Process Designs and Cost Estimates for a Medium BTU Gasification Plant Using a Wood Feedstock

Abstract

A gasification plant to effect the conversion of wood to medium-Btu gas has been designed. The Purox gasifier and associated equipment were selected as a prototype, since this system is nearer to commercialization than others considered. The object was to determine the cost of those processing steps common to all gasification schemes and to identify specific research areas. A detailed flowsheet and mass-balance are presented. Capital investment statements for three plant sizes (400, 800, 1,600 oven-dry tons per day) are included along with manufacturing costs for each of these plants at three feedstock prices: $10, $20, $30 per green ton (or $20, $40, $60 per dry ton). The design incorporates a front-end handling system, package cryogenic oxygen plant, the Purox gasifier, a gas-cleaning train consisting of a spray scrubber, ionizing wet scrubber, and condenser, and a wastewater treatment facility including a cooling tower and a package activated sludge unit. Cost figures for package units were obtained from suppliers and used for the oxygen and wastewater treatment plants. The gasifier is fed with wood chips at 20% moisture (wet basis). For each pound of wood, 0.32 lb of oxygen are required, and 1.11 lb of gas are produced. The heating value of the gas product is 300 Btu/scf. For each Btu of energy input (feed + process energy) to the plant, 0.91 Btu exists with the product gas. Total capital investments required for the plants considered are $9, $15, and $24 million (1978) respectively. In each case, the oxygen plant represents about 50% of the total investment. For feedstock prices from $10 to $30 per green ton ($1.11 to $3.33 per MM Btu), break-even costs of fuel gas range from $3 to $7 per MM Btu. At $30/ton, the feedstock cost represents approximately 72% of the total product cost for the largest plant size; at $10/ton, it represents only 47% of product cost.