Big Stone City, South Dakota

Air-Quality Control System (AQCS) Project

Client / Owner

Otter Tail Power Company, as agent for co-owners NorthWestern Energy and Montana-Dakota Utilities Co.

Architect / Engineer

Sargent & Lundy, LLC




Graycor Industrial

Project Scope

With the last crane disassembled and just 93 project staff workers on site (including subcontractors), Graycor Industrial is near completion of the Big Stone Plant Air-Quality Control System (AQCS) project near Milbank, South Dakota, for Otter Tail Power Company, owners’ agent, and plant co-owners NorthWestern Energy and Montana-Dakota Utilities Co.

In February 2013, Graycor was awarded the contract as the project’s general contractor, and in late April of that year the project began with a groundbreaking ceremony at the 475-megawatt power plant project site. Purchasing more than $20 million of additional equipment, Graycor’s team erected the AQCS, which included complete installation of all underground utilities, site grading, piling, concrete foundations, structural steel, ductwork, silos and dry scrubbers for two baghouses and two SCR (selective catalytic reduction) systems, including ACI (activated carbon injection).

Graycor also hired and managed the employees and subcontractors required to complete the project, which has been in commissioning and now startup while final tie-ins are being made before its August 2015 scheduled completion. The project has provided nearly 600 jobs, and as of late June 2015, more than 2.2 million total OSHA hours have been logged.

Graycor was chosen following an extensive evaluation process that began with 36 potential contractors, The basic criteria for contractor selection were price, commercial terms and technical specifications or work scope, which included safety performance.

Big Stone Plant’s AQCS, which must be installed by May 2017 according to Environmental Protection Agency regulations, includes about $490 million in retrofits to the plant and is expected to reduce both nitrogen oxide and sulfur dioxide by 80 to 90 percent.

Graycor has recently completed more than 20 air-quality control system projects for a variety of clients throughout the U.S. Additional information regarding the Big Stone Plant AQCS project is available at

1.6.15 – Otter Tail Power’s Big Stone Plant AQCS Project Continues to Make Progress