Investigation of co-combustion of coal and olive cake in a bubbling fluidized bed with secondary air injection

In this study, a bubbling fluidized bed of 102 mm inside diameter and 900 mm height was used to burn olive cake and coal mixtures. Tunçbilek lignite coal was used together with olive cake for the co-combustion tests. Combustion performances and emission characteristics of olive cake and coal mixtures were investigated. Various co-combustion tests of coal with olive cake were conducted with mixing ratios of 25%, 50%, and 75% of olive cake by weight in the mixture. Operational parameters (excess air ratio, secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The results were compared with that of the combustion of olive cake and coal. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion tests.For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found to be 1.35 for excess air ratio, and 30 L/min for secondary air flowrate for the combustion of 75 wt% olive cake and 25 wt% coal mixture. The highest combustion efficiency of 99.8% was obtained with an excess air ratio of 1.7, secondary air flow rate of 40 L/min for the combustion of 25 wt% olive cake and 75 wt% coal mixture.     

Combustion of olive cake and coal in a bubbling fluidized bed with secondary air injection

Combustion performances and emission characteristics of olive cake and coal are investigated in a bubbling fluidized bed. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion experiments. Operational parameters (excess air ratio (λ), secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The temperature profiles measured along the combustor column was found higher in the freeboard for olive cake than coal due to combustion of hydrocarbons mostly in the freeboard. Combustion efficiencies in the range of 83.6–90.1% were obtained for olive cake with λ of 1.12–2.30. For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found as 1.2 for λ, and 50 L/min for secondary air flowrate for the combustion of olive cake.     

Thermogravimetry-combustion characteristics of high-carbon fluidized-bed cyclone ashes

Thermogravimetry combustion characteristics are obtained for three samples of high-carbon cyclone ash, the samples received from an R & D fluidized bed combustion unit utilizing Ohio No. 6 bituminous coal with limestone. Non-isothermal TG combustion in air (at a programmed heating rate of 2 °C min^?1) for a cyclone ash (25.0 wt% carbon) indicates two chemically different combustibles, ≈82 wt% of higher reactivity with maximum weight-loss rate at 490 °C and 18 wt% correspondingly of lower reactivity at 645 °C. Isothermal TG combustion of the same ash at 700 °C and at 850 °C also indicates the presence of the two types of combustibles and gives an E_a of about 150 kJ mol^?1 for combustion of the lower reactivity type. Additionally, each of two other samples of cyclone ash (20.1 and 6.9 wt% carbon) contains two types of combustibles as evidenced by TG combustions. Non-isothermal TG combustion of feed coal, a single maximum weight loss at 440 °C, shows the coal has greater reactivity than each of the three ashes. A combustion chemistry explanation for two types of combustibles is suggested by the postulated reactivity order for the organic matter or maceral groups of coal, i.e., vitrinite ? exinite ? inertinite.     

Predicted mineral melt formation by BCURA Coal Sample Bank coals: Variation with atmosphere and comparison with reported ash fusion test data

The thermodynamic equilibrium phases formed under ash fusion test and excess air combustion conditions by 30 coals of the BCURA Coal Sample Bank have been predicted from 1100 to 2000 K using the MTDATA computational suite and the MTOX database for silicate melts and associated phases. Predicted speciation and degree of melting varied widely from coal to coal. Melting under an ash fusion test atmosphere of CO₂:H₂ 1:1 was essentially the same as under excess air combustion conditions for some coals, and markedly different for others. For those ashes which flowed below the fusion test maximum temperature of 1773 K flow coincided with 75-100% melting in most cases. Flow at low predicted melt formation (46%) for one coal cannot be attributed to any one cause. The difference between predicted fusion behaviours under excess air and fusion test atmospheres becomes greater with decreasing silica and alumina, and increasing iron, calcium and alkali metal content in the coal mineral.     

Axial and radial CO concentration profiles in an atmospheric bubbling FB combustor

Atmospheric Bubbling Fluidised Bed Combustion (ABFBC) of a bituminous coal and anthracite with particle diameters in the range 500-4000 μm was investigated in a pilot-plant facility (circular section with 0.25 m internal diameter and 3 m height). The experiments were conducted at steady-state conditions using three excess air levels (10, 25 and 50%) and bed temperatures in the 750-900 °C range. Combustion air was staged, with primary air accounting for 100, 80 and 60% of total combustion air. The effect of limestone addition was also tested.Large CO concentrations were observed inside the bed, up to 8 and 6% (v/v) in the cases of anthracite and bituminous coals, respectively. These concentrations decreased sharply as the gases emerged from the bed, and the CO flue gas concentration observed was in general less than 2000 and 4000 ppm, respectively. The CO flue gas concentration increased with air staging and with limestone addition, but decreased with either excess air or temperature increase. The observed results confirm the influence of sand particles (and probably of SO₂) in the ‘quenching’ of the oxygenated free radicals (HO and HO₂) reactions responsible for the CO oxidation inside the bed.     

