This paper presents a comprehensive experimental study of the NOX reduction through the staged injection of oxygen in the oxy-fuel combustion of lignite and wooden pellets in a 30kWth bubbling ...fluidized bed experimental facility. A number of experiments was performed examining the impact of relevant operational parameters on the efficiency of NOX reduction. Various secondary to primary oxygen ratios were selected while keeping the overall concentration of oxygen in dry flue gas the same for constant ratio of O2/CO2 concentration in fluidizing gas and for constant fluidized bed temperature. Besides that, the impact of the overall oxygen stoichiometry was studied.
The results showed that oxygen staging can be used successfully to reduce NOX emissions. Within the experiments, the NOX reduction efficiency as high as 50% was achieved (compared to the operation without oxygen staging) for the ratio of secondary and primary oxygen 0.5 in case of lignite and 1 in case of wood combustion. The extensive staged injection of oxygen also caused a significant raise of the freeboard temperature. There, it was measured about 900 °C for the lignite combustion and 880 °C in the fluidized bed, and about 950 °C for the combustion of wood and 880 °C in the fluidized bed. Oxygen staging is a promising path to reduce NOX in the oxy-fuel combustion in bubbling fluidized beds or to provide convenient conditions for the application of non-selective catalytic reduction in the freeboard section of the fluidized bed facility.
•Ready-to-operate flue gas cleaning process with CCS for natural gas CHP.•In-depth analysis of SCR, flue gas dehydration, and CO2 capture processes.•Two-step dehydration using condensation and TSA ...manage high H2O content.•4-step VSA using zeolite 13X with 15 columns can produce 90 % pure CO2 stream.•A foundationforfutureoptimization and techno-economic analysis.
This paper proposes a continuous CO2 capture system for natural gas combined heat and power (CHP) facility and explores the potential of low-temperature vacuum-swing adsorption (VSA) process for post-combustion CO2 capture (PCC) in the context of carbon capture and storage (CCS). Although VSA has strong potential for efficient CO2 capture in district-scale energy systems, previous case studies have largely focused on high-emission coal combustion sources, making VSA application challenging for these conditions. We address this issue by theoretically designing a ready-to-operate flue gas cleaning process with CCS for a medium-sized 4.3 MW natural gas CHP operating in the local industry. Our in-depth analysis examines the most critical parts such as dehydration, involving condenser and temperature-swing adsorption (TSA), and CO2 capture via VSA. The conceptual design and performance of VSA are approached by developing a mathematical model to estimate the technology size and performance. This work shows that utilising a small fraction of heat recovered from CHP-generated flue gas is sufficient to supply the necessary heat for auxiliary units, and that using natural cooling water for dehydration effectively reduces moisture (87 vol%) and energy demand for final dehydration via TSA. Furthermore, it demonstrates that 4-step VSA consisting of 15 columns using zeolite 13X can separate CO2 with a desired purity of 90.4%, meeting the requirements for CO2 storage and transportation onshore, at a 15.6% recovery rate. The cost of achieving high CO2 purity without pre-pressurising the CO2-rich flue gas, while maintaining cycle simplicity, is discussed. Overall, our paper provides a comprehensive approach to retrofitting distract-scale power plants with CO2-lean emissions, presenting a ready-to-operate flue gas cleaning technology based on real operating conditions and technical restrictions, which can contribute towards decarbonisation.
•Full oxy-fuel co-firing of sewage sludge and wooden pellets is successfully achieved.•SO2 self- retention reaches up to 55% in oxy-fuel combustion mode.•Different performance of two limestones with ...different purity is observed.•SO2 capture in oxy-fuel combustion mode is about 20 to 25% more effective.
This work presents results of an experimental investigation of a direct addition of Ca-based sorbent for SO2 capture in a bubbling fluidized bed combustion. The fuel of interest was secondary biomass represented by a dried sewage sludge, which was mixed with primary biomass represented by wooden pellets in a weight ratio of 30:70. The performance of this SO2 capture approach is compared in air and oxy-fuel combustion conditions, since the different gas environment in the bed significantly affects the SO2 capture process. The experiments were carried out in an experimental bubbling fluidized bed combustor with a power load of about 30 kW.
