Hybrid Geothermal-Biomass Power Plants: Applications, Designs and Performance Analysis

Hybrid fossil-geothermal power plants have been proposed and studied in detail for nearly one hundred years. This paper reviews the concept of superheating geothermal steam by means of a forest wood waste combustion plant. New Zealand is studied as a promising country for such plants owing to the large sustainable forestry industry and the close proximity of numerous geothermal resources. The Rotokawa I geothermal plant is used as a case study for a hypothetical biomass-geothermal hybrid plant. The original plant, a 29 MW flash-binary combined cycle was augmented by the addition of a biomass-fired superheater inserted in the main steam-line from the cyclone separator to the steam turbine. The brine from the separator was maintained but the steam expanding under dry conditions on exiting the back-pressure turbine was able to provide additional heat to two of the bottoming binary cycles. Owing to the superheat in the steam significantly more power may be generated and the expansion is completely dry allowing a higher turbine isentropic efficiency. The case study shows the hypothetical hybrid Rotokawa I plant out-generates the basic plant by 8,548 kW, a 32% gain, with 6,911 kW coming as additional output from the back-pressure steam turbine, and the rest coming from the two steam-condensate-heated binary units. Since the plant receives 19,330 kWt from biomass combustion, the extra power output may be seen as using the biomass heat with a thermal efficiency of 0.442, much higher than a conventional biomass power plant. The Second Law or exergy efficiency of the hybrid plant is comparable to that of the basic geothermal plant, but somewhat lower owing to the large temperature difference in the biomass superheater. Three more biomass-geothermal hybrid schemes are proposed and studied. System I is a double-flash geothermal plant that uses biomass energy to enhance its performance. System II consists of a single-flash geothermal plant coupled to a biomass plant superheater that delivers superheated steam to a geothermal turbine, with geothermal brine used for feedwater heating for the biomass plant. System III is mainly a biomass power plant that takes advantage of geothermal energy to enhance its performance. All three systems exhibit advantages in terms of net power generated relative to individual plants. System III was optimized for best efficiency and highest power for a given geothermal resource. The paper also includes a survey of the forest waste in New Zealand regarding its availability, cost and combustion properties. The engineering challenges facing the designer of hybrid biomass-geothermal plants are presented and discussed.