Simulation and Modelling

Parallel to the experimental work, the all different processes including the overall plant are also simulated. Two different approaches are used for the simulation:

Here only the simulation of the mass- and energy balances is described.

TUV has already established a model of the dual fluidised bed gasifier including gas treatment and gas engine in IPSEpro (flow sheeting simulation tool) to solve all mass and energy balances. Into this tool the additional gas treatment and the different applications like FT synthesis or SOFC were implemented. Using the simulation the overal processes was analysed (energetic analysis) and optimised. The integration of the different processes in the plant was investigated (optimal integration in terms of minimising energetic losses). Validation of the simulation took place with experimental data from the biomass cHP Güssing and from results of the 100kW gasifier at TUV. The validated model was also used to safeguard an up scaling of future plants and achieve reliable data for efficiencies and costs.

Description of the simulation tool

IPSE pro is an equation-oriented stationary simulation software developed for power plant simulation. Equation-oriented means that the model equations together with the information from the flow sheet form a system of equations, which is solved by numerical methods (e.g. Newton-Raphson-Algorithm). Stationary means that the simulation result represents a steady state. Thus, no time dependency is modeled. The main advantage of the equation-oriented approach is its fast convergence. This is of great importance when numerous calculation runs must be performed during parameter optimization.

Another advantage of the IPSE pro software is its modular structure as sketched in Figure 15 . The user interface (PSE) with the equation solver (Kernel) calls a model library that contains all information about the inner structure of the apparatuses used in the process. The model library can be changed or created by the user in a separate module of the package, the model editor (MDK).

Modular structure of the IPSEpro process simulation software

The standard package provided by the software developer already contains a model library with specific models for the description of conventional power plants. Streams between the apparatuses either consist of pure water, of a mixture of gases (Ar, CH4, C2H6, C3H8, CO, CO2, N2, H2, H2O, H2S, O2 and SO2 ), or are an organic mixture (elements C, H, O, N, S, free water, and ash). The thermodynamic properties of the different substances are necessary for the calculation of the energy balances as well as for the modelling of thermodynamic processes (e.g. compression, etc.). The property data are defined within the models as functions of temperature, pressure, etc., and are imported from external property libraries. Concerning substances, the standard model library incorporates a property database in the format of a dynamic link library (DLL), which is included in the package. New substances as needed for simulating the Fischer-Tropsch process can be introduced by the user since the creation of additional property-DLLs is possible. IPSE pro thus enables the user to edit the source code of the standard model as well as to create new models for special tasks. This is very important for the effective implementation of new technologies like biomass gasification or biofuel synthesis.

For the simulation of biomass gasification and related processes a special model library has been developed at TUV. The structure of the models has been expanded in order to perform exergy calculations in the streams and to cover inorganic solids for fluidized bed modeling (CaO, SiO2 , ash, and solid carbon). The solids can appear either directly as solid streams or as dust load on gas streams. The apparatuses needed in the biomass gasification process, during gas cleaning have been implemented. The models contain mass and energy balances and specific equations describing chemical conversions, splitting conditions, empirical correlations from experimental findings or data-sheets, etc.

Example of Mass and Energy balances

By the software on the one side the mass and energy balances of new processes can be calculated or the mass and energy balances of existing plants can be validated. As example in the next figure the biomass CHP Güssing was validated and based on these results the different options for optimisation were investigated and evaluated. So the most efficient optimisation needs of the biomass CHP could be identified and some of them were also already implemented.

Conclusion

Plant analysis using process simulation allows the detection of systematic errors in both simulation model and measurements. After elimination of these errors, the simulation describes the actual plant operation best within the limits of the model structure.
The actual model is always improved and further units are implemented. So at the moment the following processes based on the dual fluidised bed gasifier can be simulated: