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Fuel-rich HCCI-engines as chemical reactors for polygeneration: A modeling and experimental study on product species and thermodynamics Robert Hegner, Marc Werler, Robert Schiessl, Ulrich Maas, and Burak Atakan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02150 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017
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Fuel-rich HCCI-engines as chemical reactors for polygeneration: A modeling and experimental study on product species and thermodynamics R. Hegner1*, M. Werler2, R. Schießl2, U. Maas2, B. Atakan1 1
University of Duisburg-Essen, Institute of Combustion and Gas Dynamics, Thermodynamics, Duisburg 2 Karlsruhe Institute of Technology, Institute of Technical Thermodynamics, Karlsruhe *
[email protected] Abstract The usage of a methane-fueled homogeneously charged compression ignition (HCCI) engine process for producing base chemicals like ethylene and synthesis gas together with some work output is investigated. This polygeneration process is studied by numerical modeling, accompanied by rapid compression machine experiments. Studies include the seeding of dimethylether (DME) as a reaction enhancer which allows methane conversion already at moderate, technically easily accessible compression ratios and precompression temperatures. The concept is promising for equivalence ratios above 2, predicting product gas mole fractions of up to 25 mol% H2, 20 mol% CO and 2 mol% C2H4. These simulation results are largely consistent with the outcome of rapid compression machine (RCM) experiments, in which production of up to 20 mol% H2, 16 mol% CO and 1 mol% C2H4 was detected. In addition to studying the product composition, thermodynamic aspects of the approach were investigated by comparing the exergetic efficiency of fuel-rich and fuel-lean combustion. These calculations also confirmed the advantages of fuel rich combustion. Keywords: Polygeneration, HCCI, Exergy, Synthesis Gas, Ethylene, RCM
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1. Introduction Due to efforts to reduce the amount of CO2 exhaust in energy conversion from burning fossil fuels, the amount of energy conversion from so-called renewable energy sources increases strongly in different regions of the world, including Europe and especially Germany. The intrinsic problem of the strongly fluctuating power from wind and solar energy conversion is well known and the electrical power production is also not following the power demand within a society. Thus, flexibility in energy conversion and possibilities for energy storage are the focus of many societies. The question addressed in this paper is whether combustion processes or more generally high temperature processes in piston engines might be helpful in gaining flexibility in energy conversion, using piston engines as polygenerating chemical reactors. Polygeneration, i.e., the coupling of energy conversion and chemical conversion towards useful chemicals, is usually discussed in terms of a flexible combination of – otherwise unchanged – processes, such as steel production with electricity production, district heating, and methanol production 1–3. In contrast to this conventional coupling of separate processes, using an engine as chemical reactor would represent an integrated process, where chemical production and energy conversion are closely tied together, which may have a positive impact on the process flexibility. In order to maximize flexibility, polygeneration systems often intend to provide a well-adjusted combination of various outputs. The analyzed motor polygeneration process, would be able to provide, mechanical work, heat and a wide range of chemicals, ranging from syngas to different unsaturated or partially oxidized hydrocarbons. This could be interesting in stationary (in contrast to mobile) systems in times where the amount of mechanical power output is reduced. At such times chemicals could be produced, which could either be used in chemical industry or be burned at later times, the latter being surely less favorable. One important advantage of piston engines is the possibility to increase temperatures quite fast to high levels, such that reactions towards metastable species get quite fast, and due to a lowering of temperature along the expansion, the reaction rates of reactions which further convert the metastable species could be reduced, and the composition could be frozen to some degree. These kind of ideas are, to the best of our knowledge, quite new,
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regarding other products than synthesis gas, and thus a fundamental theoretical thermodynamical-kinetical investigation seems justified, prior to any further economic considerations. Synthesis-gas generation in engines has been studied by Karim et al. 4 with mixtures of CH4/O2-enriched air and pilot ignition by additional Diesel fuel. Yang et al. investigated HCCI combustion in rich CH4/O2 mixtures with added CO and were successful in syngas production, while Szezich investigated a spark ignition (SI) process with CH4 at low compression ratios 5,6. Morsy modeled an HCCI process with methane to find good conditions for syngas production 7 and proposed to start compression at relatively high temperatures to achieve ignition. McMillan and Lawson experimentally investigated a fuel-rich natural-gas SI process, but also modeled an HCCI process 8. With SI, they were successful in producing syngas up to Φ = 1.62. However, only few of these studies are taking thermodynamic or exergetic investigation into account and none of these examine equivalence ratios beyond Φ = 2, where higher hydrocarbons could be promising target chemicals as well. These kinds of processes, which produce higher hydrocarbons by partial oxidation of methane, are known as oxidative coupling. These processes are intensively studied, due to their economical advantages, but most research focuses on oxidative coupling of methane in reactors and the influence of various catalysts 9,10. However Swanenberg investigated the cogeneration of ethylene and work through oxidative coupling, which is a comparable approach to this study 11. In order to study the fundamentals regarding chemical kinetics and thermodynamics, an homogeneous chargecompression-ignition (HCCI) process was theoretically investigated using a single zone model, comparable to the model used by Caton and Zheng 12, but without internal gas recirculation. Product gas composition, work output and exergetic efficiency of the proposed polygeneration engine were studied theoretically. Additionally, crank angle resolved cycles and reaction path diagrams were used to gain insight into the chemical production and consumption process of higher hydrocarbons. Methane, as the main constituent of natural gas, was taken as main fuel, and since it was expected that the needed initial temperatures to achieve a reasonable conversion would get unreasonably high, also mixtures with 10% dimethylether (CH3OCH3, DME) addition were investigated. Typical engine parameters of actual diesel engines were investigated numerically,
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with respect to compression ratio and engine speed. In order to compare the numerically predicted results, of the exhaust gas compositions, experiments were conducted under simplified, well-understood physical conditions in a rapid compression machine. These experiments were carried out in a wide range of equivalence ratios, ranging from 1 to 15.
