Biomass Feasibility Project Final Report - Xcel Energy

Manures
Variability. Depending on their species, housing systems, diets, and digestions, livestock produce
manures that contain different moisture levels and BTU densities. Beyond that inherent
variability, certain amendments like rinse water, wood shavings, straw, and corn stover that are
added to manure for easier handing or livestock comfort, introduce further inconsistencies.
Limited data. Calculating manures for BioPET from available data can be a challenge. The
starting point is the NASS database of Minnesota livestock production numbers by county, but
limitations in those data force us to derive figures from other clues as well. For example, since
NASS doesn’t track all animal species found on farms, we model numbers and sizes of animals
based on our knowledge of the way livestock are raised. Since NASS takes inventories of various
species of livestock at various times of the year, the data do not provide a snapshot. Some data
are not reported at all because of privacy.
Inferences. We have to infer the number of livestock on farms in each county from no less than
sixty columns of figures on thirty-five classes of livestock raised or fed in Minnesota. In some cases
we base our categories on amounts of corn, soybean meal, and oats consumed by those thirtyfive
animal classes. In other cases we base estimates on the number of days in a year that a
particular class of animal is alive on a Minnesota farm to eat feed and excrete manure. We
gather estimates of days on feed from published budgets and from University of Minnesota
animal scientists who have specific information on livestock. We assume that some classes of
livestock, like dairy cows, are alive 365 days a year, while others, like broilers, live just 56 days. We
calculate pounds of manure excreted daily for each class of livestock on a wet basis and a dry
basis. Excreted manure of poultry tends to be 25% total solids, while swine manure tends to be
10% total solids (MWPS, 2004).
Our numbers for animals of a particular class reflect either NASS data or estimates by the project
team. The number of animals in each county is derived from the various crop reporting districts
of the state. Final values show the number of tons of manure annually produced by the classes
of livestock in each county. We show estimates for both wet and dry tons in order to help the
developer assess transportation costs and calculate BTUs on a dry-ton basis.
The Conservation Reserve Program (CRP)
The federal Conservation Reserve Program (CRP) pays landowners to convert their cropland to
permanent cover, such as grassland or trees, for a term of ten or fifteen years. Under current
program rules, lands in CRP cannot be harvested. But if that policy changes, they could grow
biomass for energy.
We calculate potential energy contents of these lands by first aggregating all 25 official cover
types (warm season grass, wetlands, wildlife plots, cool season grass, etc.) into the three main
categories of grass, trees, and other, and then applying a common yield estimate for each of
these broad categories. For grasses, we estimate 2.2 tons per acre per year based on the
research of Tilman, Reich, Knops, Wedin, Mielke, and Lehman, 2001. For brushland, we settle on
1 ton per acre per year based on a low-yield estimate in the forest softwood feedstock
calculations. We then calculate the energy content of each broad category using the
parameters mentioned previously.
Identifying Effective Biomass Strategies: Page 27
Quantifying Minnesota’s Resources and Evaluating Future Opportunities