Design Basis of the Boiler
The 2011 NREL corn stover ethanol plant proposal suggests the use of a combustor capable of handling wet solids, where a fan moves air into the combustion chamber [4]. According to this design, treated water enters the heat exchanger circuit in the combustor, where it is boiled and superheated to high-pressure steam. A multistage turbine and a generator can be used to generate electricity from high — pressure steam. Steam extracted from the turbine at two different conditions is available for use in the ethanol distillation process. In the final stage of the turbine, the remaining steam is taken down to a vacuum and condensed with cooling water for maximum energy conversion. The condensate is returned to the boiler feed water system along with condensate from the various heat exchangers in the process. The steam turbine turns a generator that produces
AC electricity for all users in the plant. The balance of electricity is assumed to be sold to the grid, providing a co-product credit.
NREL technical analysis recommended a Towerpak Stirling power boiler system for the generation of steam from solid waste with 44% moisture content [4]. This system features a live-bottom grated fuel bin to ensure drying and complete combustion of the wet solid fuel. The system was quoted to produce 525,000 lb/h (239,000 kg/h) of steam at 850°F (454°C) and 900 psig in the 2,205 dry corn stover US ton/day plant. Boiler efficiency, defined as the percentage of the feed heating value that is converted to steam heat, is ~80%.
Flue gas from the combustor can be used to preheat the entering combustion air which enters a spray dryer for flue gas desulfurization (FGD). All of the sulfur entering the combustor is converted to sulfur dioxide, and its concentration in the flue is expected to be >1,800 ppm. This level of SO2 requires FGD. The proposed flue gas desulfurization technique involves spraying lime (calcium hydroxide) into the flue gas as a 20 wt% slurry at 20% stoichiometric excess. Thereby flue gas desulfurization converts 92% of the SO2 into calcium sulfate, which falls out the bottom of the spray dryer.
In a cellulosic ethanol plant of this capacity, carbon monoxide is assumed to be generated at a rate of 0.31 kg/MWh, nitrogen oxide (Nox) is generated at 0.31 kg/MWh. NOx formation is a complicated mechanism and depends on the feed, combustion temperatures, excess air rate, combustor design, and the presence of flue gas cleanup devices like flue gas desulfurization [5].
Additionally, according to the NREL proposed design of the combustor-boiler system, of the superheated steam leaving the boiler, 12% is extracted from the turbine at 175 psig (13 atm) and 268°C (514°F) for feeding to the pretreatment reactor and the boiler feed water economizer. An additional 35% is extracted at 125 psig (9.5 atm) and at 164°C (327°F), which is used in the distillation and in the deaerator. The rest of the steam is condensed at -13 psig (0.1 atm). The condensate is pumped back to the boiler.