1 st Post Combustion Capture Conference Heat requirement of CO 2 absorption by aqueous ammonia Henrik Jilvero*, Fredrik Normann, Klas Andersson, Filip Johnson Division of Energy Technology, Department of Energy and Environment, Chalmers University of Technology, Gothenburg, SE-412 96 "Keywords: Chilled Ammonia, Post Combustion Capture, Ammonia" Aqueous ammonia has lately emerged as a promising absorbent of CO 2 in post-combustion capture applications, often referred to as the chilled ammonia process (CAP). The heat required for regeneration, i.e. the reboiler duty (kJ/kg CO 2 ), is a crucial parameter for all absorbents. For the CAP, the required reboiler duty presented in literature ranges from 1,500 to 4,200 kJ/kg CO 2 [1-6]. This is a wide range with crucial consequences for the feasibility of the process. The aim of this work is to review the different sources in order to explain the large difference in heat requirement. Figure 1 presents the reboiler duty as a function of ammonia concentration (mole NH 3 /kg H 2 O) and CO 2 -loading (mole NH 3 /mole CO 2 in the lean solution) from six different investigations [1-6]. The investigations may be divided into three categories depending on method applied to decide the reboiler duty: 1) simulations assuming CO 2 to be absorbed solely by ammonium carbonate crystals (Reaction 1) [2], 2) equilibrium based process simulations [1,3-5], 3) pilot plant studies [6]. Group 1 shows on the lowest regeneration heat, around 1,500 kJ/kg CO 2 [1]. Group 2, range from 2,000 to 4,000 kJ/kg CO 2 highly dependent on the ammonia concentration [2-5]. So far, pilot plant data is only recorded at low ammonia concentrations (2 to 5wt%) and the lowest registered regeneration heat is around 4,000 kJ/kg CO 2 [6], which agrees with the equilibrium calculations under similar conditions. The ammonia concentration mainly affects the thermal performance through the increase in mass flow when the ammonia concentration declines; hence the sensible heat requirements are increased. However, the ammonia concentration also affects the chemistry. The low sensitivity of the reboiler duty above 7 moles NH 3 /kg H 2 O (CO 2 - loading of 0.25) shown by the equilibrium calculations is caused by the precipitation of ammonium bicarbonate at these ammonia concentrations. The reduced sensible heat requirements are counteracted by the heat of dissolution of the solid ammonium bicarbonate. At a CO 2 -loading of 0.15, precipitation does not occur. The regeneration heat is related to the heat of reaction. Carbon dioxide may be absorbed trough Reactions 1-4. Reaction 1 is the absorption by ammonium carbonate, which is slow and has a low heat of reaction. Reactions 2-4 include free ammonia, which makes them faster but with a significantly larger heat of formation. Thus, if the process is forced through Reaction 1 a considerably lower reboiler duty is achieved. The high reboiler duty of the pilot plant [6] is caused by the low ammonia concentration which promotes Reaction 3 and increases the sensible heat through a larger amount of water in the system.