The following is taken from the Larimer county website:
"We generally only allow clean, dry slash to be burned; never trash. Certain activities are exempt from needing a burn permit from the health department.
Most outdoor burning in Larimer County requires a permit signed by both your local fire department and the Larimer County Department of Health and Environment. The fire department evaluates applications for potential fire safety issues, while the health department evaluates air quality issues that can impact human health.
Open burning in Larimer County is regulated by the Colorado Department of Public Health and Environment/Air Pollution Control Division under Colorado Air Quality Control Commission's Regulation One and Regulation Nine. Open burning without a permit is illegal and can result in fines of up to $10,000 per day."
Prin the application form available here and mail it to:
Larimer County Department of Health and Environment
1525 Blue Spruce Drive
Fort Collins, CO 80524
Visit the Larimer County Department of Health environment at the address listed above and fill out an application.