Prior to imaging, the samples, placed on a conductive carbon tape, were sputter-coated by gold and palladium for 90 s under Ar plasma to improve their conductivity.
Results and Discussion. The first step of the study consisted in the analysis of the thermal activation process. The thermal activation of sodium bicarbonate, i. The most important outcome of thermal activation is its effect on sorbent morphology. As shown in the micrographs reported in Figure 4 , sodium bicarbonate panel a is a nonporous solid.
During the decomposition reaction, the release of ca. As already noticed by Hartman et al. Figure 4 also reports the XRD patterns of the samples. Regardless of the activation temperature in this case, the temperature of the isothermal TG run , the chemical species originated from the decomposition of sodium bicarbonate is sodium carbonate.
The only noticeable difference between the samples activated at different temperatures, which underwent the same storage conditions in the time between their preparation and the XRD analysis, is a higher intensity of the peaks associated with the hydrated form of sodium carbonate for the samples obtained at a lower activation temperature.
This is again indicative of the higher surface area and, hence, of a higher adsorption capacity of sodium carbonate activated at lower temperature. To highlight the role of thermal activation in enhancing the reactivity of the sodium-based sorbent toward acid gases, a dedicated fixed bed experimental run in ramp of temperature was carried out.
Thermal activation appears crucial in triggering fast desulfurization of the gas stream. Thus, the onset of significant SO 2 sorption coincides with the onset of thermal activation. The promoting effect of the thermal activation is related to the increase of the porosity and surface area of the sorbent caused by the release of H 2 O and CO 2 , and not to the chemical nature of the product of activation, Na 2 CO 3. The reactivity of activated sodium carbonate toward HCl and SO 2 was tested in isothermal sorption runs.
All the runs were carried out heating the sorbent to the desired reaction temperature in pure nitrogen and providing an additional activation time of 40 min before starting the reaction. The duration of this stabilization time was chosen to facilitate the thermal decomposition of NaHCO 3 to Na 2 CO 3 , while avoiding the exposure of the sample to prereaction sintering for an unrealistically high time.
Figure 7 a shows the breakthrough curves obtained for HCl for runs carried out at different temperatures with a gaseous mixture having an inlet HCl concentration of ppm. Figure 7 b shows the corresponding results obtained with a gas mixture having an inlet SO 2 concentration of ppm.
These concentration values were selected as representative of the HCl and SO 2 content in flue gas generated by municipal or hazardous waste incinerators. In light of the considerations of section 3. Figure 7 c reports the calculated sorbent conversion after 3 h. It is well-known that in this range there are no thermodynamic limitations to acid gas abatement: from a thermodynamic point of view, reactions R2 and R3 should produce the almost complete removal of HCl and SO 2. The low conversions calculated at the higher temperatures might suggest that physical adsorption is actually taking place instead of chemical reaction.
The investigation of the reaction via FTIR spectrometry allows clarifying this aspect by collecting spectra of the gaseous species leaving the reactor. With reference to the reaction with SO 2 , Figure 8 a shows that CO 2 released by reaction is detected at all the temperatures tested, in an amount that is closely related to the solid conversion shown in Figure 7 c, confirming the occurrence of reaction R3.
In addition, the composition of the solid samples after the reaction with SO 2 , probed by XRD, also confirms the occurrence of reaction R3. Thus, only a weak scattering from Na 2 CO 3 is present in this case. Therefore, both the analyses of evolved gas species and resulting solid phases clarify that the sorption of acid gases is a chemical process across the entire range of temperatures tested. However, this is certainly not the case in the experiments of Figure 7 b, which were carried out with nitrogen as carrier gas, thus not allowing reaction R4 to occur.
The presence of oxygen is clearly shown to slow down the overall reaction rate of SO 2. This is likely due to the additional diffusional resistance in the transport of SO 2 to the reaction interface provided by the formation of Na 2 SO 4.
As such, it tends to form a rather compact barrier layer, impervious to diffusion. Figure 9 b shows the effect of a different particle size of sorbent on SO 2 removal. Similarly, Figure 9 c shows the effect on SO 2 removal of a higher SO 2 inlet concentration ppm. Clearly enough, breakthrough takes place earlier than for a SO 2 inlet concentration of ppm, since the amount of sorbent in the bed was unchanged, but the final sorbent conversion and its dependence on temperature are not affected by this variation.
Excluding effects of particle size and concentration of gaseous species, the declining reactivity of activated Na 2 CO 3 with reaction temperature has to be ascribed to morphological changes in the sorbent. Figure 4 in section 3. The loss of available surface area for reaction has an evident effect on the acid gas removal performance. The decrease of surface area and, thus, of the reactivity of the sorbent might be also accentuated by the nature of the reaction product formed by acid gas sorption, hence explaining the different behavior of SO 2 and HCl.
The present results evidenced that the temperature range at which sodium bicarbonate injection is highly effective is narrow, comprised between the lower limit of thermal decomposition to sodium carbonate and an upper limit deriving from the sintering of the newly formed sodium carbonate. Operating far from the optimal temperature implies an increasing need of excess sorbent. Considering the results of Figure 7 c on the final sorbent conversion at different temperatures, it can be estimated that the removal of 1 kg of HCl would require 2.
Pursuing the maximum utilization of the sorbent is a key aspect in the optimization of flue gas cleaning operation. Feeding excess sodium bicarbonate to the DSI system not only results in a higher reactant cost per unit of acid gas removed, but also causes an increase in the generation of process residues, the disposal of which is a significant environmental drawback of dry acid gas removal systems. In most cases, a further constraint to the operating temperature of DSI systems is given by mercury Hg removal, which is typically performed by means of activated carbon, injected in DSI together with the acid gas sorbent.
Thermal decomposition is necessary to form the porous structure in activated sodium carbonate that promotes sorbent reactivity. Yet, significant sintering of nascent sodium carbonate is detected even at temperatures markedly lower than the Tammann temperature of the material. Sintering reduces the sorption capacity of activated Na 2 CO 3 toward acid pollutants. From the viewpoint of process optimization, this information is useful for the identification of optimal operating conditions in flue gas treatment systems.
From the viewpoint of sorbent optimization, the inherent limitations of natural sodium bicarbonate that emerged in this study suggest the potential for synthetic approaches.
Sorbent modification methods aimed at improving the resistance of activated Na 2 CO 3 to sintering could be envisaged to harness the benefits of improved kinetics at higher temperatures, while avoiding the insurgence of adverse morphological changes. Author Information. The authors declare no competing financial interest.
Estimating source strengths of HCl and SO 2 emissions in the flue gas from waste incineration. Moreover, since only This simple method is easily applicable and the estimated results could provide scientific basis for the appropriate design and operation of the APC systems as well as corrosion control of heat recovery systems. Characterisation of acid pollutant emissions in ceramic tile manufacture.
Google Scholar There is no corresponding record for this reference. Hydrogen chloride emissions from combustion of raw and torrefied biomass.
Fuel , , 37 — 46 , DOI: Elsevier Ltd. Elevated emissions of hydrogen chloride HCl from combustion of biomass in utility boilers is a major issue as it can cause corrosion and, in combination with the high alkali content often encountered in these fuels, it can also deposit molten alkali chloride salts on the boiler's water tubes. Such deposition can impede heat transfer and cause further corrosion. It monitored the HCl emissions from torrefaction of biomass and, subsequently, the comparative HCl emissions from combustion of both raw and torrefied biomass.
Results showed that during torrefaction most of the chlorine of biomass was released in the gas phase, predominately as HCl. Consequentially, combustion of torrefied biomass, which contained less chlorine than raw biomass, generated significantly lower HCl emissions than raw biomass, particularly so for biomass of low alkali content.
This observation complements previous findings in this lab. Both of these findings enhance the appeal of torrefied biomass as a substitute fuel in utility boilers. Energy Fuels , 29 , — , DOI: Air pollution control systems in WtE units: an overview.
Waste Manage. All WtE waste-to-energy plants, based on combustion or other thermal processes, need an efficient gas cleaning for compliance with legislative air emission stds. The development of gas cleaning technologies started along with environment protection regulations in the late s. Modern APC air pollution control systems comprise multiple stages for the removal of fly ashes, inorg.
The main technologies and devices used for abatement of the various pollutants are described and their basic principles, their peculiarities, and their application are discussed. Few systems for cleaning of synthesis gas from waste gasification plants are included. Examples of APC designs in full scale plants are shown and cautious prospects for the future development of APC systems are made.
Environmental and economic performance assessment of alternative acid gas removal technologies for waste-to-energy plants. Sustainable Prod. Consumption , 16 , — , DOI: Talanta , , — , DOI: Elsevier B. Described is the quant.
Back titrn. A similar trend was obsd. In replacement titrn. The titrn. American Chemical Society. The decompn. A particularly slow increasing-temp. Efficient removal of the gaseous products eliminated possible equil. A near-first-order reaction rate equation was presented for the decompn. This correlation makes it possible to predict the reaction rate as a function of temp.
In combination with its integrated form, it can readily be used, for example, in modeling or design of the decompn. It is believed that such a highly alk. An empirical model was proposed to correlate the exptl. In addn. Study in an Integral Fixed-Bed Reactor. Study in an integral reactor. By using the results of a math. With Na2CO3 prepd. H2O vapor at a partial pressure of Air Pollut. Control Assoc. The results are presented of the reactivity of NaHCO3 and its thermal decompn.