Co-combustion of peach and apricot stone with coal in a bubbling fluidized bed

In this study a bubbling fluidized bed combustor (BFBC) having an inside diameter of 102 mm and a height of 900 mm was used to investigate the co-combustion characteristics of peach and apricot stones produced as a waste from the fruit juice industry with coal. A lignite coal was used for co-combustion. On-line concentrations of O₂, CO, CO₂, SO₂, NO_X and total hydrocarbons (C_mH_n) were measured in the flue gas during combustion experiments. Variations of emissions of various pollutants were studied by changing the operating parameters (excess air ratio, fluidization velocity, and fuel feed rate). Temperature distribution along the bed was measured with thermocouples.     

Axial concentration profiles and N2O flue gas in a pilot scale bubbling fluidised bed coal combustor

Atmospheric Bubbling Fluidised Bed Coal Combustion (ABFBCC) of a bituminous coal and anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility (circular section with 0.25 m internal diameter and 3 m height). The experiments were conducted at steady-state conditions using three excess air levels (10%, 25% and 50%) and bed temperatures in the 750–900 °C range. Combustion air was staged, with primary air accounting for 100%, 80% and 60% of total combustion air.For both types of coal, virtually no N₂O was found in significant amounts inside the bed. However, just above the bed-freeboard interface, the N₂O concentration increased monotonically along the freeboard and towards the exit flue.The N₂O concentrations in the reactor ranged between 0–90 ppm during bituminous coal combustion and 0–30 ppm for anthracite. For both coals, the lowest values occurred at the higher bed temperature (900 °C) with low excess air (10%) and high air staging (60% primary air), whereas the highest occurred at the lower bed temperature (750 °C for bituminous, 825 °C for anthracite) with high excess air (50%) and single stage combustion.Most of the observed results could be qualitatively interpreted in terms of a set of homogeneous and heterogeneous reactions, where catalytic surfaces (such as char, sand and coal ash) can play an important role in the formation and destruction of N₂O and its precursors (such as HCN, NH₃ and HCNO) by free radicals (O, H, OH) and reducing species (H₂, CO, HCs).     

Air and oxy-fuel combustion characteristics of biomass/lignite blends in TGA-FTIR

Pyrolysis and combustion behavior of indigenous lignite, olive residue and their 50/50 wt.% blend in air and oxy-fuel conditions were investigated by using thermogravimetric analyser (TGA) combined with Fourier-transform infrared (FTIR) spectrometer. Pyrolysis tests were carried out in nitrogen and carbon dioxide environments which are the main diluting gasses of air and oxy-fuel environment, respectively. Pyrolysis results of the parent fuels and the blend show that weight loss profiles are almost the same up to a temperature of 700 °C in these two environments, indicating that CO₂ behaves as an inert gas in this temperature range. However, further weight loss takes place in CO₂ atmosphere at higher temperatures due to CO₂-char gasification reaction which leads to significant increase in CO and COS formation as observed in FTIR evolution profiles. Comparison between experimental and theoretical pyrolysis profiles of the blend samples reveals that there is no synergy in both atmospheres. Combustion experiments were carried out in four different atmospheres; air, oxygen-enriched air environment (30% O2-70% N₂), oxy-fuel environment (21% O₂-79% CO₂) and oxygen-enriched oxy-fuel environment (30% O2-70% CO₂). Replacing N₂ in the combustion environment by CO₂ causes slight delay (lower maximum rate of weight loss and higher burnout temperature) in the combustion of all samples. However, this effect is found to be more significant for olive residue than lignite. Elevated oxygen levels shift combustion profiles to lower temperatures and increase the rate of weight loss. Combustion profiles of olive residue/lignite blends lie between those of individual fuels. Comparison between experimental and theoretical combustion profiles and characteristic temperatures of the blend samples indicates synergistic interactions between the parent fuels during co-combustion of olive residue and lignite.     

Coal combustion characteristics in a circulating fast fluidized bed

The effect of coal size (0.73–1.03 mm), excess air ratio (1.0–1.4), operating bed temperature (750–900‡C), coal feeding rate (1–3 kg/h), and coal recycle rate (20–40 kg/h) on combustion efficiency, temperature profiles along the bed height and flue gas composition have been determined in a bubbling and circulating fluidized bed combustor (7.8 cm-ID x 2.6 m-high). Combustion efficiency increases with increasing excess air ratio and operating bed temperature and it decreases with increasing particle size in the bubbling and circulating fluidzing beds. In general, temperature profiles and combustion efficiency are more uniform and higher in a circulating bed than those in bubbling bed. Combustion efficiency also increases with increasing recycle rate of unburned coal in the circulating bed. The ratio of CO/CO₂ of flue gas decreases with increasing bed temperature and excess air ratio, whereas the ratio of O₂(CO + CO₂) decreases with bed temperature in both bubbling and circulating fluidized beds.     

Impact of air staging along furnace height on NOx emissions from pulverized coal combustion

Experiments were carried out on an electrically heated multi-path air inlet one-dimensional furnace to assess NO_x emission characteristics of an overall air-staged (also termed air staging along furnace height) combustion of bituminous coal. The impact of main parameters of overall air-staged combustion technology, including burnout air position, air stoichiometric ratio, levels of burnout air (the number of burnout air arranged at different heights of the furnace), and the ratios of the burnout air flow rates and pulverized coal fineness of industrial interest, on NO_x emission were simulated to study in the experimental furnace, as well as the impact of air staging on the carbon content of the fly ash produced. These results suggest that air-staged combustion affects a pronounced reduction in NO_x emissions from the combustion of bituminous coal. The more deeply the air is staged, the further the NO_x emission is reduced. Two-level air staging yields a greater reduction in NO_x emission than single-level air staging. For pulverized coal of differing fineness, the best ratio between the burnout air rates in the two-level staging ranges from 0.6 to 0.3. In middle air-staged degree combustion with f_M = 0.75, pulverized coal fineness, R₉₀ (%), has a greater influence on NO_x emission, whereas R₉₀ has little influence on NO_x emission for deep air-staged degree with f_M = 0.61. Air-staged combustion with proper burnout air position has little effect on the burnout. For overall air-staged combustion, proper burnout air position and air-staged rate should be considered together in order to achieve high combustion efficiency.     

A comparative reactivity and kinetic study on the combustion of coal-biomass char blends

The combustion behavior and kinetics of various biomass chars, a lignite and a hard coal char and their blends were investigated. Pure fuel chars were compared to blended chars with respect to their performance during combustion. Non-isothermal thermogravimetry experiments were performed in air atmosphere, over a temperature range of 25-850 °C and at a heating rate of 10 °C/min. Kinetic evaluation was performed using a power law model. Reaction kinetic parameters were obtained by modeling the combustion of biomass and coal chars as a single reaction, with the exception of lignite and olive kernel chars, the combustion of which was modeled by two partial reactions. A single reaction model was used in the case of coal-wood char blends, while for the lignite-biomass char blends two partial reactions were used. Reactivity was assessed using the specific reaction rate, as a function of conversion. Biomass chars were generally more reactive than those of hard coal and lignite. The combustion behavior of the blends was greatly influenced by the rank of each coal (hard coal or lignite) and the proportion of each component in the blend. Combustion performance of the blends showed some deviation from the expected weighted average of the constituent chars. An attempt was made to estimate the kinetics of the blends using, as a basis, the parameters estimated for the individual components. In this case, because of the interactions between the components of the blends, the kinetic parameters needed to be slightly modified. Alteration in reactivity was more pronounced in the case of lignite-biomass chars than coal-wood chars.     

Leaching characteristics of fly ash from fluidized bed combustion thermal power plant: Case study: Çan (Çanakkale-Turkey)

