(547e) Energy Efficiency Calculation of a Combined Heat and Power(CHP) Plant Integrated with Torrefaction Process Using Aspen PLUS

Authors: 
Yun, D. Y., Chung-Ang University
Lee, C. J., Chung-Ang University
Bach, Q. V., Chung-Ang University
According to the climate policy in the European Union, we should reduce the greenhouse gas emission and increase the portion of renewable energy in the final energy. So, biomass is recognized as an important renewable energy source and contributes to approximately 10% of the global energy consumption. Combustion of biomass is almost used for heat generation in the small scale plants, or combined heat and power (CHP) plant. Torrified biomass shows similar characteristics those of coal, like comparable heating value, hydrophobicity, good grindability and so on. Therefore, Torrified biomass can be co-combusted with coal.

Torrefaction is a pretreatment method for improving biomass’s thermal property as a fuel. The procedures of torrefaction process is as follows : drying, torrefaction, cooling. The process is simulated in ASPEN PLUS V8.8. A drier using air is simulated in our model. Through the drier unit, the feedstock biomass moisture contents is reduced from 50wt% to 10 wt%, wet basis. Torrefier is the main reactor of this process, where the biomass feedstock is thermally treated to produce torrefied biomass and by-products. Due to the complexity of torrefaction, it is difficult to use a pre-defined reactor in Aspen Plus to model this process. Consequently, it normally requires several blocks and calculators with FORTRAN codes to simulate this reactor. Reduction of mass and energy yield and improvement in heating value of the torrefied biomass are observed and optimal temperature of torrefaction are between 275 and 278 degrees. Total Energy consumption and process energy efficiency is calculated from the model. The drier consumes 76-80% of the total heat demand.

A CHP plant model is also simulated in ASPEN PLUS. There are three part of the process : fuel combustion, steam generation, steam turbine cycle. In fuel combustion part, we use Yield reactor and Gibbs reactor to simulate fuel combustion with calculator box. To make superheated steam, the boiling feed water pass an economizer, evaporator and superheater. Superheated steam is led to a High pressure(HP) turbine and the outlet stream is separated back to boiler to reheat the used steam. The reheated steam is injected Intermediate pressure(IP) turbine with HP turbine outlet stream. Finally the steam pass through the low pressure(LP) turbine and condenser. The condensed water is heated by the steam extracted from each turbine, pass boiling feed water pump, recycled to the economizer. From the CHP model, we can calculate electric efficiency and total energy efficiency. Conventional CHP plant’s electric efficiency is about 30 to 40% and total energy efficiency about 70 to 80%. And will perform a case study to replace 25%, 50%, 75%, 100% of the fossil fuel in boiler using the torrefied biomass. Compared with coal, torrefied biomass have competitive both efficiencies on the model and reduce the emission of CO2.

Within the CHP plant using steam turbine, the possible spots of integration of torrefaction in CHP plant are various because of operating temperature of torrefaction including drier, usually 100 to 300 degrees. In flue gas from the combustion, we can easily find the integrating points due to stability of the flue gas temperature. So, we devide the scenarios of integration: the flow after HP turbine, IP turbine and flue gas stream. After this, we calculate energy efficiency of the integrated process, and determine whether the torrefied biomass can replace fossil fuel 100%.