Ind. Eng. Chem. Res. 2007, 46, 7811-7819
7811
Modeling of Blast Furnace CO2 Capture Using Amine Absorbents Finn Andrew Tobiesen,*,† Hallvard F. Svendsen,‡ and Thor Mejdell† SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway, and Department of Chemical Engineering, Norwegian UniVersity of Science and Technology, NTNU, NO-7491 Trondheim, Norway
The performance of amine absorption technologies developed for iron works is evaluated for two base cases, conVentional blast furnace top gas and nitrogen free blast furnace top gas with shaft injection. The study includes well-known absorbents like monoethanolamine (MEA) and methyldiethanolamine (MDEA)/piperazine, as well as 2-amino-2-methyl-1-propanol (AMP). A rate-based closed-loop model was developed for AMP based on a previously developed rigorous model. For the AMP model, equilibrium data have been obtained in our own laboratories. Simulations show that it is very favorable with intercooling of the absorber. Inserting a precooler at the gas inlet to the absorber does not cause significant operational cost improvements. For higher piperazine activator concentrations, up to 20 wt % Pz, lower regeneration duties as well as lower circulation rates are achieved. A comparison between the amines shows that the AMP is most energy efficient, followed by MDEA/Pz and MEA. By combining the best amine with process intercooling, a heat requirement of only 2.2 MJ/(ton of CO2 recovered) is possible for treatment of conventional blast furnace top gas. 1. Introduction In the past decade, there has been great focus on greenhouse gas emissions from the power industry. The possibility to capture and store CO2 into geological reservoirs has triggered the interest to explore possible schemes for capturing CO2, including precombustion, oxy-fuel, and postcombustion routes. Large research programs have been initiated all over the word. In most comparative studies, the postcombustion route, where the exhaust gas from coal or gas-fired gas plants is captured at atmospheric pressures, has been found to be the most inexpensive alternative (at least in a near-term perspective). One example is the recent published report from the Carbon Capture Project,1 which was performed by eight of the world’s largest energy companies. In addition, this report concluded that postcombustion capture with chemical absorption also has a substantial potential for improvement. These improvements may be achieved by use of more suitable amine mixtures, by better energy optimization of the absorber/desorber process, and by improved thermal integration with the power plant. Lately, there has been an increased focus also on other large stationary CO2 sources like the cement and steel industries. One example is the ULCOS (Ultra-Low CO2 Steelmaking) project, which is the largest integrated project within the EU 6th framework program. In the present study, some results from this project are presented where the applicability of using chemical absorbents for extracting CO2 from blast furnace gas is evaluated. The blast furnace gases (BFGs) to be treated have some important differences compared to exhaust gas. One aspect is the gas composition where BFGs, contrary to exhaust gases, do not contain oxygen. This leaves the amines less susceptible to degradation. Another aspect is the partial pressure of CO2, which in BFG is about 100 kPa as compared to 4 kPa from gas turbine exhaust gas and 13 kPa from a coal-fired power plant. On the other hand, the CO2 partial pressure is considerably lower than those usually encountered in natural gas production, in * To whom correspondence should be addressed. E-mail: andrew.
[email protected]. Phone: +47-98283947. Fax: +47-73592786. † SINTEF Materials and Chemistry. ‡ Norwegian University of Science and Technology.
ammonia production, and in precombustion power schemes. Consequently, because the optimal amine and process configuration is dependent on the partial pressure of CO2, one might expect that the optimal solution for steel plant blast furnace gases may be quite different from these previously studied cases. In the present study, we will, therefore, look at three important issues: (i) the effects of using various amines and amine blends; (ii) the effects of different process configurations; and (iii) the effects of optimizing process parameters. The overall goal is to lower the total cost of the CO2 separation while keeping certain specifications provided by the ULCOS project. 1.1. Specific System Description. In Figure 1, a typical CO2 capture plant for flue gas cleaning is illustrated. The process consists of four main parts: (1) Gas pretreatment. In the figure, this is shown as a wet cooler to reduce the flue gas inlet temperature to 40-50 °C and a blower (or compressor) for increasing the pressure to counteract the pressure drop in the column. For the base case defined in the ULCOS project, we will discuss the necessity of these equipment units. (2) CO2 removal step. This is performed by an absorption column. A water wash section is included in order to reduce the loss of amine to
