(421ai) Generation Behavior of Heat-Stable Salt In Novel Chemical Solvent for CO2 Capture Process From Blast-Furnace Gas

Blast-furnace gas is known to be one of the large-scale CO₂ emission sources, as well as flue gases emitted from coal-fired power plants and cement manufactures; these CO₂ emission sources contribute to increasing the global warming. Carbon Dioxide Capture and Storage (CCS) process is currently brought to international attention as one of the most feasible CO₂ reduction technology, and thus the practical application of chemical CO₂ solvents for a CO₂ separation and capture process within the CCS technology is desired, which have high efficiency, high stability and low cost.

Currently, we are developing original novel amine solutions as chemical CO₂ solvents for the CCS process from blast-furnace gas in cooperation with Nippon Steel Corporation in Japan as a part of “CO₂ Ultimate Reduction in Steelmaking Process by Innovative Technology for Cool Earth 50 (COURSE50)” project; this project is promoted under the initiative by the Japanese government. The gas generated within blast-furnaces contains a large amount of CO, commonly about 20 vol%, which is under reducing condition. When the blast-furnace gas is treated within an amine solution to absorb and separate CO₂, formic acid is known to be generated due to dissolution of CO into the amine solution. There is concern that the generated formic acid reacts with the substrate amine in the chemical CO₂ solvent to form deteriorate materials, known as heat-stable salts, even more than that CO₂ absorption is inhibited by amine consumption and/or decline in pH value due to the formation of formic acid. However, there is little report on the behaviors of CO containing in blast-furnace gas, generated formic acid and heat-stable salts, and also, on the effects of these materials on performances of the chemical CO₂ solvents. In this presentation, we will report the generation behavior and the estimated molecular structure of a heat-stable salt arising from coexistence between formic acid and the substrate amine contained in one of our developed solvents.

A certain secondary amine (Amine-R1) has been employed as the main substrate of our developed CO₂ solvent (R1). A bench scale test and a pilot scale test with actual blast-furnace gas have been carried out toward R1. Formic acid is observed to accumulate at the rate of approximately 0.25 mg-ion/L/hr (= 1.2 × 10^-6 mol-HCOO^-/mol-amine/hr) in the case without any pretreatments toward supplied blast-furnace gas, such as a desulfurization and a denitrification. It is considered that formic acid generated in R1 accumulates with repetition of increase and decrease through CO₂ absorption and desorption cycles, judging from changes with time of its concentration.

Laboratory scale tests have been carried out to examine deteriorations of Amine-R1 arising from coexistence of formic acid; these tests are what CO₂/N₂ mixture gas is aerated through the solutions of Amine-R1 previously containing definite quantities of formic acid at given temperatures and pressures. By GC-FID analysis, a peak of novel product (HSS-R1) that is unobservable without the formic acid addition has been detected. The concentration of HSS-R1 at 32 hr after the beginning of the test shows a nearly linear increase with increase in the concentration of the added formic acid. Additionally, the concentration of HSS-R1 after 32 hr increases with the test temperature in a range from 100 °C to 150 °C, which represent a desorption temperature of the CO₂ separation and capture process. HSS-R1 has been observed in the results of the bench scale test and the pilot scale test; these results of the actual gas tests are consistent with the linear correlativity between the concentrations of HSS-R1 and the added formic acid shown in the laboratory scale tests. In the laboratory scale tests, the concentration of HSS-R1 shows a convex-upward increase with time and stays constant after 16 hr, and the formic acid concentration shows a reversely convex-downward decrease. These results indicate that HSS-R1 is the heat-stable salt generated by the reaction between Amine-R1 and formic acid; this reaction is considered to be endothermic and balanced reaction. The generation rate of the heat-stable salt HSS-R1 in the pilot scale test without the pretreatments is estimated to be approximately 1.25 × 10^-5 Area-%/hr as a ratio to the initial concentration of Amine-R1.

The deterioration of Amine-R1 is accelerated by the coexistence of formic acid because of the conversion from Amine-R1 to HSS-R1. In the laboratory scale tests, the deterioration ratios in the cases to add formic acid of 0.5 g/L and 5.0 g/L are 0.09% and 0.31%, respectively, while that in the case without formic acid is 0.08%; these added amounts of formic acid should correspond to the accumulation of it through the pilot scale test for 2000 hr and 20000 hr, respectively.

The heat-stable salt HSS-R1 has been proved to be formamide of Amine-R1 by accurate mass analyses with HPLC-MS and decoding fragment peaks of GC-MS; this molecule is a tertiary amine expressed as HCO-[Amine-R1]*, where [Amine-R1]* is deprotonated Amine-R1. Therefore, HSS-R1 is considered to be generated in a dehydration reaction of Amine-R1 formate.

Acknowledgment

This work was financially supported by the COURSE 50 project founded by the New Energy and Industrial Technology Development Organization, Japan.

The bench scale test and the pilot scale test with actual blast-furnace gas have been carried out by Nippon Steel Engineering co. ltd., which takes charge of the process developments.