Plasma torch for biomass pyrolysis

Abstract

Our world abounds in diffuse sources of energy, and the engineer's challenge is to concentrate that energy, and convert it to a useful form. When America's farmers grow corn, half of the non-root mass is left to decompose in the field. This corn stover (cobs, stalks and leaves) represents a convenient and plentiful source of energy. The scope of this paper describes undergraduate research into a key component of an apparatus which has the potential to make small- and medium-sized farms net exporters of energy. Although large-scale gasifiers are already used to convert municipal solid waste to combustible synthesis gas (carbon monoxide plus hydrogen), system efficiencies do not scale well to the average farm operation. A common design is the up-draft gasifier, which partly combusts the feedstock while producing the syngas. Direct combustion is less efficient than complete gasification; and up-draft gasifiers further dilute the output gas with non-combustible nitrogen from the air. A different approach, pioneered by our company and patent pending, uses a novel apparatus to efficiently convert farm biomass to syngas, which can then be burned in a generator set to provide electrical power and heat. In order to achieve this efficiency, a plasma torch is used to rapidly pyrolyze corn stover, producing high BTU content syngas and a dry ash rich in soil nutrients. A multidisciplinary team of four undergraduate researchers converted a commercially-available plasma cutter into a plasma torch for use in a farm-scale biomass syngas reactor. This hands-on research project combined the best available published literature on plasma cutters and torches with the accumulated experience of professional engineers from industry to create a working prototype. Following each major development, the student team reviewed their work, and near-term plans, with a review board. Fabricating the plasma torch required the use of computer-aided design tools, and close interaction with the model makers who ran the computer numerical controlled milling machine. Anticipating the bizarre behavior of plasmas, the students integrated the ability to tweak their design after the first operational tests. After the lengthy design process, the test-and-refine phase proceeded quickly. Each student contributed to the understanding and improvement of the performance, engendering a spirit of teamwork, appreciation for diversity, increased problem solving ability, and an appreciation for the value of a thorough design phase. In this paper we briefly review the plasma torch biomass reactor concept, explain the lessons learned by the students, and address the value gained by the company. © American Society for Engineering Education, 2008.