Investigating Retrofitting an Existing Geothermal Power Plant with a Biomass Gasifier for Additional Power Generation

Abstract

This paper investigates different hybrid configurations for integration of biomass gasification into an existing geothermal power plant in order to generate additional power. Gasification is a promising energy technology for New Zealand and a hybrid plant in areas where supplies of wood residue and geothermal resources coexist represents a potentially attractive first step in its utilization. Four hybrid configurations were examined to retrofit a biomass gasifier into the Wairakei Geothermal Field. Each of the configurations were evaluated against operational constraints that exist in the field..
Configuration 1: Superheating of geothermal steam for more efficient power generation. A maximum of 12 MWe additional power generated with a 14 t/hr wet wood input with a wood-electricity energy efficiency (LHV) of 31.4%.
Configuration 2: Syngas fired heating used to boil condensate available on site for additional steam generation. A maximum 6.5 MWe of additional power generated with a 16 t/hr wood input with a wood-electricity energy efficiency (LHV) of 15.5%.
Configuration 3: Boiling separated geothermal water immediately after the first separation stage. A maximum 7.3 MWe of additional power generated with a 16 t/hr wood input with a wood-electricity energy efficiency (LHV) of 17.5%
Configuration 4: Heating of separated geothermal water available at Wairakei to increase power generation at an existing binary power plant. A maximum 1.4 MWe of additional power is generated with a 3.5 t/hr wood input with a wood-electricity energy efficiency (LHV) of 15.4%
The resulting electrical efficiencies are generally lower than for gasification gas engine power plants, but are encouraging in suggesting a novel route to the deployment of biomass gasification plant in NZ. The hybrid concepts examined show significant merit in terms of improving current geothermal steam utilization at Wairakei.