Combustion and boiler performance. Boiler performance at 50% biomass thermal input During the Advanced Green phase, wood pellets were fired on the boiler rows 2 and 3. The lower row 1 was used for coal. The existing coal burners and part of the pulverized coal transport system were reused for wood. The wood combustion was problematic in the Advanced Green configuration. Owing to the large size of the biggest particles, the worse particle size distribution and the irregular shape of wood particles compared to PC particles, the fuel feeding resulted rather difficult, especially where the pipes bend. In order to avoid pulsing and sedimentation in the fuel lines and to overcome the pressure losses, the feeding velocity had to be kept very high (30 m/s or even faster). This resulted in a too high velocity at the burner outlet, causing the flame to be very unstable and not attached to the burner. The pulverized wood was even “projected” in the boiler, which caused flame impingement to the opposite and side walls, a bad combustion with a large amount of UBC in the bottom and fly ash, post-combustion in the convective part and increased slagging and fouling in the combustion chamber as well as at the convective part inlet. The burnout was not complete at 43 meters and it is known that secondary combustion of coarse particles on heat transfer surfaces can create local reducing zones that favour slagging. Boiler performance at 100% biomass input After changing the burners and modifying the boiler for the Xxx Xxxxx configuration as described in chapter 3, the combustion resulted much more stable, with much less slagging, UBC in the ashes, no flame impingement. Slagging and fouling As main conclusion of the deposit analyses on the probes, it came out that there was a spread of molten salts which causes a sulphate layer on the metal surface, with cracks and loss of the oxide layer as a consequence. The particles were large and sintered, and problematic to clean. The recommendations were to maintain a stable and constant firing system, to avoid fluctuating temperatures, and to be sure that the protective oxide layer on the tubes was good before co-combustion of biomass. Another finding was the confirmation of the post-combustion of particles in the convective part. The burn-out was not complete at 43 meters and it is known that secondary combustion of coarse particles on heat transfer surfaces can create local reducing slagging. zones that favour The ash fusion temperature, more specifically the initial ...
Combustion and boiler performance. The boiler efficiency resulted not influenced by a biomass thermal share as low as 10%. Mean values of the flue gas temperatures at various heights along the boiler, as well as after the ESPs, were measured. No significant changes were observed during co-firing. The flue gas volume showed only one small decrease of 2% during co-firing. With regard to the composition, it resulted that: - The oxygen content was almost constant and within the range 5-7% during the high- demand hours, rising to more than 11% during the night. This increase can be correlated to the corresponding decrease in load and to changes in the lignite quality. - The CO emissions, which are an indication of the combustion efficiency, showed no significant changes during for co-firing. - The SO2 emissions fluctuated greatly for co-firing, while coal firing presented a more clear figure, with sporadic peaks upwards. In all cases, however, SO2 emissions remained much lower than the limit of 400 mg/Nm3. The fluctuations can be attributed to changes of the CaO content in the ash, which can capture SO2 and form CaSO4. Furthermore, decreases in dust emissions concurring with SO2 increases and vice versa can be observed. In fact, SO2 influences the electrical conductivity of the flue gas, so that higher contents improve the ESP performances. - The influence of the co-firing on the NOx-emission was (490±10) mg/m³ without co- firing and (458±32) mg/m³ with co-firing. In this context the origin of the biofuel may be important: agricultural waste produced under application of fertilisers may influence the production of NOx during the thermal utilisation. Furthermore, NOx is a function of the excess air, which increases in low-load operation caused by the cooling air for the burners not in operation. - Dust emissions exhibited large variations during co-firing, as a result of lignite quality changes and ESP operation. A large decrease of dust emissions was observed upon firing of lignite with lower calcium content; this was accompanied by an increase of SO2 emissions, as discussed above. Concluding, no negative effects of biomass co-firing on load, emissions or other operational parameters were observed. Unburned material content in the fly ash depending on the grindability also appears to remain the same, although a more intensive sampling should clarify the issue. The only noticeable difference was an increase of UBC (and LOI) in the bottom ash during cardoon co-firing. The UBC resulted nearly doubl...
Combustion and boiler performance. As part of the activities planned within the DEBCO project, a series of 11 tests was carried out at RDF co-firing shares of 0-5% on a thermal basis (8.2% on a mass basis) and at the average unit load in the range 301-307 MWel. It clearly resulted that the impact of the RDF co-firing on the boiler performance was negligible at the tested ratios. There are no clear trends of the oxygen concentration in the flue gas nor of the flue gas flow rate with the RDF co-firing level. The measured flue gas temperatures at the ECO inlet, at the SCR inlet and outlet, at the air heater inlet and outlet as well as at the chimney were within very close bands and there are no clear trends in the flue gas temperatures with the RDF co-firing levels. This indicates that, with regards to the boiler convective section, there were no major differences in the fouling factors nor in the heat absorption resulting at the tested RDF co-firing ratios. The most reliable indicator of the heat absorption performance of the furnace is provided by the measured superheater spray flow rates, and it is clear from the measured data that there is no significant trend. This indicates that there is no evidence to suggest that the co-firing of the RDF material had any significant impact on the furnace performance. No clear variation of the total losses either appeared. They were estimated including the dry gas loss, the moisture loss, the humidity loss and the R&U losses, and depend principally on the moisture content of the fuel, the flue gas oxygen concentration and the flue gas temperature at the air heater outlet. On the contrary, the UBC loss could not be included since no relevant data is available for the trial periods. The NOx, O2 and CO concentrations in the flue gas were measured downstream the ECO with a 5-point grid, both on the left and the right duct during a dedicated measurement campaign. The UBC was determined in the chemistry laboratory in the fly ash samples collected in both ducts downstream the ECO. From a general point of view, the performed analyses indicate no significant differences between the emission values in the only coal and in the RDF/coal co-firing configurations with the same setting of the burner corner ports: - The NOx emission is within the normal range of fluctuation experienced without co- firing. - The difference of the steam consumption for the soot blower is negligible. - The CO values resulted always under the lawful limit. - The UBC in the ESP ash is withi...
Combustion and boiler performance. During the testing activities, the risk of ash deposition and corrosion was simply minimised by producing fresh steam at relatively low super-heating temperature. No problem in reaching the nominal steam temperatures was observed. The flue gas temperature at the boiler exit was in the design range (150 °C). The NOx concentration resulted higher than with only coal and some combustion optimisation is needed. Slagging and fouling was not noticed thanks to the low furnace flue gas temperatures and to the careful selection of biomass with high ash initial deformation temperature. By now, the biomass co-firing has become a common technique at this plant. Co-firing tests resulted successful also at Mátra PP, where a mixture of wood-chips and saw- dust was burned with lignite in a heat ratio of 10% in boilers designed for PC with the combination of tangential and grate combustion.