Sewage sludge is typical for its high ash content and has sufficient dimensions and attrition resistance to create the fluidized bed without the support of external bed materials. However, it typically consists of S-containing organic substances, resulting in the presence of SOX in the off-gas. A reference case was initially conducted without any sorbent addition in order to investigate the effect of sulfur self-retention in the fuel ash. The sulfur retention in the ash reached almost 45 % in air- and up to 55 % under oxy-combustion.
In the next phase, the performance of Ca-based sorbents in sulfur capture was evaluated. The sorbents were represented by two types of limestone with diverse CaCO3 content produced at different locations. The experiments were carried out with three Ca/S mole ratios (1.5, 3, and 4.5). The effect of the fluidized bed temperature was also investigated as the most important parameter.
The SO2 capture ratio (used as a performance indicator) was calculated based on emission factors, due to the different specific volumes of flue gas in the air and oxy-fuel modes. The results show in general a significantly higher SO2 capture ratio under oxy-fuel conditions. The differences are more significant in lower Ca/S ratios, where the SO2 capture ratio increased from 46 % under air conditions to 75 % under oxy-fuel for Ca/S = 1.5. In the case of the highest Ca/S ratio examined (Ca/S = 4.5), the increase in SO2 capture was from 90 % to 95 %.
•Experimental investigation of direct SO2 capture in oxyfuel bubbling fluidized bed.•Sulfur self-retention is up to 45% in the fuel ash.•SO2 capture optimum temperature at 880 °C, does not depend on ...sorbent properties.•Direct SO2 capture can achieve requirements for CO2 purity at Ca/S = 5 to 6.•High BET surface of the sorbent is important in non-calcining conditions.
This work presents research results of direct sorbent addition for SO2 capture from a lignite coal combustion in a bubbling fluidized bed (BFB). SO2 capture ratio in air and in full oxyfuel combustion conditions is compared in real environment, using a 30 kW BFB combustor. The first part studies the sulfur self-retention in the fuel ash. It shows significantly more efficient SO2 capture increase from 15% in air conditions to 45% in oxyfuel conditions. The next part concerns evaluation of optimal temperature for the SO2 capture and a sorbent performance is evaluated in the last part. Two varieties of sorbents based on CaCO3 were used, differing significantly in purity, geological age and BET surface area. The optimal fluidized bed temperature for SO2 capture was for air combustion about 840 °C and for oxyfuel combustion about 880 °C. The sorbent performance was studied at three different Ca/S molar ratios (1.5, 3 and 5). The results show the fundamental differences between air and oxyfuel conditions in real combustion environment, and also reveal the differences between the sorbents. In general, significantly higher SO2 capture ratio was reached in oxyfuel conditions. At Ca/S equal to 5, it is possible to reach the SO2 capture as high as 98% in oxyfuel mode. The BET surface of the sorbent is important mainly at lower Ca/S molar ratios. The difference in SO2 capture between the sorbents is below 10% relative at Ca/S molar ratio 3 and it further decreases with increasing Ca/S ratio.
•Bed-to-surface heat transfer in a BFB was investigated in air and oxyfuel modes.•Correlation equations were examined for convection caclulation.•Experimentally found convection is 120–180 W/m2/K in ...air and 180–230 W/m2/K in oxyfuel.•Convection is more affected by the fluidization velocity in the oxyfuel mode.•Packet theory can be applied with experimental constants to calculate the convection.
