Case study: Development of a swath harvester for small diameter (

Abstract

A 216 kW biomass harvester, modified from a John Deere silage harvester, was field tested in Texas and New Mexico to examine the productivity and cost of harvesting shrubs and small trees for energy purposes. The harvester was tested on mesquite ( Prosopis glandulosa var. glandulosa) in Texas and salt cedar ( Tamarisk pentandra) and pinyon ( Pinus edulis)/juniper ( Juniperus monosperma) stands in New Mexico. While the harvester severed and chipped a few individual 20-cm basal diameter trees, the harvester was much more efficient harvesting dense stands of small trees that were less than 10-cm in basal diameter. During the course of these trials, major modifications were made to increase the efficiency of the cutterhead and the materials handling system. After these modifications, when harvesting mesquite stands less than 10-cm in basal diameter, the machine harvested at the rate of 0.95 ha h −1 with a fresh weight harvest production of 7050 kg h −1. Using $70 h −1 operating cost data for similar commercial equipment, we estimated an energy cost of $1.00 kJ −1 which compares favorably to energy sources such as low sulfur Wyoming coal, natural gas and fuel oil. The swath harvester described here did not bale the chips. However, baling trials with 3 commercial balers found that 2 commercial balers could pick up dense windrows of the chips and make a satisfactory 300 kg square bale and a 595 kg round bale. The large square bale with a density of 319 kg m −3, provides an opportunity for full load potential of flat bed truck trailers to be realized. As the harvester is built on an agricultural frame, it is not sufficiently robust to operate in a forestry environment. A commercial version will need to be built on a high clearance, heavy duty frame with 4 wheel drive similar to a forestry skidder. Brown Bear Corporation, the company that manufactures the cutterhead, is anxious to build this harvester for a purchase price of about $280000. We estimate that an annual demand for about 12000 Mg of biomass at $9 per green Mg will be necessary to justify the purchase of the first harvester. The market potential for non-energy related biomass i.e., potting soil base, landscape mulch, wood chips for bioremediation, mesquite barbecue products, appears sufficiently great in some locations to justify purchase of the first commercial version of a harvester.