It is known that the concentration of elements of fly ash varies due to the used-coal and the used-lime qualities varying in different periods. In the Çan Thermal Power Plant (CTPP) located at northwestern Turkey, Çan (Çanakkale) basin coals, which are classified as lignite to sub-bituminous C coal with high total sulphur (0.4-12.22%) and a broad range of ash contents (3.2-44.6%) are mainly used. Performed studies reveal that some toxic elements exit in the coal, including As, U and V. Also, while the As, Cu, Co and Hg contents in coal increases, the sulphur contents in coal also increase. Additionally, trace elements that have inorganic compounds in coal are mobilized into air during the combustion process. This poses a big risk for human health and keeping the environment when Çan Basins low quality lignite is burned, it's the fly ash that contains several toxic elements which can leach out and contaminate the water resources.In this study, toxicity tests were conducted on the fly ash samples that were obtained from the fluidized bed combustion of Çan Thermal Power Plant. The results showed that water temperature, pH and the quality of the limestone used were the most important factors affecting the leaching properties. Concentration of some toxic elements found in the fly ash, such as; As, Cd, Cr, Pb, Se and Zn were analyzed. Concentration richness of some heavy metals were attributed to the increase of water temperature, especially when pH is lower than 5. At pH = 5 value, there is no clear explanation of each heavy metal presence in the fly ash from fluidized bed combustion thermal power plant.     

Experimental investigation of the combustion of bituminous coal in air and O2/CO2 mixtures: 2. Variation of the transformation behaviour of mineral matter with bulk gas composition

Combustion of a Chinese bituminous coal was carried out in a laboratory-scale drop tube furnace (DTF) to clarify the variation of ash properties with bulk gas composition. The combustion conditions tested include three bulk gases, air, 21% O₂/79% CO₂ and 27% O₂/73% CO₂, two furnace/gas temperatures close to the fluidized bed reactor temperature range, 1073 K and 1273 K, and three particle residence times. Apart from bulk properties analysis, individual ash particles and the original mineral species in coal were characterized using Computer - Controlled Scanning Electron Microscopy (CCSEM). The results indicate that, under the given experimental conditions, shifting bulk gas from air to O₂/CO₂ mixtures is insignificant in terms of the elemental composition of bulk ash, in agreement with the literature. However, changes in the properties of individual species/metals are noticeable, including the extent of the vaporization of volatile elements, ash particle-size distribution (PSD), crystallization extent of K alumino-silicate associate, pyrite decomposition and oxidation rate and formation propensity of liquidus in ash. These changes were mostly considered to be caused by the evolution of included mineral grains in the distinct char particles in the O₂/CO₂ environment. Reduction in char particle temperature with bulk gas shifting from air to O₂/CO₂ mixtures was primarily crucial, which, however, could be overweighed by the existence of a fairly strong local reducing condition on the char surface in O₂/CO₂. Consequently, vaporization of the volatile elements such as Na and P was promoted; formation of the crystalline leucite in air was in contrast inhibited. Furthermore, the extent of coalescence of included minerals and oxidation rate of pyrite (or its derivative, pyrrhotite) were also influenced by char consumption rate, i.e. the receding extent of char surface. These parameters exerted a combined effect on ash formation, requiring detailed mathematical modeling to describe the dynamics of the formation of oxy-fuel ash. This study also indicated that the differences of ash properties formed between air and O₂/CO₂ mixtures can be greatly reduced and eventually eliminated by increasing furnace temperature. Increase in the turbulence of gas flow should also benefit the elimination of the side effects of local reducing gases on char surface.     

Mode of occurrence of arsenic in feed coal and its derivative fly ash, Black Warrior Basin, Alabama

An arsenic-rich (As = 55 ppm) bituminous feed coal from the Black Warrior Basin, Alabama and its derivative fly ash (As = 230 ppm) were selected for detailed investigation of arsenic residence and chemical forms. Analytical techniques included microbeam analysis, selective extraction, and As K-edge X-ray absorption fine-structure (XAFS) spectroscopy. Most As in the coal is contained in a generation of As-bearing pyrite (FeS₂) that formed in response to epigenetic introduction of hydrothermal fluids. XAFS results indicate that approximately 50% of the As in the coal sample occurs as the oxidized As(V) species, possibly the result of incipient oxidation of coal and pyrite prior to our analysis. Combustion of pyrite and host coal produced fly ash in which 95% of As is present as As(V). Selective extraction of the fly ash with a carbonate buffer solution (pH = 10) removed 49% of the As. A different extraction with an HCl-NH₂OH mixture, which targets amorphous and poorly crystalline iron oxides, dissolved 79% of the As. XAFS spectroscopy of this highly acidic (pH = 3.0) fly ash indicated that As is associated with some combination of iron oxide, oxyhydroxide, or sulfate. In contrast, a highly alkaline (pH = 12.7) fly ash from Turkey shows most As associated with a phase similar to calcium orthoarsenate (Ca₃(AsO₄)₂). The combined XAFS results indicate that fly ash acidity, which is determined by coal composition and combustion conditions, may serve to predict arsenic speciation in fly ash.     

