Two coal-consuming cement plants in Montana, run by Ash Grove Cement and Holcim (US) Inc., have gotten a pass from the U.S. Environmental Protection Agency when it comes to the question of whether they need to change their coal use due to regional haze concerns.
EPA on April 20 published in the Federal Register a proposed regional haze Federal Implementation Plan (FIP), since the state of Montana decided not to submit to the agency its own revised State Implementation Plan. In its proposed FIP, which is now up for public comment, EPA found that the Ash Grove and Holcim plants are Best Available Retrofit Technology (BART)-eligible. Under BART, the agency reviews three different pollutants, NOx, SO2 and particulates.
The EPA focus for the Ash Grove Cement plant near Montana City was the facility’s long wet kiln, which is the primary source of SO2 and NOx emissions. EPA identified the following available SO2 control technologies: dry absorbent addition (DAA), fuel substitution, raw material substitution, lime spray drying (LSD), semi-wet scrubbing, and wet scrubbing. Ash Grove currently uses a mix of coal and petroleum coke as the primary kiln fuels. In 2008, Ash Grove used 50% petroleum coke, 41% coal and 1% natural gas. The sulfur content of the petroleum coke was 5.2% and the sulfur content of the coal was about 0.8%.
“If sulfur in fuel input to the kiln were reduced by burning a different blend of coal and coke with lower sulfur contents, a reduction in SO2 emissions would be expected,” EPA wrote. “We considered two different options for fuel switching. Option 1 would use 62% coal with 0.8% sulfur and 38% coke with 5.2% sulfur. Option 2 would use 100% coal that has a lower sulfur content (0.7%), and a higher Btu value.”
The kiln inherently acts as an SO2 scrubber, since some of the sulfur that is oxidized to SO2 is absorbed by the alkali compounds in the raw material fed to the kiln. So, considering the scant regional visibility improvements from further controls, EPA proposes that BART for SO2 is no additional controls at Ash Grove. It is proposing a BART emission limit of 11.5 lb/ton clinker (30-day rolling average).
As for NOx emissions at Ash Grove, EPA is proposing that the NOx BART emission limit should be based on what can be achieved with low-NOx burners and selective non-catalytic reduction (SNCR). Ash Grove currently has an electrostatic precipitator (ESP) for particulate control from the kiln, so EPA is proposing a BART limit based on the current system.
Holcim (US) Inc.’s Trident cement plant is located near Three Forks. EPA’s analysis focused on the kiln as the primary source of SO2 and NOx emissions. EPA identified these SO2 control technologies: wet scrubbing, semi-wet scrubbing which for this source is the same as spray dryer absorbers (SDA), fuel switching (lower sulfur fuel), and hot meal injection. Hot meal injection is the hot-meal bypass in a kiln system, where calcined hot meal produced in the kiln is, in part, discharged in front of the kiln entrance after the precalcining process, so that the hot meal can scrub some of the SO2 generated from the kiln feed.
Holcim currently uses a mix of 60% low-sulfur coal and 40% petroleum coke as the primary fuels for the kiln. The sulfur content of the petcoke is about 5.3% and the coal is about 0.8%. EPA considered two different options for fuel switching. Option 1 would use 75% coal with 0.8% sulfur and 25% petcoke with 5.3% sulfur. Option 2 would use 100% coal, which has a lower sulfur content (0.8%) than petcoke. For fuel substitution to 100% coal with 0.8% sulfur content, EPA relied on Holcim’s estimate of 62% control effectiveness. For fuel substitution to 75% coal with 0.8% sulfur content and 25% petcoke with 5.3% sulfur content, EPA relied on Holcim’s estimate of 32% control effectiveness.
“We have concluded that wet scrubbing, SDA and fuel switching are not cost effective control technologies and would not provide substantial visibility benefits,” said EPA about the Holcim plant. “Given the minimal visibility improvements associated with SO2 controls, we propose that the SO2 BART emission limit for the kiln at Holcim should be based on current emissions….”
For NOx at Holcim, EPA proposes that BART is low-NOx burners plus SNCR. Holcim currently has an ESP for particulates and EPA is proposing a standard based on current emissions.