2. Theoretical Background and Modeling In order to describe and simulate the processes inside the engine, time-dependent chemical kinetics simulations were performed, by developing a homogeneous, single-zone engine model. The simulation consists of a compression and expansion stroke, starting at the bottom dead center (BDC) with closed valves. All concentration changes due to chemical reactions during the engine cycle were calculated in kinetical models using elementary reaction mechanisms. The crucial point in achieving substantial conversion in these kinds of processes, is to produce sufficiently high radical concentrations and temperature after compression that finally leads to ignition. Unfortunately most methane reactions have high activation energies, which necessitate high temperatures at the end of the compression stroke to overcome these barriers, thus, this also leads to a need of high initial temperatures for the fresh gas mixture. It should also be kept in mind that the isochoric heat capacity cv of the initial gas mixture and thus the ratio (cv+R)/cv changes systematically with the air to fuel (A/F) ratio , since the cv values of the diatomic air constituents are lower than the ones of the fuel molecules. At constant compression ratio, this leads to a reduction in the maximum temperature of an unreacted air/fuel mixture at the end of the polytropic compression with increasing equivalence ratios. The equivalence ratio Φ is defined as the amount of air that would be necessary for stoichiometric combustion, divided by the amount of air in the actual combustion case. This leads to higher needed initial temperatures with equivalence ratio in order to ignite the mixture after compression. The exhaust gas of conventional piston engines is rarely utilized in further processes, since the exhaust gas species are usually already fully oxidized and not of interest for chemical processes. However, the exhaust gas of the proposed polygeneration systems contains high amounts of chemically useful species and unconverted energy. In order to consider this amount of energy and to evaluate the process, the exergy concept was used 13, also called available energy. Unlike energy, which is just converted to other kinds of energy, exergy can be
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actually destroyed or ‘lost’. This exergy loss is caused by irreversibilities and can be used to evaluate the conversion of fuel exergy to work and (chemical and thermal) exergy of the exhaust gas. The losses in exergy Ev are easily calculated from the Gouy-Stodola theorem 13 = ,
(1)
from the ambient temperature Tsur, here assumed to be 288 K and the entropy generation Sirr of the engine cycle, calculated from the second law of thermodynamics. The respective exergetic efficiency is calculated with eq. (2), using the fuel masses of methane and DME and their respective, specific standard exergies of 52.281 MJ/kg for methane and 30.953 MJ/kg for DME, taken from 14 . η = 1 −
.
(2)
The total work could be calculated from the calculated pressure and volume along the cycle = −
!"#°
#°
.
