3.3 Conducted experiments
3.3.2 Experiments investigating SO x retention in ash and deposits
3.3.2.1 Experimental parameters
Combustion parameters for the experiments studying SOx retention in ash and deposits at the 500 kW KSVA and the 30 MW “Schwarze Pumpe” plants are summarized in table 3.6. In the oxy-fuel experiments, oxidant O2concentrations were between 25 and 28.9·10−2 mm33, wet. In most oxy-fuel experiments, oxygen was mixed with the recirculated flue gas before injecting it via the burner. Only the oxy-fuel experiments with lignite L2 were conducted injecting pure oxygen through dedicated nozzles of the burner. In air fired experiments, the exit O2 concentration was approx. 3.2 to 3.7·10−2 mm33, dry, while in the oxy-fuel tests at KSVA it was 4.3 to 4.9·10−2 mm33, dry, and in the 30 MW oxy-fuel experiment it was 6.9·10−2 mm33, dry. The experiments represent typical air and oxy-fuel combustion conditions, and hence, the results are suitable for a comparative evaluation. The higher oxygen concentrationsy¯O2,oxid,wet in the oxidant gas in oxy-fuel experiments correspond to increased flue gas residence times in
Table 3.6: Combustion parameters for experiments studying SOx retention in ash and deposits at the500 kW KSVA and the30 MW“Schwarze Pumpe” pilot plant including mode of O2 addition, thermal powerP¯th, fuel feedM¯˙RC, volumetric flow of clean CO2 for sensor purging and fuel dosingV¯˙CO2, volumetric fraction of air leakagey¯leak,dry, and overall O2 concentration in the oxidanty¯O2,oxid,wet and in the flue gas exiting the furnace y¯O2,out,dry.
P¯th M¯˙RC V¯˙CO2 y¯leak,dry y¯O2,oxid,wet y¯O2,out,dry fuel mode O2add. MW ht mh3(ST P) 10−2 mm33
L1 air - 0.31 0.0606 - - 20.9 3.2
oxy premixed 0.31 0.0611 33 6.1 28.9 4.6
L2 air - 0.29 0.0490 - - 20.9 3.5
oxy direct 0.29 0.0495 42 7.1 25.0 4.9
C4 air - 0.33 0.0385 - - 20.9 3.7
oxy premixed 0.33 0.0385 38 6.1 28.0 4.3
L3 oxy premixed 22.6 3.8500 n.a. n.a. 28.5 6.9
yleak,dryfor KSVA tests estimated on basis of the measured H2O and/or CO2concentrations in the flue gas n.a. = analysis not available
the furnace26. In table 3.6 also information on the volumetric flow of clean CO2 for sensor purging and fuel dosingV¯˙CO2, and the volumetric fraction of air leakagey¯leak,dry is given, which is required for the calculation of sulfur retention in KSVA oxy-fuel experiments. For the experiments conducted at the 30 MW “Schwarze Pumpe” pilot plant, no information onV˙CO2 andyleak,dry is available and those values could only be estimated.
3.3.2.2 Details on ash and deposit sampling
Ash and deposit samples and associated calculations: In experiments investigating SOx retention in ash and deposits at the KSVA and the “Schwarze Pumpe” plants, process ashes were sampled from the facilities’ air/gas preheater (GH), bottom ash (BA), and different ESP precipitation fields (E1, E2, E3). In addition, entrained ash and deposits were sampled at various locations in the furnaces of both facilities. Table 3.7 summarizes details on all samples of entrained ash (A) and cooled (C) and uncooled (UC) deposits included in this thesis. The sampling locations for the lignite fired experiments have been selected to represent flue gas
Table 3.7: List of and details on entrained ash (A) and cooled (C),and uncooled (UC) deposit samples from the500 kWKSVA and the30 MW“Schwarze Pumpe” pilot plant (i.e.
fuels, combustion modes, sampling levels, distances from burner, andϑFG).
furn. level bur. dist.
plant fuel mode m ϑFG samples
KSVA L1
air
Lev11 1.99 1110◦C U
Lev15 2.67 1020◦C U
Lev26 5.67 750◦C U
oxy
Lev11 1.99 1125◦C U
Lev15 2.67 1035◦C U
Lev26 5.67 750◦C U
KSVA L2
air Lev11 1.99 1095◦C U, C, A
Lev26 5.67 750◦C U, C, A
oxy Lev11 1.99 1055◦C U, C, A
Lev26 5.67 730◦C U, C, A
KSVA C4 air L20 3.63 915◦C U, C
oxy 3.63 895◦C U
KSVA C4, Ca(OH)2 furn. inj.