This work presents an experimental determination of heat transfer coefficient between the dense zone of a bubbling fluidized bed combustor and an immersed heat exchanging surface in oxy-fuel combustion mode, air mode being used for a comparison. The experimental work was carried out using a 30 kW scale autothermal bubbling fluidized bed combustor with real wet flue gas recirculation. A lightweight ceramic aggregate was used as the bed material and wood pellet as the fuel, the testing range of the bed temperature and fluidization velocity was within a typical range of fluidized bed combustors. For heat transfer measurement purposes, a single-loop water-cooled heat exchanger was developed. Convection and radiation were evaluated for both combustion modes. The convection in air mode reaches about 120 to 180 W/m2/K, while in the oxy-fuel mode it reaches about 180 to 230 W/m2/K. Radiation is not particularly affected by the combustion mode, reaching 80 to 120 W/m2/K for both modes.
The correlation equations to determine convection were examined and compared with the experimental data. The general shortcoming of most of the available equations is a disagreement in trend between the predicted and experimentally measured convection correlated with the fluidization velocity. The Packet model, which was identified to predict this trend correctly, was fitted with the experimental data to determine new constants. The modified model is capable of predicting convection with a 2 and a 9 % deviation for air and oxyfuel combustion modes, respectively.
This paper reports on a full-scale investigation of a possible increase of the SO2 capture ratio in current semi-dry flue gas desulphurization (FGD) technology. The FGD unit is used for two 130 t/h ...steam PC boilers burning a blend of lignite and hard coal with dry ash-free sulphur content at 1.83%. The FGD unit has been designed to reach an SO2 emission limit of 1350 mg/Nm3. The aim of the experimental work presented in this paper was to investigate the possibility of reaching SO2 emission targets of 500 and 200 mg/Nm3. The investigation sought to determine the real correlation of SO2 capture ratio with Ca/S and with the difference of dry-bulb temperature and dew point in the absorber ΔtAD. Generally, the SO2 capture correlation with Ca/S has flat characteristics at a capture ratio >90%, which is required to reach the 200 mg/Nm3 target. In this case, lowering the ΔtAD in the absorber has only a weak effect. The 500 mg/Nm3 target requires an SO2 capture ratio of about 80%; in this case lowering the ΔtAD by about 7 °C increases the SO2 capture by 10% points at the same Ca/S ratio.
•Increase of SO2 capture in a semi-dry FGD was investigated.•SO2 emission targets of 500 and 200 mg/Nm3.•Getting closer to the dew point by 7 °C gains about 10% capture increase.•Flat characteristics of Ca/SSO2 capture correlation at high capture ratios.
•Emission factors from small scale biomass combustion were measured.•Different load regimes and wooden and agricultural biofuels were tested.•Definition of testing conditions must be present.•Results ...are consistent in NOx and SO2 emission factors with literature.•CO emission factors significantly differ.
The paper presents results of determination of emission factors for CO, NOx and SO2 from combustion of wooden and various sorts of agricultural biofuels in a commercially available small scale pellet boiler. The emission factors (EF) were determined at three different power loads of the boiler and are presented in mass, LHV and utilizable energy form. The results show significant influence of operating conditions as well as conditions of the boiler itself. Comparison of the results with various literature data shown significant impact of the experimental procedure on CO EFs and turned out that definition of the experimental conditions is essential for relevance of the presented data. EFs for NOx and SO2 were found to be particularly affected by nitrogen and sulphur content in the biomass respectively.
•Hg capture using solid sorbents was tested for FB lignite combustion.•Two alternative sorbents, commercial AC as reference.•SO2, temperature and feeding rate correlation.•The sorbents capture 15–45 ...% of the Hg.
This paper presents an experimental study of two of alternative sorbents for mercury capture from the lignite combustion flue gas. The sorbents are based on calcium hydroxide and aluminosilicate, using different modifications. A commercially available activated carbon (AC) with bromine impregnation was selected as the reference sorbent. This sorbent is a state-of-the-art solution for mercury removal in industrial combustion facilities. However, its application is costly for power plants because of the market price of the AC and because the presence of carbon in the fly ash can complicate its further use, which gives a strong motivation for exploring new alternatives. Experimental investigation was carried out in a 500 kW pilot-scale bubbling fluidized bed combustor using a lignite with naturally high mercury content. A injection system of our own design and construction was used to supply the sorbents to the flue gas downstream of the combustor at a flow rate in the range of 100–500 mg/m3N of the flue gas. Continuous measurement of mercury concentration was used upstream the sorbent injection and downstream the fabric filter, simultaneously. Different sorbent injection rates and flue gas temperatures at the sorbent injection point were examined.