(3)
The calculations were performed using the framework of the reactor model implemented in Cantera 15 within Python. Here a homogeneous gas mixture within the reactor is assumed for each state, thus, concentrating on the main chemical processes and neglecting all complications due to transport processes. In contrast to such a single zone approach, HCCI engines are often modeled using multi-zone-models, to account for inhomogeneities, especially with respect to the in-cylinder-temperature. However, due to the high computation times associated with multi-zone-models, and our finding that the main trends are well reproduced in single-zone models, the present study focusses on discussing single-zone-model results, while multi-zone-model calculations will be part of future investigations. The reaction mechanism and the thermodynamics database were taken from the USC mechanism 16 for the neat methane calculations and from the ether mechanisms 17,18 if DME was part of the fuel. The oxidizer was assumed to be dry air throughout. Table 1 Engine specifications and operation conditions. Parameter Displacement VD Bore d / stroke s Comp. ratio ε
Specification 400 cm3 79.5 / 80.5 mm 16.5; 22.0
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Engine speed N Intake pressure p0 Coolant temperature TK
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3000 rpm; 600 rpm 101.3 kPa 373 K
In order to simulate the process including the chemical reaction kinetics, further information and some assumptions about the simulated engine and operation conditions are necessary, which are given in Table 1. In order to simulate compression and expansion the piston speed, and thus the cylinder volume, as a function of crank angle and thus time, has to be calculated. This was calculated with following equation 19: *
$%&' = $( ∗ + ∗ sin%&' ∗ /1 +
123%4'
: %5 673896%4''6
;,
(4)
with the mean piston speed $(, the crank angle & and the constant < = 3.5. The following set of differential equations describing the conservation of species and energy are solved simultaneously by Cantera: @AB @C
= DE ,
(5)
with the effective rate of formation DE of a species i, calculated by the sum of all rates of reactions j that contribute to the formation or consumption of this species, and V the (time dependent) cylinder volume, computed from the piston speed, assuming a cylinder-shaped combustion chamber. O
O
M′
′′ ′ − J I ' ∏ P L IBN Q, DE = ∑I R GHI %J I
(6)
where the total number of reactions and species are NR and NS respectively. The stoichiometric coefficients of species i in reaction j are referred to as T ′′ for products and T ′ for reactants. The conservation of energy is
described in eq. (7), where U stands for the molar enthalpy of species i, R is the universal gas constant, T the
temperature and p the pressure. VE represents the temporal change of moles, calculated with eq. (5) and L̅X the
molar isochoric heat capacity of the mixture. @Y @C
=
∑B%AE B ZB 7AE B [Y'7\ E 7]E ^̅_
.
(7)
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The heat flow rate `E (describing heat losses of the gas to the cylinder wall) was estimated using Woschni’s semi-empirical formulae for the convective heat transfer coefficient 20. This model is used to simulate an HCCI-engine process with various fresh gas conditions. The objectives of the simulation are to predict the output of such a polygeneration process and to estimate the production of syngas and hydrocarbons in order to identify optimal operation conditions. However, the mechanisms are not well validated for these fuel rich conditions, so that validation experiments were carried out in a rapid compression machine.
3. Rapid compression machine experiments A rapid compression machine (RCM) was used to study the exhaust gas compositions after compression ignition of methane/DME mixtures. This machine was described in detail in 21, therefore, the experimental setup is only discussed briefly here. The RCM is a piston-cylinder device for gas compression, which, in contrast to a reciprocating engine, performs a single compression stroke before holding the compressed, hot gas at isochoric conditions. The combustion chamber is designed to minimize physical effects like heat loss and fluid flow. For this, the piston head has a creviced shape to suppress the input of cold boundary layer gas into the hot core by an evolving roll-up vortex 21. This enables us to study chemical kinetics in the RCM under HCCI-like conditions but with a simpler setup and more well-defined initial and boundary conditions, allowing highly reproducible measurements. The RCM operation conditions, regarding geometry, inlet temperature, compression ratio and compression time can be found in Table 2. Table 2 RCM operation conditions
Parameter Bore d / stroke s Comp. ratio ε Compression time tC Intake temperature T0 Coolant temperature TK
Specification 80.05 / 73.48 mm 7.5 32 - 35 ms 432 - 450 K 432 - 450 K
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Sampling and ex-situ analysis of the in-cylinder gas mixture is possible. Gas analysis was performed by a micro-gas chromatograph (Agilent 490 Micro GC). In an RCM experiment, first a gas with the desired ratio of methane, DME and air was premixed in a mixing tank at ambient temperatures. This homogeneous fuel/air mixture was filled to the preheated, initially evacuated RCM combustion chamber up to the desired initial pressure and is also preheated during this step. From kinetic modeling at these initial temperatures and reaction times of minutes, it was concluded that DME should not react prior to compression. Afterwards, the piston is driven into the chamber, thus compressing the mixture and increasing pressure and temperature. At the isochoric conditions after compression,, temperature and pressure within the core gas first stay almost constant; after a certain ignition delay time, auto-ignition occurs, causing a rapid rise in temperature and pressure. A typical experimental pressure trace is shown along with the pressure trace of a non-reacting mixture in Figure 1. Post-ignition pressure decay is observed due to heat losses. Over time, the mixture cools down to the initial temperature. After this cooling time, a sample was delivered to the micro GC for analyzing the final gas composition. The reacting mixtures were measured at an equivalence ratio of Φ=2 under conditions denoted as mixture number 2 in the subsequent Table 3, with an N2/Ar-ratio of 10/90. In the unreactive mixture, oxygen is replaced with additional nitrogen, while all other operation parameters and mole fractions were held constant.
Figure 1 RCM pressure evolution for a reacting methane/air mixture with Φ=2 and an unreactive measurement, (T0=436 K, p0=0.7 bar , tC=32 ms).