air L20 3.63 940◦C U, C
oxy 3.63 925◦C U
OxyPP L3 oxy Lev4 ≈5.5 1100◦C U, A
Lev8 ≈10 750◦C U, C, A
26On basis of the volume reduction, residence times in air firing can be estimated to be approx. 27 % (L1), 16 % (L2), 25 % (C4), and 27 % (L3) below those in oxy-fuel firing
temperatures in radiative (approx. 1100◦C) and convective sections (approx. 750◦C) of power boilers. Sampling locations for coal C4 have been selected to be about 1.5 m downstream the DSI sorbent injection location to allow for a comparison of the deposit composition with and without sorbent injection. The temperature at this location corresponds also to the convective section of a power boiler. Temperature controlled deposit samples, so called “cooled deposits”, were collected using a probe whose surface was controlled to temperaturesϑProbe between 550◦C and 650◦C to simulate superheater surface temperatures. Entrained ash and uncooled deposits have been sampled at KSVA and the oxy-fuel pilot plant “Schwarze Pumpe” subsequently at the same measurement ports, while cooled deposits were collected at neighboring sampling ports at the same furnace levels. The obtained ash and deposit samples have been analyzed for their chemical composition (i.e. main ash-forming oxides) and in addition, selected samples were subjected to electron micro-probe analyses. Based on the chemical composition, the extent of sulfation of the samples can be evaluated. For this purpose, molar Ca/S, Mg/S, Na2/S, and K2/S ratiosαi are calculated, according to equations 3.6, 3.7, 3.8, and 3.9. In these calculations, γA,db refers to the ash content determined for the ash and deposit samples to account for any unburned fuel in the samples.
αCa/S = γA,db
γS,db ·MM,S · xCaO,A
MM,CaO (3.6)
αMд/S = γA,db
γS,db ·MM,S · xMдO,A
MM,MдO (3.7)
αNa2/S = γA,db
γS,db ·MM,S · xNa2O,A
MM,Na2O (3.8)
αK2/S = γA,db
γS,db ·MM,S · xK2O,A
MM,K2O (3.9)
Limitations in respect to ash mass balances at KSVA: During the air and oxy-fuel ex-periments with lignites L1 and L2 and coal C4 at KSVA, process ashes from all ash drains (bottom ash, ash from gas preheater, ESP ash27) were collected and the sample masses were determined, summed up, and compared against the ash input with the fuel. The recovery ratios for the ash were relatively low (42-86 %) and deviations between corresponding air and oxy-fuel tests carried out subsequently within one test campaign were high (e.g. 50 vs. 69 % and 86 vs. 42 %, respectively). It can only be speculated for the reasons, e.g. too low sampling durations and considerable ash accumulation in furnace and flue gas ducts. It is known from the KSVA facility that several hours of experimental operation are required for the ashes in the system to be moved to the respective ash discharge systems. Those ash travel times are
27The mass of ash accumulated in the fabric filter over several hours of operation was non-existent or very low (few hundred grams) in the experiments and was not further considered.
impacted by changed flue gas flow rates and corresponding gas velocities in flue gas ducts in air and oxy-fuel operation. Changed gas velocities may also impact the ash fractionation between bottom ash, gas preheater ash, and the different ESP ashes, which is an uncertainty in the comparative assessment of the process ashes from air and oxy-fuel operation. Due to the low ash recovery ratios and the observed deviations between combustion modes, no experimentally determined process ash production rates etc. are included in this thesis. For the same reasons, the assessment of process ash compositions is focusing on qualitative changes in the character of sampled ashes, omitting a detailed quantitative evaluation. It should be highlighted that all process ashes included in this thesis were carefully sampled at the end of longer periods of stable operation (i.e. between 8 h and 20 h for all experiments except the air fired DSI experiments that had to be stopped after approx. 3.5 h, due to a mill damage). To avoid a possible contamination of the ash samples by carry-over of ash accumulated in the flue gas ducts during previous experiments, the samples were always obtained from the top of each ash container. In general, the qualitative results in respect to sulfur retention in ashes from the experiments at KSVA are in agreement to the findings based on more reliable flue gas concentration measurements.