Electricity produced by Fischer-Tropsch (FT) process are generated by generators. Key components of the power generation of the renewable energy consist of a boiler, gas turbine generator, Heat Recovery Steam Generator (HRSG) and steam turbine generator with condenser. The power block resembles a typical combined cycle system with an auxiliary fuel gas fired boiler to supplement the steam turbine capacity.
The following key components are illustrated using the assumed inputs: A renewable energy plant converting 1000 TPD (dry) urban green waste can generate 600 BPD of Fischer-Tropsch fuels and 16.5 MW of export electricity to yield 466 bbl/day of "green diesel" or syngas and 166 bbl/day of Naphtha.
A fired boiler produces high pressure/temperature (900 psig, 750°F) superheated steam to augment the steam turbine power generation capabilities of the power block. The boiler will have a rated capacity of 58 million btu/hr utilizing fuel gas from the mix drum and distribution header. Principal sources of fuel gas are Fischer-Tropsch off gas, product upgrading off gas and Hydrogen pressure swing adsorption (PSA) unit tail gas. Natural gas is added at the mix drum to supplement fuel gas demand during startup and low fuel gas production periods.
Saturated steam at 900 psig from the partial oxidation reactor steam generator is fed to the boiler and superheated to the 750°F temperature required for the steam turbine. Superheating of the 87,200 lbs/hr of saturated steam requires nominally 11.5 million btu/hr of the boiler firing capacity. The remaining 46.5 million btu/hr of capacity generates an additional 35,000 lb/hr of superheated steam. The large steam superheating load in the boiler will limit the maximum steam temperature attainable.
During startup, the boiler fueled with natural gas produces steam for catalyst activation and initial heating of the production units. Additionally, the boiler is utilized to support maintenance and turnaround activities.
In order to control emissions to Best Available Control Technology (BACT) levels, low-NOx combustion provisions are provided in the burner system as well as a Selective Catalytic Reduction (SCR) system fitted with a CO catalyst to further condition the boiler exhaust gas prior to release to atmosphere.
Combustion Gas Turbine
Fischer-Tropsch tail gas and cleaned Syngas are routed to the power generation system where it is mixed and burned in the combustion gas turbine to generate 16.5 MW of electricity. A booster compressor increases the pressure of the fuel gases feeding the gas turbine. Syngas and FT tail gas have a low heating value compared to standard natural gas. Hydrogen content in the fuel results in a high combustion temperature. Due to the low heating value of the fuel and hydrogen content, standard low-NOx combustion systems with nitrogen or water injection cannot be utilized in the gas turbine to offset the effects of the high combustion temperature. Projected NOx emissions from the unit are estimated at 72 ppmv @ 15% oxygen. Carbon Monoxide (CO) levels are projected to be 50 ppmv. The gas turbine will utilize natural gas for startup.
Heat Recovery Steam Generator (HRSG)
A HRSG is utilized to capture the gas turbine expanded exhaust heat. The HRSG is utilized to preheat feedwater, generate steam and superheat the steam generated for use in the steam turbine. Turbine exhaust generates 48,500 lbs/hr of 900 psig, 750 °F superheated steam in the HRSG.
Because of the high NOx and CO levels from the gas turbine-generator combustion, a Selective Catalytic Reduction (SCR) system with CO catalyst is integrated into the HRSG to provide BACT to the emissions.
The total steam produced (171,900 lb/hr) in the boiler and HRSG is expanded in a condensing steam turbine generator set to produce additional electrical power of about 18.5 MW. Total electrical generating capacity from the facility is 35 MW. Internal power requirements are nominally 20 MW for a net export power production from the integrated system of 15 MW.
The steam from the low-pressure turbine exhaust is condensed by the air cooled heat rejection system. Condensate is routed to the condensate drum for return to heat recovery and the plant header. A vacuum system removes non-condensibles to maintain the required pressure level at the turbine exhaust.