5.59 1.62 Fe Alumino-silicates 3.83 5.62 Ca/Fe Alumino-silicate 0.34 0.15 Alumino-silicates (low fluxes) 42.89 44.27 Alumino-silicates (mid fluxes) 24.94 29.95 Alumino-silicates (high fluxes) 0.38 0.58 Ca Alumino-silicates 1.81 1.71 Calcium oxides 1.04 0.11 Ca/Mg sulphates (1) 0.99 2.57 Ca/Mg silicates 0.51 0.54

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» ~ 75 – 80 % of the calcium in the ashes is incorporated into the alumino-silicates with little variation in firing conditions. » ~ 60 % of the sulphur is incorporated in the alumino-silicates under both firing conditions. » The additional sulphur captured in the oxyfuel ash is in the form of calcium sulphate.

˃ A slight increase in the sulphur capture of the alumino-silicates but not substantial.

SO3/(CaO+MgO) Ratio % of calcium sulphate particles Air Fired Ash Oxyfuel Fired Ash 0-0.1 43.42 7.93 0.2-0.3 19.40 14.97 0.4-0.5 19.17 63.92 0.6-0.7 16.63 12.13 0.8-0.9 0.46 1.05 >1 0.92 0.00

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Sulphur in the ashes with dolomite and limestone additions have increased from around 1.1-2.0 % in the ash with no additive to around 6-8% with the additives.

˃ Ca/Mg sulphates increased from 1.0 & 2.6 % for the tests with no additive to levels

• f 10.0 and 12.3 % for the air and oxyfuel firing tests.

» Some interaction between the injected sorbent particles and the alumino-silicates but comparable between firing conditions;

˃ Calcium alumino-silicates increased from 1.8 & 1.7 wt. % for air & oxyfuel firing tests without additives to 5.1 & 5.2 wt. % with limestone addition. ˃ Dolomite showed a similar trend with higher Ca alumino-silicate contents of 3.2 & 3.7 wt. % for air and oxyfuel firing respectively.

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» Limestone showed a greater capture potential than dolomite. » For both limestone and dolomite particles the sulphur capture was generally higher under oxyfuel fired conditions.

˃ More pronounced for dolomite sorbent particles.

» Calcium hydroxide showed little variation between firing conditions.

10 20 30 40 50 60 70 80 90 100 0.2 0.4 0.6 0.8 1 1.2 Particle Percentage (%) SO3:(CaO+MgO) Ratio Dolomite AF Dolomite OF Limestone AF Limestone OF

Engineering Doctorate ( EngD) Centre Efficient Fossil Energy Technologies

» The four sorbents all offer potential as SO3 control sorbents under oxyfuel fired conditions. » The post furnace sorbents have been shown to be most effective for SO3 control. » The oxyfuel ashes generally showed;

˃ A reduced iron oxide content. ˃ A greater sulphur content.

» The additional sulphur in the oxyfuel ashes was present as calcium sulphate not incorporated into the silicate material. » For both limestone and dolomite particles the sulphur capture of the particles was generally higher under oxyfuel fired conditions.