May 1, 2010

Dry Injection of Trona for SO3 Control

Dr. Yougen Kong, PE and Michael D. Wood, Solvay Chemicals Inc.

In 2006 and 2007, POWER ran a three-part series on the formation of SO₃, O&M issues caused by SO₃, and sorbent injection control for SO₃ control. Three years later, many plants still struggle with their SO₃ mitigation systems or remain undecided on which mitigation path to follow. This article explores the advantages of dry sorbent injection technology.

The formation mechanisms and high costs of controlling SO₃ emissions have been thoroughly discussed in a series of three POWER articles on "SO₃ ’s impacts on Plant O&M" (Part I in October 2006, Part II in February 2007, and Part III in April 2007). In that series, the impact of SO₃, in sufficient concentrations, was shown to penalize plant heat rate, increase plant operation and maintenance (O&M) costs for back-end equipment through the formation of weak acids that are very corrosive, and increase fouling of air heaters and selective catalytic reduction (SCR) catalysts due to the reaction of SO₃ with ammonia. In the three years since those articles were published we still find many utilities struggling to implement SO₃ mitigation measures.

One of the key conclusions presented in the final article in the series was that injection of a carefully selected finely atomized sorbent, such as trona, is very effective in mitigating the formation of SO₃. Dry sorbent injection (DSI) technology for SO₃ control is gaining popularity in the utility industry due to its low capital cost, small installation footprint, ease of operation, and flexibility to adapt to fuel changes.

Trona is a naturally occurring mineral that is produced in Green River, Wyoming (Figure 1). Interestingly, milling of trona is not necessary, as it is already produced as a fine powder. Although milling trona can increase its SO₃ removal efficiency, the cost to do so must be balanced against the added cost of equipment and maintenance.

1.    Raw trona. A microscopic view of raw trona. Courtesy: Solvay Chemicals Inc.