NiO/Al2O3 oxygen carriers for chemical-looping combustion prepared by impregnation and deposition-precipitation methods

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using γ-Al₂O₃ and α-Al₂O₃ as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a thermogravimetric analyzer and methane combustion selectivity towards CO₂ and H₂O, attrition rate, and agglomeration behavior were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests.XRD and TPR analysis showed the presence of both free NiO and NiAl₂O₄ phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl₂O₄ that affected negatively to the OC reactivity and methane combustion selectivity towards CO₂ and H₂O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on γ-Al₂O₃.The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process. OC prepared by the AI and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on γ-Al₂O₃ with NiO content lower than 30 wt%, and OC prepared by the DP method on α-Al₂O₃ with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using γ-Al₂O₃ as support were higher than the ones prepared using α-Al₂O₃ as support, and in general the attrition rates of all the OC were low.The OC prepared by AI, HI or DP methods on α-Al₂O₃ as support had appropriated characteristics to be used in the chemical-looping combustion process.     

Fluidized bed gasification of Kingston coal and marine microalgae in a spouted bed reactor

Steam gasification experiments were performed using a low-rank coal from South Australia, a marine microalga, and a blend of leached microalgal biomass and coal, in a spouted, fluidized bed reactor. The effect of different operating conditions – air-to-fuel ratio (A/F), steam-to-fuel ratio (S/F) and bed temperature (T_b) – on the producer gas composition was investigated. Producer gas compositions were analyzed and samples of bed material were also examined to identify ash components formed during each experiment. The optimum operating conditions for coal gasification, in this system, were identified to occur with A/F = 1.82, S/F = 0.75 and T_b = 850 °C. These conditions resulted in a producer gas with the highest heating value (per mass of fuel fed), the highest extent of carbon conversion and the optimum H₂:CO ratio for Fischer–Tropsch synthesis. In addition, preliminary attempts to gasify a sun-dried marine microalga are reported. The dried biomass, sieved to 1.0–3.35 mm, was gasified with air and steam. Preliminary experiments, utilizing the as-received biomass, proved unsuccessful due to rapid bed sintering. Leaching of the algal biomass to remove the extra-cellular salt and co-gasification of the resultant biomass (10 wt%) with low-rank coal also proved unsuccessful due primarily to blockages of the downstream product lines most likely due to attrition of the algae feed in the screw feeder and elutriation from the bed.     

In situ gasification and CO2 separation using chemical looping with a Cu-based oxygen carrier: Performance with bituminous coals

This paper is concerned with the chemical looping combustion of coal in a technique whereby the fuel is gasified in situ using CO₂ in the presence of a batch of supported copper oxide (the “oxygen carrier”) in a single reactor. As the metal oxide becomes depleted, the feed of fuel is discontinued, the inventory of fuel is reduced by further gasification and then the contents are re-oxidised by the admission of air to the reactor, to begin the cycle again. A catalyst support, impregnated with a saturated solution of copper and aluminium nitrates, acted as a durable oxygen carrier over numerous cycles of reduction and oxidation, using air as the oxidant. Two bituminous coals (Taldinskaya, Russia, and Illinois No. 5, USA) were investigated and compared with a lignite (Hambach, Germany). The lignite was highly reactive and was gasified completely by 15 mol% CO₂ in N₂ at 1203 K and 1 bar, so that there was no build up of char in the bed. The bituminous coals produced chars much less reactive than the lignite char, so that there was a steady accumulation of char in the bed with number of cycles, with the degree of accumulation being dependent on the reactivity of the char. Since the kinetics of gasification by CO₂ of the chars from either bituminous coal were slow, their rates were controlled by intrinsic chemical kinetics and were not affected by the ability of the oxygen carrier to alter the rates of external mass transfer when gasification is rapid. However, it is likely that rates of gasification in the presence of the carrier are still larger than in its absence, owing to the overall lower [CO] present in the bulk of the fluidised bed during chemical looping. At the temperature used, the carrier was cycling between Cu and Cu₂O, since CuO is only stable if the partial pressure of O₂ exceeds 0.03 bar at 1203 K. The CuO decomposes to Cu₂O and O₂ relatively rapidly at these temperatures, once the oxygen concentration is effectively zero. It was impossible to ascertain in our experiments whether the oxygen so generated, after the switching of the air for nitrogen before the start of the succeeding cycle of gasification, made any substantial difference to the reactivity of the char present in the bed. The rate of oxidation of the carrier was found to be much more rapid than the rate of oxidation of the inventory of char. This allows a preferential oxidation of the carrier and most likely accounts for why progressively less CO and CO₂ is produced during successive cycles with short periods of oxidation: the increasingly reduced carrier reacts more rapidly than the char. There was no obvious impact from the sulphur contained in the fuels, but longer-term testing is needed. No agglomeration between the carrier particles and the ash was observed, despite the high temperatures during oxidation.     

