Biomass Harvesting in Western USA: An Overview

by R Franklin, CADDET US National Team

Introduction

In the United States, the most active promotion of demonstrated biomass energy technologies occurs through the Western Region Biomass Energy Program (WRBEP). Funded by the Department of Energy and administered by the Western Area Power Administration in Colorado, the programme is a collection of biomass energy development projects in 13 western states. This article highlights just a few of these diverse projects, which deal with a range of biomass resources including cotton stalks, root-fuel, animal manure and mesquite shrub.

Shrubby Biomass

In Texas and New Mexico, WRBEP has supported field trials of a harvester that efficiently cuts shrubby vegetation. This project also assessed the production costs and usability of three types of vegetation for fuel: mesquite, pinyon juniper and sagebrush.
 In Texas alone, it is estimated that harvesting mesquite could yield up to 2 x 10 to the power 10 GJ at 19 GJ/tonne. After harvesting, the biomass can be shredded for direct combustion or gasified to produce biofuel.

Animal Waste

Another project in Texas aims to develop a series of anaerobic digestion ponds to produce biogas, algae and fish. The expanding aquaculture market presents an export opportunity for companies that can improve manufacturing methods for cultured fishery products.
 Manure and water are combined in a mixing tank to produce slurry. This is pumped to a digestion pond where it is retained for 17 days for methane capture. The slurry then flows, by gravity, to another pond for second-stage digestion to grow microalgae. The residue from this process is then used to assist in fish culturing.
 In a separate project, in Colorado, a use is being developed for the 12 million tonnes (dry weight) of pig manure produced in the USA every year. Iowa State University is to design and build a field-scale anaerobic sequencing batch reactor on a pig farm near Wellington. The demonstration system will be operated at various hydraulic and organic loadings and temperatures to evaluate its performance.

Cotton Stalks

Arizona farmers grow, on average, more than 160,000 hectares of cotton annually. State law requires that waste cotton stalks are ploughed under the fields, but this is energy-inefficient, can lead to soil erosion and stirs up unacceptable levels of dust. The specific energy of cotton stalks is in the range 17-18 GJ/tonne, which compares favourably with wood. The WRBEP therefore supports a project testing the most efficient ways to harvest this potentially useful resource.
 Two extraction devices and three collector/baler systems were tested:
Four-row cotton-stalk puller
 This device was developed specifically for the project. It mounts onto a standard tractor and uses pairs of counter-rotating pneumatic tyres to pull each row of stalks from the ground. The stalks are thrown up to the rear, where they are funnelled together on the ground by a pair of shields, ready for collection.
Two-row uprooter-shredder-mulcher
 This Israeli-built equipment undercuts the stalks and pulls them from the soil. For this project, the implement was modified so that the stalks are chopped and blown into a following wagon, rather than onto the ground.
Round-balers
 Originally developed for forage harvesting, these roll the cotton stalks into cylindrical bales, bound with twine. Balers are available in large or small sizes.
Forage harvester
 This implement picks up and chops previously extracted stalks and blows the material into a wagon.
Cotton module makers
 These are designed to produce large, dense packages of harvested seed cotton. The project evaluated half-length modules made from cotton stalks.
 The four-row cotton-stalk puller, combined with a small round-baler, appears to be the most promising combination; however, the forage harvester recovered a greater percentage of available stalks. Economic evaluation of both harvesting systems is currently under way, as several variables determine the best system for a specific farmer.

Root-fuel

Many Navajo Indian homes are heated by burning wood or coal. A project in New Mexico is examining the potential of root-fuel from the buffalo gourd, a native feedstock, as an alternative. Advantages of root-fuel include quicker growth and cleaner combustion than many wood fuels.
 In addition to the technical aspects, the project has endeavoured to increase Navajo tribespeople's awareness of indoor air pollution from wood and coal smoke.

Food-processing Waste

Waste streams from food-processing operations are being used to generate methane in an anaerobic digestion facility at the University of Texas. The digester is heated with energy from a saline gradient solar pond.

For more information on these or the other projects in the WRBEP, contact the CADDET US National Team in Golden, Colorado.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.