Chapter 2 presents a description of the backpressure CHP plant considered in this thesis.
The design-point and part-load modelling of the main boiler and steam cycle components is described, as well as the constraints likely to limit the plant operation at minimum and maximum load, or when another process is integrated to the plant. The annual district heat
load profile, and the multi-period approximation of the plant annual operation is also presented in this chapter.
Chapter 3 presents the thermochemical biomass conversion modelling. The mathematical models of the drying, heating, conversion and cooling processes, as well as the mass and energy yields as functions of operating parameters are described. Configurations of the stand-alone carbonization plants and the relevant component models are also covered in this chapter.
The technical studies of integration of both thermochemical conversion processes to the CHP plant are described in Chapter 4. For both processes, the results of the initial technical analysis at one or a small number of operating points are presented. The impact of carbonization temperature is also evaluated. On the basis of the initial technical studies, the most promising cases are selected for more detailed study. These cases are analysed in more detail by using the multi-period model described in Chapter 2. The results are used to obtain data on how each of the integrated plant configurations affect the CHP plant operation, and to find the total annual fuel and feedstock consumptions, and the amounts of heat, power and biochar produced. Chapter 5 presents the economic analysis of those cases that were chosen for detailed operational and annual net production and consumption analysis in Chapter 4.
Chapter 6 presents the condenser heat transfer modelling. After starting with the initial heat transfer model of the large condensing power plant sea water condenser, the validation and analysis of the necessary level of modelling detail, the chapter continues to describe how this model was adapted for a district heat condenser model to be used in optimization, which is then described in Chapter 7. The mechanical sizing and cost model of the condenser is described, followed by the optimization algorithms used, and the implementation of the objective function and the optimization.
Finally, chapter 8 summarizes the main findings of the thesis. The answers found to the research questions are presented here. In some cases also some significant limitations imposed by the assumptions applied in the work were found, and these are also explained.
The thesis concludes with a description of some of the main issues still requiring further work.
2 CHP plant and multi-period model
A small modular biomass-fired backpressure plant with a 29 MW thermal output bubbling fluidized bed (BFB) boiler based on the one described by Komulainen (2012) is considered in this study. At design point conditions the plant has a net output of 20 MW district heat and 8 MW electricity. The turbine has a partial admission regulating stage and separate high-pressure (HP) and low-pressure (LP) parts with an extraction at the HP exhaust, controlled by the LP turbine inlet valve. The design point extraction pressure is 8.5 bar. There is a single backpressure DH condenser.
The schematic diagram of the CHP plant is shown in Figure 2.1; the design point operating parameters and ambient conditions are summarized in Table 2.1. IPSEpro process simulation software has been used to model the CHP process. The model was developed initially for Publication III and later improved for Publication IV. This chapter describes the final model of Publication IV, which was also used in publications V, VI and VII to obtain the main results of the thesis.
To consider the annual variation of the DH load, ambient conditions and fuel properties, a multi-period model was implemented. Off-design models of the main components were developed to evaluate the performance of the plant at varying loads and ambient conditions. The multi-period model is presented in chapter 2.1, followed by a description of the design-point and off-design modelling of the boiler in chapter 2.2, and the steam cycle in chapter 2.3.
Figure 2.1: Schematic diagram of the CHP plant model.
2.1
Multi-period modelA district heat load duration curve was approximated by a peak load of 35 MW, 20 MW heat load at 1800 hours, linear reduction to 2.6 MW at 7890 hours, and finally steady 2.6 MW load for the remaining summer hours. This was represented by two full-load periods, P1 and P2, followed by a steadily reducing DH load at 4 MW intervals (P3 to P6) until the summer period, which was split to a low-load P7 and minimum-load P8.
The moisture of the wood chips was assumed to increase towards winter. The temperature of the boiler fuel and HTC feedstock was set at the average ambient temperature of each
period. The combustion air temperature, taken from the boiler room, was assumed to be 20 °C higher than the ambient temperature. The ambient temperatures were based on 30-year monthly average temperatures gathered by the Finnish Meteorological Institute (Finnish Meteorological Institute, 2015) for Jyväskylä, a city in central Finland. The DH water output and return temperatures were based on ambient temperature according to Koskelainen et al. (2006). The data for fuel properties and temperature levels for each period are listed in Table 2.2; load curve approximation and multiperiod approximation of heat and power production are plotted on Figure 2.2.
Table 2.1: Main characteristics of the CHP plant at the design point.
Category Parameter Quantity
Fuel and ambient conditions
Wet-basis fuel moisture MC Fuel LHV, moist fuel / dry matter Ambient temperature
Regulating stage isentropic efficiencys,R
HP turbine isentropic efficiencys,HP
LP turbine isentropic efficiencys,LP
Extraction pressure Condenser Back pressure
DH water output/return temperature
0.80 bar 90/50 °C
Deaerator Pressure 5.6 bar
Generator Gross electric power 8.66 MW
CHP plant parameters
Net electric power District heat (DH) power Electrical efficiency el
Total CHP efficiency tot
8.00 MW 20.00 MW 24.0 % 86.0 %
* Valves wide open
Figure 2.2: Annual district heat load variation (black) and production of district heat (red) and electricity (blue) in the CHP plant.
0 10 20 30 40
0 2000 4000 6000 8000
District heat; electricity [MW]
Time [h]
Table 2.2: Summary of the load points and their durations during a year in multi-period approximation of annual plant operation.
Parameter P0 P1 P2 P3 P4 P5 P6 P7 P8
The boiler model consists of furnace, superheater and economizer components, and an IPSEpro standard library heat exchanger representing the air preheater (luvo). A steam coil air heater (SCAH) is available as well.
The furnace module determines all boiler losses except the stack loss. The losses at the design point are stack loss stack = 2.5 MW (Tstack = 150 °C); radiation loss rad = 0.1 MW; blowdown loss at 1% of feedwater flow bd = 0.1 MW; ash heat loss ash = 0.02 MW; unburnt loss ub = 0.2 MW; and other losses 1% of fuel power, other = 0.3 MW.
These yield a design-point boiler efficiency of b = 0.88, defined as
f f
a awhere subscripts LS, FW, f and a refer to live steam, feedwater, fuel and air.
The radiation and conduction losses loss,rad [kW] were assumed constant, and estimated from net output at maximum continuous rating b,MCR (European Committee for Standardisation, 2003),
rad = 0.0315 b,MCR 0.7. (2.2)
Ash losses ash consist of the sensible heat lost with ash removal as bottom ash at bed temperature Tbed, and fly ash from the filters at stack temperature Tstack :