Co-combustion of rice husk with coal in a cyclonic fluidized-bed combustor (ψ-FBC)

Thailand is well-endowed with renewable energy resources. In Thailand, rice husk, a by-product of the rice-milling process and one of the most potentially sustainable cultivated biomasses, has an annual energy equivalent of 6.6 × 10⁷ GJ. Using rice husk alone, however, can be problematic, particularly if there is a deficit during the off-season. Coal, the most abundant fossil fuel, has thus been considered an appropriate supplementary fuel. This paper describes the combustion characteristics of co-firing rice husk with bituminous coal in a 120 kW_th-capacity cyclonic fluidized-bed combustor (ψ-FBC), and how excess air ratios and fuel blends impacted emissions and combustion efficiency (E_c). Overall, excess air and blending ratios did not have tremendous effects on E_c, easily achieving >97%. Radial temperature profiles revealed that vortex combustion prevailed along the combustor walls. Concurring with axial temperature profiles, axial O₂ profiles suggested that the combustion was confined chiefly to regions under the vortex ring. Despite massive CO production in the lower section, CO emissions were satisfactory (range 60-260 ppm, at 6% O₂). Due to the high bed temperatures, NO_x appeared rather high (260-416 ppm, at 6% O₂). Not only were NO_x emissions affected by coal ratio, it were also highly reliable on the operating conditions. SO₂ emissions varied directly, but not proportionally, with the sulfur content of the fuel mixture.     

Combustion and air emissions from co‐firing a wood biomass,a Canadian peat and a Canadian lignite coal in a bubbling fluidised bed combustor

The effects of particle size, fuel blending ratio, moisture content and excess air ratio on combustion efficiency and air emissions (CO₂, CO, SO₂ and NO_x) from the co‐combustion of white pine or peat with a Canadian lignite coal, were examined in a pilot‐scale bubbling fluidised bed combustor. Pelletising was important for the efficient combustion of wood due to its high volatile content. Co‐firing lignite and pine pellets gave a proportional reduction in SO₂ and NO_x emissions with blending ratio, while co‐firing of peat and lignite resulted in increased SO₂ emissions, but decreased NO_x emissions. Moisture promotes combustion but with increased CO emissions, and results in increased NO_x emissions, and decreased SO₂ emissions. High excess air decreased CO, but moderately increased SO₂ and NO_x emissions. © 2011 Canadian Society for Chemical Engineering     

Characterization and evaluation of fly-ash from co-combustion of lignite and wood pellets for use as cement admixture

Conventional coal fly-ash (CFA) and two coal-biomass fly-ashes (CBFAs) were obtained at a thermoelectric power station (Atikokan, Ontario) from combustion of undiluted lignite coal and co-combustion of lignite coal with up to 66% wood pellets (on a thermal basis). Fly-ashes were characterized and analyzed for use as cement admixtures. Co-combustion did not markedly change the fly-ash composition, owing to an extremely low ash content of wood pellets compared to lignite coal; toxic metals and minor elements were within ranges reported for other coal fly-ashes. All fly-ashes had losses on ignition (LOI) <1 wt% and therefore complied with ASTM LOI regulations for use in concrete. All fly-ashes contained major amorphous phases, along with quartz and periclase. Partial substitution of cement with fly-ash (up to 40 wt%) had a moderate effect on the entrained air content of mortars (up to 2.5%), but this difference vanished upon addition of air entraining agent (0.6 mL/kg of cementitious material). Substituted mortars exceeded 75% of the strength of ash-free mortar after 28 days of curing (therefore meeting ASTM requirements for strength development), and by 90 days, met or surpassed 100% of the strength of ash-free mortar. Amending mortar with 20 wt% CFA or CBFA had no effect on its durability following repeated freeze-thaw cycles when air content was kept constant. Also, no micromineralogical differences were observed between hydrated CFA- and CBFA-amended mortars, with fly-ash particles reacting with Ca ions originating from dissolution of cement clinker or calcium hydroxide.