Alstom’s chilled ammonia CO2-capture process advances toward commercialization

Dr. Robert Peltier, PE

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Big retrofit market

The industry may have put new coal projects on hold while it deals with carbon paralysis, but greater commercial opportunities for carbon capture lie with the future retrofitting of many of the 1,100-plus existing U.S. coal plants. Whether you prefer your carbon legislation with a cap-and-trade or a tax flavor, the aftertaste will be bitter: the need to build a small refinery on the power plant’s grounds.

Considerable chemical processing is needed to implement all of the post-combustion carbon capture processes that have proven their worth in the lab or at pilot scale and are now advancing toward commercial viability (POWER, October 2006, p. 60). Two processes that seem to have gathered the most steam in the marketplace are the chilled ammonia process favored by Alstom Power (see sidebar) and Powerspan’s Electro-Catalytic Oxidation (ECO) process, which was recently upgraded to include CO₂ removal and relabeled ECO2. Powerspan and FirstEnergy Corp. plan to demonstrate the ECO2 process at a 1-MW (equivalent) pilot scale at the utility’s R.E. Burger plant in Ohio early this year (POWER, October 2007, p. 54).

There’s no doubt that Alstom is about to enter the flue gas treatment market; the company continues to fund an extensive R&D program whose target is to make a CO₂ capture system commercially available before the end of 2011. The evolution of Alstom’s business development plans for its chilled ammonia systems has been transparent from the start:

A 5-MW (equivalent) pilot plant with EPRI and We Energies.
A 5-MW demonstration plant for E.ON in Sweden.
A 30-MW (equivalent) product validation unit for American Electric Power (AEP), followed by the design, construction, and commissioning of a commercial-scale (up to 200 MW) unit by 2011.
A 40-MW (equivalent) product validation facility for Statoil in Norway.

Inside Alstom's chilled ammonia CO₂ capture system

You don’t need a degree in chemical engineering to understand Alstom’s chilled ammonia CO₂ removal process—but it wouldn’t hurt. First, let’s break the entire process down into three separate process blocks (Figure 1) and follow the exhaust gas as it leaves the plant and is treated to remove its CO₂.

1. Coming to a coal plant near you? Schematic of a commercial chilled ammonia CO₂ capture system added between a plant’s existing flue gas scrubber and stack. Source: Alstom

One key point about Alstom’s chilled ammonia design is that it does not require extremely low levels of SO₂ removal from flue gas. If the candidate plant already has a scrubber operating at a 95% removal rate, and its steam system can be reconfigured to accommodate the process steam demand, a chilled ammonia system may be just the ticket, assuming there’s sufficient space for it.

Step 1: Cool and clean the gas

The first step is to cool and clean the flue gas, which typically is at 120F to 140F, is water-saturated, and contains residual amounts of SO₂, NOx, HCl, and particulate matter. Both steps can be accomplished by injecting refrigerated water directly into the gas stream. As the gas is cooled, much of its water content condenses out, carrying the residual contaminants with it. The water is then evaporated in cooling towers, substantially reducing the total flue gas volume. The cooled flue gas leaves as a chilly (35F) and dry (<1% moisture) gaseous substance.

Cooling the flue gas first pays big dividends by reducing the size and cost of equipment required downstream. For example, if the volume of the saturated flue gas is one-third smaller at 32F than at 140F, a smaller, cheaper induced-draft fan would suffice. Flue gas cooling itself consumes only 1% to 2% of the plant’s power output.

Step 2: Absorb the CO₂

The second process step is CO₂ absorption, which is similar to the SO₂ absorption common at many coal-fired plants today. After the 35F flue gas enters the bottom of the absorber vessel, it is forced upward against the current of a slurry containing a dissolved and suspended mix of lean ammonium carbonate (AC) and rich ammonium bicarbonate (ABC). Chemical reactions remove over 90% of the CO₂ in the flue gas, leaving it only with nitrogen, excess oxygen, and low concentrations of CO₂. Any residual ammonia is captured by a cold-water wash and returned to the absorber.

Step 3: Separate the CO₂

The third step of the process takes the CO₂-rich slurry at 1,200 to 1,500 psi (anticipated for commercial use or for transportation to an enhanced oil recovery process or sequestration) from the ABC-rich output of the high-pressure pump and directs it to a heat exchanger. The heat exchanger dissolves the slurry into a clear solution at about 175F and sends it on to the high-pressure regenerator, where additional heat is added by a reboiler to strip away the CO₂ gas. The only by-product of the entire process is a small amount of water; it can either be treated by the plant’s wastewater system or recycled and reused.

A baseline study on the auxiliary load and cost of a full-scale CO₂ capture project found that retrofitting a 462-MW supercritical pulverized coal–fired boiler operating at 40.5% net thermal efficiency would result in only small performance penalties (see table).

The performance penalties of a chilled ammonia CO₂ separation system. Source: Alstom Power