Simultaneous capture and mineralization of flue gas CO₂ at point sources is invaluable in addressing green house gas problems. We have developed a novel accelerated mineral carbonation (AMC) process for point source flue gas CO₂ (US Patent Number: 7,879,305 B2, 2011).

This process utilizes coal combustion by-products i.e., fly ash particles and the flue gas to produce environmentally safe and stable carbonate minerals. The AMC process, called SequesTech, has been successfully tested at the pilot scale at 2,200 MW coal-fired power plant in the western USA. In addition to mineralization of CO₂, the AMC process also mineralizes flue gas SO₂ and Hg. To our knowledge, this is the first research project which demonstrates direct capture and mineralization of significant amounts of flue gas CO₂, SO₂ and Hg from a point source under actual field conditions.

Introduction
Flue gas emissions from various industrial processes (e.g., coal-fired power plants, cement plants, paper mills, steel plants, oil shale incinerators, and municipal and medical solid waste incinerators) are a major source for the release of anthropogenic CO₂ into the atmosphere. Concurrently, these industrial processes also generate significant quantities of solid residues as by-products. To address the anthropogenic CO₂ problems, multiple CO₂ capture and storage (CCS) processes are required [1]. As a result, different CO₂ capture technologies and storage processes are under evaluation. However, CO₂ capture technologies and storage processes have limitations for widespread practical use due to the requirement of separation of CO₂ from the flue gas, compression of CO₂, and transportation of CO₂ to a site where it can be safely stored or used for mineral carbonation processes [2]. Furthermore, CO₂ separation and capture technologies are limited by the flue gas SO₂, because SO₂ is known to affect the performance of the solvents use. Here we show evidence suggesting that significant quantities of flue gas CO2 as well as SO₂ and Hg can be directly captured (without separation) and mineralized by the fly ash particles under actual field conditions (fig.

1).

Materials and Methods
We designed and developed a pilot scale process to directly capture and mineralize flue gas CO₂. The AMC pilot process consists of three process vessels (fig. 2): moisture reducing drum (MRD), heater/humidifier, and fluidized-bed reactor (FBR). Flue gas was withdrawn from the stack (after the power plants wet scrubber process for removing SO₂) and was fed to the MRD. The MRD captures droplets of water entrained in the flue gas to protect the blower placed between the MRD and the heater/humidifier. The heater/humidifier enables control of flue gas moisture and temperature. The reactor's top inlet was connected to the fly ash hopper to deliver a required amount of fresh ash particles into the reactor. The fly ash particles were fluidized by the flow of flue gas through a distributor plate in the FBR, ensuring proper mixing and good contact between the fly ash particles and the flue gas. A control valve and a pressure transmitter were used to set the pressure inside the FBR. A particulate removal cyclone in the reactor separated fly ash particles from the exiting flue gas. The pressure drop across the distributor plate, the fluidized bed, and the cyclone were measured by differential pressure transmitters. The temperature at various points inside the humidifier and the reactor were measured by thermocouples. The flue gas was continuously monitored by an industrial grade multi-gas analyzer (Horiba VA-3000). It was connected to the inlet and outlet lines to monitor the real-time concentration of CO₂, NO_x, and SO₂ in the flue gas. 

We conducted pilot scale studies at Jim Bridger Power Plant Unit 2, Point of Rocks, Wyoming, USA. We conducted seven test runs to study the AMC process. The temperature, flue gas moisture content, and ash amount (kgs) ranged between 35-92°C, 5-16 %, and 100-640 kgs, respectively. The pressure was 115.1kPa and flow rate was ~9 SCMM. Experiments were conducted from 0-120 minutes. Fly ash samples were collected from the reactor sample port over a period of 0-120 minutes. Control and treated fly ash samples were analyzed for total inorganic carbon (C), sulfur (S), and mercury (Hg).

In order to estimate the costs associated with a full-size reactor, the capital investment and operating costs of the pilot reactor were first estimated. The total capital investment for the pilot reactor was estimated using the Percentage of Delivered-Equipment Cost method which uses the delivered equipment costs as a basis for estimating total capital investments [3]. The operating cost for the pilot reactor were estimated using cost data gathered from the operation of the pilot reactor and industry standards for this type of chemical process. This information was combined with depreciation rates developed from the capital investment analysis to determine total production costs including depreciation. The capital investment and operating cost estimates for the pilot reactor were then scaled up to a full-sized reactor using the six-tenths factor rule [3]. This information was used to develop a simple spreadsheet model to conduct a break-even analysis of the process. The data from the break-even analysis was then incorporated into a cash flow model to consider the financial feasibility of the process in terms of the time value of money.