1. Thermodynamic analysis of a water tube boiler operating on
natural gas with flue gas recirculation and an economiser
Fraser McGill
Division of Electronic Engineering, Physics and Renewable Energy, School of Engineering, Physics, and
Mathematics, University of Dundee, Dundee DD1 4HN, Scotland
Introduction
The climate of the planet Earth is changing. To say that the human race is not the only contributor to climate change but that the human race is
definitely contributing would be a statement that is unanimously accepted. Natural gas is an important fossil fuel with an increasing rate of
consumption. The increased demand for natural gas is because natural gas burns with less greenhouse gas emissions than other fossil fuels.
Therefore, through transitions to a more environmentally friendly fuel, natural gas can be the first step.
As the supply of fossil fuels peak and then reduce, the cost will only ever increase, this coupled with additional climate taxation of fossil fuels will
drive the market to change their fuel choice or increase their efficiency.
The idea of thermodynamic analysis has been applied to many combustion related processes with the aim of increasing the efficiency of the fuel use.
Through an extensive literature search, the thermodynamic analysis of the above named water tube boiler configuration has not currently been
published, this highlights the individuality of the honours project. The boiler configuration to be analysed is commonly located at industrial
operations where steam is used as a heat transfer mechanism.
Table 1
Averaged natural gas composition for Scotland in 2010 [1]
Carbon Neo- Iso- N-
% Nitrogen Methane Ethane Propane Iso-butane n-Butane Hexane
Dioxide Pentane Pentane Pentane
Average 0.8756 2.4414 88.1825 6.1176 1.7854 0.1772 0.2925 0.00002 0.0467 0.0478 0.0332
Max 1.1468 3.1870 89.4403 7.1864 2.3847 0.2370 0.4362 0.0001 0.0740 0.0776 0.0525
Min 0.7109 1.8239 86.6052 5.3103 1.2087 0.0857 0.1251 0.0000 0.0106 0.0086 0.0037
Figure 1
Boiler tubes, water containing drums and combustion gas path
Analytical Method
During a controlled test where the boiler modulation was held at 95%, a ‘snapshot’ of the process variables was captured. The subsequent use of the data began with Psychrometric
analysis of the combustion air, followed by Stoichiometric combustion analysis specific to the measured gas composition for that day. The initial calculations created the foundation of
information that allowed for the application of the thermodynamic laws.
The boiler was deconstructed into manageable control volumes upon which thermodynamic analysis took place, these control volumes are depicted in Figure 2. The analysis used was
the first law of thermodynamics and the second law of thermodynamics. The second law of thermodynamics allowed for the analysis of Entropy generation in the form of Exergy
analysis.
The second law of thermodynamics result can be represented as an Irreversibility per kilogram of steam output or water input and an Irreversibility per kilogram of fuel consumed. Any
decrease of the Irreversibility will directly translate to an increase in the second law efficiency and indeed the first law efficiency.
Figure 3
Pressure – Enthalpy diagram for the steam raising process
1000
Example of Exergy calculation for substance k relative to reference condition r ─── Temperature in °C
─── Vapour fraction
─── Saturated liquid
─── Saturated vapour
������������ = ������������ ℎ������ − ℎ������ − ������������ ������������ − ������������ 100
������
Triple point
32.8 Liquid saturation line
33 21barg saturated liquid
Feed Pump
20.55
Economiser
Pressure - bar(a)
10 Economiser to Boiler
Boiler
Exergy Analysis Vapour saturation line
Process
The application of exergy analysis consists of using a combined form of the first law and second law of thermodynamics to D/A Feed pump
establish the performance of a system compared to the theoretical reversible limit. The exergy of a substance is the amount 1
D/A
of work which can be produced by the substance as it comes from an energy state that is higher than a reference condition
to an energy state at the reference conditions. Exergy is not subject to conservation laws and it can be described as being
increased or decreased during a process. The further a process veers from the ideal case, the more of a substance’s exergy is
destroyed. Exergy essentially measures how well energy is transformed and it can give an indication of the performance of
the transformation with respect to the ideal case. [2] 0.1
0 500 1000 1500 2000 2500 3000 3500 4000
Enthalpy - kJ/kg
In the analysis that has been undertaken, the area of greatest interest with respect to exergy analysis is the combustor
control volume (Control volume 2 in Figure 2). The greatest exergy destruction takes place in the combustor control
volume, therefore, developing a slower reaction with more than one stage will improve the exergy efficiency of combustion. Future work
With reference to Figure 3, a future improvement to the steam
raising process would be to decrease the pressure that the feed
water is supplied to the boiler as the additional work currently
Figure 2 provided to the water by the pump serves little use. The feed water
Control volumes for the thermodynamic analysis pressure need only be at a pressure greater than the steam
generation pressure. The difference could be as small as 1bar but to
allow for a factor of safety of 3, this could increase to 3bar of a
Air intake Combustion Boiler heat exchange Economiser heat exchange Flue Gas Recirculation takeoff difference. This would create a yearly saving of 67MWh of
mAI + mFGR = mCA mCA + mF = mPC mPC = mBE mBE = mFG mFG - mFGR = mSG electricity.
mAIhAI + mFGRhFGR = mCAhCA mCAhCA + mFhF = mPAhPA mPChPC = mBEhBE – QBHE mBEhBE = mFGhFG – QEHE mFGhFG - mFGRhFGR = mSGhSG
QBHE = QSTM + QHTLS-B QEHE = QE + QHTLS-E Further refinement of the thermodynamic model could be achieved
if there were less assumptions, however this would require
Air In, AI Combustion Air, CA Products of Combustion, PC Boiler Exhaust, BE Flue Gas, FG Stack Gas, SG intrusive measurements of pressure and temperature with the
combustion chamber and a shift towards a computer based model.
1 2 3 4 5
The novel proposal of thermodynamic analysis of a water tube
boiler operated on natural gas with flue gas recirculation and an
economiser is a complicated scenario compounded by daily
variation in it’s operating conditions. The variables of the operating
Flue Gas Recirculation, Fuel, F Steam, STM
Economiser in, Flue Gas Recirculation, conditions would constantly create a unique conclusion if the
Economiser out, E-OUT E-IN FGR
FGR analysis were performed with a timescale of days to weeks. Any
stable model yielding a replicable result would be required to
ascertain large data sets over a timescale of months to years. Such a
Steam generation Feedwater preheating data set and timescale lends itself well to computer based
mE-OUT = mSTM - mBLD mE-IN = mE-OUT modelling.
mSTMhSTM = mE-OUThE-OUT - mBLDhBLD + QSTM mE-INhE-IN = mE-OUThE-OUT + QEHE
Acknowledgements
Supervisor: Dr R.A. Gibson
Special thanks are extended to Gerry Bannister of Michelin Tyre Company
References
[1] National Grid daily calorific values (2010).
[2] M.A. Rosen, I.Dincer, M.Kanoglu Energy Policy. 36, 128-137 (2008).
Degree Programme: BSci Renewable Energy