In the temp. Kinetics of the sodium carbonate — sulfur dioxide reaction. AIChE J. The rates of reaction of the anhyd. Na2CO3 with HCl and its mixts. A relation was found between the reactivity of the solid and the method of prepn. The active form of Na2CO3 exhibits towards HCl a reactivity which is orders of magnitude higher than that of the inactive form.
The degree of gas purifn. Air Waste Manage. The sorption of HCl by thermally decompd. NaHCO3 was investigated using a fixed-bed reactor contg. The influence of particle diam. The results showed that HCl sorption increased strongly with increasing temp. Reaction kinetics of hydrogen chloride with sodium carbonate. The study, conducted in a fixed bed multilayer reactor, investigates the influence of temp. Results show that the carbonate conversion and the HCl collection efficiency reach the max.
Furthermore results show that the neutralization reaction is weakly influenced by the sorbent particle size, although some differences are appreciable only at low temp. The exptl. A first order reaction with respect to hydrogen chloride concn. Duo, W. The concn. The optimum temp. This has been attributed to a combination of reaction kinetics and sintering. A math. The coeff. Chloridization rates of sodium hydrogen carbonate calcines were detd.
The expansion of the solid phase vol. Up to advanced stages of the reaction, the rate-decaying behavior of the chloridization reaction can be approximated by first-order kinetics as a function of solids conversion or the elapsed time of reaction. The reaction between HCl gas and Na2CO3-based sorbents was rapid; a high degree of sorbent utilization was attained.
The unsteady-state sorption of HCl gas in a column packed with reactant solids can be described by a pair of partial differential equations; their anal. Unsteady-state exptl. Effective reaction rate consts. Presented, tractable expressions can readily serve as a rational basis for conceptual design and effective operation of packed-bed reactors for deep removal of HCl gas from hot producer gas. Energy Eng. Energy Fuels , 32 , — , DOI: Influence of flue gas cleaning system on the energetic efficiency and on the economic performance of a WtE plant.
Gas cleaning systems of MSW Municipal Solid Waste incinerators are characterized by the process employed to remove acid gases. In some recent plants beside a wet cleaning system, a dry neutralization with Ca OH 2 is used. The goal is to reduce the amt. The influence of these different technologies on the elec.
The effects of the different gas cleaning systems on elec. The difference of efficiency between the most advantageous technol. A simple economic anal. Municipal solid waste incineration MSWI is a method of waste valorization whose overall sustainability depends on the effective removal of the gaseous contaminants generated.
Hydrogen chloride HCl is a typical pollutant formed in waste combustion. Dry processes based on its reaction with basic powders such as calcium hydroxide are among the state-of-the-art best available technologies for MSWI flue gas treatment. An exptl. A lab. Solid reaction products were characterized using thermogravimetric anal.
A sensitivity anal. The results allow an improved understanding of the heterogeneous reaction process that is applied in acid gas dry removal processes. ChemSpider Database. CaO-based CO 2 sorbents: From fundamentals to the development of new, highly effective materials.
Topics discussed include: thermodn. For years researchers have been working to build experimental technology that would capture CO2 from chimneys and flue pipes before it gets into the atmosphere. The gas would then be buried permanently, deep underground. Indeed the promises made in Paris Climate Agreement to limit global temperatures to "well below" 2 degrees will need significant deployment of CCS.
The equipment and chemicals that need to be built on to a power station to extract the CO2 are known to be expensive and cumbersome. While governments and industry both recognise the benefits of such a system, they have been unwilling to put up the large amounts of cash needed.
Scientists are now looking at other materials to see if they can achieve the same impact but at a lower cost. They've created microcapsules that have a liquid solution of sodium carbonate also known as soda ash at their core, surrounded by a polymer shell that allows CO2 to flow through. The tiny blue balls sit in an aqueous solution which prevents them from sticking together.
When they come in contact with CO2 they change colour to a yellowish brown. Exposure of sodium carbonate to CO2 and water creates sodium bicarbonate - otherwise known as baking soda. The scientists say that current CCS techniques using chemicals like monoethanolamine are toxic and expensive and create as many problems as they solve. The LLNL team believe that baking soda can do an effective job at a much lower price.
One advantage of the microcapsule approach is that recovery of the captured carbon is done by heating the baking soda solution, and that means the scientists get a more pure form of the gas - which increases its value for other industries. But with production of the capsules limited to about a kilogram a day, the research team understand that this is a long term project.
Thanks to 3D printing and advanced computational techniques, the researchers in California are not limited to capsules. They are developing a range of materials that can be built and tested quickly thanks to the technology. You can almost think of it as a CO2 sponge. The idea is that the researchers can customise carbon capture materials to fit chimneys or flue pipes.
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