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Gas samples were analyzed for a wide range of conditions, as shown in Table 3. To reach high final temperatures and pressures at moderate compression ratios, the specific heat capacity of the mixture was lowered by replacing the inert gas nitrogen partially or completely by argon. Fuel/air equivalence ratios φ from 1 to 15 were investigated experimentally; Table 3 Mixtures and conditions investigated in the RCM. Mix No. 1 2 3 4 5
Φ 1 2 6 7 15
CH4/DME / mol% 90/10 90/10 90/10 90/10 90/10
N2/Ar / mol% 100/0; 50/50; 0/100 100/0; 10/90 0/100 0/100 0/100
TTDC / K 755 - 985 640 – 750 710 - 780 675 -730 675 - 725
pTDC / bar 10 10 10 10 10
4. Results and discussion 4.1. Preheat temperatures The purpose of the engine is to convert methane via partial oxidation. If the fresh gas is supplied at ambient temperature, achievable conversion rates within the timescale (≈20 ms) of a normal engine cycle are quite low. Thus, higher temperatures are necessary to reach significant conversion levels. A series of calculations was performed to evaluate the minimal, initial temperature to reach ignition after compression as a function of equivalence ratio. In this context, ignition is identified as a sudden, steep increase in temperature combined with a decrease in fuel mole fractions. The initial temperatures were increased iteratively until this abrupt rise in temperature was observed. The obtained minimum initial temperatures are shown in Table 4. As assumed, the temperatures remain very high (>550 K) at high equivalence ratios and the investigated fixed engine speed and compression ratio from Table 1. The required temperatures even increase with increasing Φ since the heat capacity of the fresh gas increases with Φ. Typical initial temperatures for methane-fueled HCCI-engines in the lean regime are between 450-525 K 22. In order to provide comparable temperatures for fuel-rich mixtures several parameter modifications were investigated. Thus, the first idea was to investigate an increase in compression ratio to 22, which seems to be realistic, and which would increase the temperature at the end of the compression stroke. This leads to some
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reduction in minimum initial temperatures, by 60-80 K, but further reductions are needed. A further idea was to reduce the engine speed, leading to longer reaction times. This has an effect which is comparable to the change in compression ratio at low equivalence ratios, but is more effective at higher equivalence ratios. Finally, the addition of 10 mol% DME with unchanged initial parameters, regarding compression ratio and engine speed, has the strongest effect on the necessary initial temperatures. Table 4 Effect of equivalence ratio and operation conditions on minimal preheat temperature (in K) required for autoignition. Φ
0.50 0.77 1.06 1.36 1.68 2.02 2.38 2.76 3.17 3.61 4.08 4.59 5.13 5.71 6.35 7.04 7.79 8.62 9.52
CH4, N=3000 rpm, a = 16,5 530.0 545.0 560.0 585.0 610.0 635.0 660.0 680.0 700.0 725.0 750.0 775.0 800.0 820.0 840.0 860.0 880.0 900.0 920.0
CH4, N=3000 rpm, a = 22
CH4, N=600 rpm, a = 16,5
450.0 465.0 480.0 505.0 530.0 555.0 580.0 595.0 610.0 640.0 670.0 690.0 710.0 735.0 760.0 785.0 810.0 835.0 860.0
470.0 485.0 500.0 520.0 540.0 560.0 580.0 600.0 620.0 642.5 665.0 682.5 700.0 720.0 740.0 770.0 800.0 820.0 840.0
90/10 mol/mol CH4/DME N=3000 rpm, , a = 16,5 440.0 442.5 445.0 447.5 450.0 455.0 460.0 465.0 470.0 475.0 480.0 495.0 510.0 535.0 560.0 600.0 640.0 680.0 720.0
Consequently, all investigated parameter modifications lead to decreasing preheating temperatures. A combination of all modifications would reduce the preheat temperatures to values close to ambient temperatures but would, according to the model predictions, also reduce the hydrocarbon yield dramatically. Thus, these results will not be presented here.
4.2. Simulated exhaust gas composition
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The minimum preheat temperatures, as given in Table 4 are used for simulations to determine the product gas composition. The calculated mole fraction of carbon monoxide, hydrogen, acetylene and ethylene at the end of a cycle (BDC) as a function of the initial equivalence ratio are shown in Figure 2, together with the relative methane conversion for the base case with neat methane as fuel, 3000 rpm engine speed and a = 16.5.
Figure 2 Mole fraction of product gas species (left scale) and relative methane conversion (right scale) as a function of equivalence ratio for neat methane as fuel, 3000 rpm engine speed and a = 16.5; initial temperatures see Table 4
The predicted output of both synthesis gas (hydrogen and carbon monoxide) and the hydrocarbons are quite high, making them promising target chemicals. For both species, the mole fractions obtained from the kinetic simulation (solid lines) are plotted against the equilibrium mole fractions at TDC (dashed lines). The highest mole fractions of
