In addition to its efficiency and low capital costs, a gasification combined-cycle system would allow the use of locally grown biomass energy crops in addition to biomass wastes of the mill to generate electricity. The excess power that is generated can be sold to local customers. A majority of the recovery furnaces and conventional power boilers in existing pulp and paper plants are 20 to 30 years old and more than half of them will need to be replaced or upgraded in the near future. There is an opportunity to install new gasification technology in these plants during this capital replacement cycle. The biomass gasification combined cycle system, originally developed by Battelle Columbus Laboratory, works by contacting partially dried biomass residues with hot sand and a conveying stream of steam in a gasifier.
Cyclones separate the product gas from the sand and hot char that are entrained in the gas stream.
The sand and char are returned to a combustor where the char reacts with air to reheat the sand, which is then returned to the gasification vessel. The medium-Btu gas is passed through a heat recovery unit and a water scrubber to remove condensable organic matter. Waste heat is recovered at various points in the process for the generation of steam or for use in process heating applications. It was confirmed that the medium-Btu gas generated could be substituted for fossil fuels and is free of atmospheric nitrogen. A computer model was used to explore the economics of the biomass gasification, combined-cycle technology.