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Ammonia Loss

https://doi.org/10.2516/ogst/2013160

“Ammonia is very volatile and characterised by high equilibrium pressure even in the CO2 loaded aqueous ammonia solution. Figure 9 shows predicted partial and total pressure at the ammonia concentration of 28% and temperature of 8C; Figure 10 predicted equilibrium CO2 and NH3 partial pressure as a function of CO2 loading at the ammonia concentration of 5 wt% and the temperature of 10, 20 and 30C; Figure 11 predicted equilibrium NH3 and CO2 partial pressure as a function of CO2 loading at the ammonia concentration of 2.5 and 5% and the temperature of 10C. The conditions for Figure 9 are relevant for the chilled
ammonia process in which precipitation of products is taken into account while the conditions for Figures 10 and 11 are for processes at ambient temperatures, used by Powerspan, CSIRO and KIST at lower ammonia concentration in which precipitation did not take place. It is clear in all cases, that NH3 vapour pressures are significant at CO2 loading below 0.5.
As a result, ammonia will inevitably slip to flue gas in the absorber and to the CO2 product stream from the stripper in the capture process and measures will have to be taken to recover the ammonia. The operational and capital costs for an ammonia recovery process will be determined by the amount of ammonia slip and the impact of the operational parameters. Despite the fact that it is widely acknowledged that ammonia loss is a big challenge to the aqueous ammonia based CO2 capture process, there are limited studies on the ammonia loss rate in the literature.
For the chilled ammonia processes, the results available in the literature are all from the modeling work. Mathias et al. (2010) studied the effect of ammonia concentration in the gas phase at the outlet of absorber. They found out that the NH3 slip from the absorber is only weakly dependent on the NH3 concentration in the range of 15-26% and depends mainly on temperature. At the absorber temperature of 10C and atmospheric pressure, the NH3 concentration is approximately constant at 2 230 ppmv at ambient pressure, the ammonia concentration of 26% and CO2 loading of ca. 0.4 in the lean solvent and reduced to 242 ppm if absorber temperature can drop to 1.1C. The equilibrium NH3 partial pressure for a solvent at the ammonia concentration of 28% and CO2 loading of 0.4 and 8C is more than 0.05 bar equivalent to 50 000 ppm at atmospheric pressure (Fig. 9), according to Darde et al. (2009). The large discrepancy between the two references is most likely indicative of the uncertainty around the
thermodynamics of liquid-solid systems. Versteeg and Rubin (2011) did a thorough analysis of effect of ammonia concentration (0-30%), CO2 loading (0.25 to 0.67), absorber temperature (5 to 20C) on ammonia slip in a chilled ammonia process in which rich
solvent is not recycled to absorber. An increase in ammonia concentration and absorber temperature as well as a decrease in CO2 loading can lead to an increase in ammonia slip. Darde et al. (2012) also modeled the chilled ammonia process at the low ammonia concentrations up to 12% and investigated the effect of ammonia concentration, CO2 loading, absorber temperature and
process configuration. In a configuration close to the configuration proposed by Alstom (Fig. 2), it has been found that with increase in ammonia concentration in the solvent from 4 to 11%, the ammonia concentration in the gas stream leaving the absorber varies between 4 500 and 19 000 ppm. In Darde’s work, a murphy tray efficiency was introduced to account for deviation from the equilibrium. Budzianowski (2011), Niu et al. (2012) and Yu et al. (2012c) conducted experiments to investigate ammonia
loss at low ammonia concentrations and relatively high absorber temperatures. All results from these studies
observed the trends similar to the modeling studies described above, but the ammonia loss is much higher.
For example, Niu et al. (2012) studies the absorption at the ammonia concentration of 1-11 wt%, the CO2 loading of 0.12 and the temperature of 24C and the ammonia concentrations at outlet were 1-14 vol%. Similar to research work by Niu et al., Yu et al. used a pilot scale research facility to investigate the effect of a number of parameters. The pilot plant trials show that the conditions which typically enhance CO2 absorption rate such as an increase in ammonia concentration and absorption temperature and a decrease in CO2 loading tend to increase ammonia loss significantly. Among many parameters which affect ammonia loss, temperature is one of the most sensitive parameters. As shown in Figure 12, with the increase in solvent temperature,
CO2 absorption rate in absorber 1 remains similarly but ammonia loss rate increases significantly. From the ammonia loss point of view, the absorption temperature should be as low as possible. However, it is expected that a considerable amount of additional energy is required to produce chilled water. Dave et al. (2009) simulated the aqueous ammonia based process by keeping the flue
gas and lean solvent temperatures to the absorber at 10C and solvent concentration at 2.5 and 5 wt%. The electricity consumption is estimated to be more than 1 400 KJ/kg CO2.
In addition, the research by Budzianowski (2011) and Qi et al. (2013) suggested that the NH3 desorption process is a fast process and controlled by the vapour liquid equilibrium which could guide future research on process modification to reduce ammonia slip in absorber. Ammonia is also present in the CO2 product and requires removal. Yu et al. (2012a) investigated the effect
of operation conditions on ammonia concentration in the CO2 product. To reduce ammonia concentration,
the high pressure operation and low temperature in the over-head condenser will help reduce ammonia concentration in the CO2-product. However, this can cause solid precipitation in the stripper where solvent regeneration leads to generation of vapour which contains significant amount of ammonia and CO2 gas apart from water vapour. When the temperature of vapour drops
in the condenser/reflux line or even in stripper when the regeneration stops, part of vapour will condense,
leading to formation of the solution/droplet in which ammonia concentration is very high (more than 10 wt%). Since partial pressure of CO2 is high in stripper, this facilitates formation of a highly carbonated ammonia solution which reaches the ammonium bicarbonate solubility limit. The precipitation of solid will block the stripper condenser and reflux line, causing a shut-down of the plant. Blockage also occurs in instrument tubing, leading to false readings and affecting the operation. For ammonia processes which operate at higher ammonia concentrations, the blockage could also occur in the absorber packing.”

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