The mercury capture ratio was evaluated considering the Hg balance in the combustor. It was shown that the mercury self-capture by the fly ash in the fluidized bed combustion system varies approximately from 30 to 90 %, depending on the SO2 concentration. The Hg capture ratio for calcium hydroxide was in a range of approximately 15–25 %, while for aluminosilicate it was 25–45 %, determined by the actual conditions of the flue gas and the initial Hg concentration. This is in the range of the reference AC, which typically reaches from 25 to 95%. The increased injection rate of the sorbents was reflected in the Hg capture ratio, but its promotion was observed only in the case of the aluminosilicate sorbent. A correlation of the Hg capture ratio with the flue gas temperature at the injection point was also clearly identified only for the aluminosilicate sorbent, when an increase in the temperature led to increasing in the capture ratio.
•The experimental data were obtained using 30 kWth BFBC simulating real combustors.•The observed correlations were explained using 1-D numerical model.•Significant sensitivity of NOX formation ...towards the excess of oxygen and the fluidized bed temperature was detected.
This paper reports results of experimental and numerical studies of NOx formation in a 30 kWth lab-scale bubbling fluidized bed running in oxy-fuel mode. The numerical model is based on the GRI-mech 3.0 mechanism to compute the kinetics of homogeneous reactions of volatiles and char combustion products and takes into account the flue gas recirculation. The impact of the oxygen excess and of the fluidized bed temperature was examined. Both the numerical simulations and the experiments shown a significant correlation of NOX formation with the excess of oxygen, where a higher oxygen concentration enhances the fuel-bound nitrogen oxidation to NOX. It was also found that there is no correlation of the NOx formation and resulting emissions with the fluidized bed temperature in temperature range typical for bubbling fluidized bed combustors (840–960 °C), but the numerical simulations showed an increased NOX concentration when the temperature raised more (up to 1360 °C). The agreement of experimental and numerical results shows that the numerical model can provide useful insight into the mechanism of NOX formation.
This paper presents an experimental study of various bed materials for oxy-fuel combustion of biomass in a bubbling fluidized bed (BFB). A silica sand with three particle size distributions (PSDs) ...and a lightweight ceramic aggregate (LWA) made of thermally expanded clay with two particle size distributions were chosen as potentially suitable materials. The experiments of biomass combustion in oxy-fuel mode with these materials were carried out to study their suitability as fluidized bed materials. The experiments were performed using a 30kWth lab-scale BFB facility with A1 wood pellets as a fuel.
The fluidized bed temperature was controlled at approx. 800°C and the volumetric fraction of O2 in dry off-gas at 10%. The resulting volumetric fraction of CO2 in dry off-gas was approximately 85%. At these conditions, there were no problems with sintering and forming of agglomerates. In the case of ceramic materials, it was possible to control the combustion process in a significantly wider range of operating parameters. Compared to silica sand, the experimental facility could be operated at a lower excess of oxygen (below 6% in dry off-gas) without significant influence on the temperatures in the facility. In terms of the consumption of the combustion facility itself, ceramic materials seem to be more suitable, as they are lighter. The pressure drop of the fluidized bed made of LWA is lower than that of the bed of the same volume made of silica sand, which means that less power is required to drive the fluidization fan.
•Experiments performed using a 30 kWth BFB combustor with real flue gas recirculation.•Silica sand and lightweight ceramic aggregate (LWA) were tested as BFB materials.•Both materials were found to be suitable for the oxy-fuel combustion of wood pellets.•No issues related to agglomeration were observed.•LWA allows for about 29% lower load of the fluidizing fan compared to silica sand.
