CS-048: Natural Gas Combustion Modelling

CHALLENGE

The main challenge in this case study is the application of Flownex^® to:

• conveniently and easily specify and model natural gases;
• analyze natural gas properties;
• conveniently and easily specify ambient conditions, using ambient air as the oxidant;
• perform the combustion process and extract meaningful results; and
• analyze the flue gas emissions.

BENEFITS

By creating compound components for the specification and analysis of gas compositions, and by wrapping the Flownex® Adiabatic Flame model and some associated scripts in another compound component, Flownex® may be used to perform extremely powerful combustion modeling in a very simple and efficient manner. Furthermore, this basic combustion model may be used in conjunction with other heat transfer and fluid flow processes to create very comprehensive, yet easy to use models of industrial applications. This combined capability of modeling combustion, fluid flow and heat transfer is not commonly available in other design tools.

SOLUTION

Flownex^® could effectively be used to model natural gas combustion processes.

INTRODUCTION

The Flownex® Adiabatic Flame model is based on the NASA Glenn Chemical Equilibrium Program CEA2 and supports a large range of fuels. The development of the natural gas combustion model has only attempted to implement a combustion model for typical natural gas compositions. It would be easy to add capabilities for other fuel components, whether gaseous or liquid.

The development of this model essentially comprised of three basic fields of development. Firstly, some effort went into defining fluid tables for the selected natural gas components (listed in Table 2) in such a way that accurate interpolation would result at low partial pressures and at temperatures exceeding those expected during combustion. Secondly, a suite of compound components were developed to assist with the convenient specification and analysis of the gas components. Thirdly, a Simple Burner model was developed by wrapping the Flownex Adiabatic Flame model in a compound component together with a script to enable the specification and calculation of typical natural gas burner performance parameters.

MODEL

The gas tables were created in Aspen HYSYS for each component and range in pressure from 0.0001 kPa to 200 kPa. The assumption is that the combustion process itself will be atmospheric, around 100 kPa (abs). Similarly, the gasses are defined between -10ºC and 2500ºC, so care should be taken to ensure gas inlet temperatures are not sub-zero to be safe.

Unfortunately, only hydrocarbon gasses up to Octane (C8) are supported by the NASA Glenn Chemical Equilibrium Program CEA2 on which this model is based. The combustion model also produces NO, O and OH. These fluids are not in the fluid mixture, so a warning will be produced as a result. The mass (and mol) fractions of these fluids are added to that of Argon and hence the mass fraction of Argon will appear to increase from inlet to outlet.

Flownex^® deals with combustion gas mixtures via the specification of the mixture mass fractions at the boundaries. Similarly, the combusted flue gas may be analysed at any node downstream of the Adiabatic Flame model. As gas compositions are commonly specified in mol fractions rather than mass fractions, a suit of 5 scripts, each wrapped in a compound component for convenience has been developed. These serve as inputs and outputs of information to and from the burner compound component.

CASE STUDY

As an example, a hypothetical natural gas which contains all the gas species supported by the model is combusted using wet atmospheric air as the oxidant. The results are then compared with the results of other available software.

Data Specification 

The example uses the following data specification:

Table 1: Input Data

RESULTS COMPARISON & DISCUSSION

Air Psychrometry

The simplest compound component in the natural gas combustion suite is the Air Psychrometry component which implements equations from ASHRAE. Given typical site conditions – altitude, temperature and relative humidity – this component aims to calculate the ambient pressure and ambient air composition. This information is then assigned to the combustion system’s air inlet boundary component. Using the inputs given in the table above, calculations from this component are compared with results from other software in the following table:

Table 2: Atmospheric and Psychrometric Calculations

1. WinHeat Thermal Rating Suite

2. ASHRAE Dayton Chapter (www.daytonashrae.org/psychrometrics.shtml)

3. Psychrometric Calculator (www.psychrometric-calculator.com)

It is shown that the Flownex^® Air Psychrometry component provides accurate calculations of atmospheric air properties as well as composition.

NG Mol to Mass

The following results were obtained using the Flownex® model’s NG Mol to Mass compound component to convert the gas composition as supplied by the user from mol% to mass% and assign it to the fuel gas inlet boundary. This conversion has been repeated in HYSYS and is compared below: 

Table 3: Natural Gas Composition Conversion from Mol% to Mass%

It is shown that the mol% to mass% conversion agrees well with the same conversion done in HYSYS.

NG Combustion Props

The gas composition is also supplied to the NG Combustion Props compound component via a data transfer link to calculate the gas heating values and other properties. The results are tabled below and compared to results from HYSYS (amongst others).

Table 4: Natural Gas Properties

1.  Aspen HYSYS

2. Heat & Mass Balance by Phillip Dane, ABM Combustion Pty Ltd

These properties are not used in the actual combustion calculations implemented by the Basic Burner compound component as it relies on the underlying NASA Glenn Chemical Equilibrium Program CEA2. Although the results are reasonably close, they only serve to assist the user with additional information.

The small differences between the heating values may be attributed to the Flownex NG Combustion Props component being based on the GPSA Data Book gas property tables (www.gpsa.gpaglobal.org).

The NG Combustion Props component also performs air-fuel ratio calculations, the results of which are used to specify the combustion airflow at the combustion air inlet boundary. As such, the required excess air flow percentage is also specified in this component. These air-fuel ratio calculations have been compared against the results of other commercial software, the results of which are presented below.

Table 5: Natural Gas Air-Fuel Ratio Requirements

As shown, good agreement is also achieved between the Flownex NG Combustion Props component and WinBurn. Further verification of the air-fuel ratio calculation is also provided below with the combustion calculation comparison, using WinBurn and HMB.

Basic Burner & NG Flue Gas Analysis

As previously discussed, the Basic Burner compound component is simply a wrapper around the Flownex Adiabatic Flame element, which itself simply implements the NASA Glenn Chemical Equilibrium Program CEA2. The results of the Basic Burner component is the heat release and of course the conversion of the natural gas components to products of combustion.  The results of the Basic Burner and NG Flue Gas Analysis components are therefore best verified together.

This NG Flue Gas Analysis compound component is used to analyse natural gas products of combustion. The flue gas composition is transferred from a node downstream of the Basic Burner via a data transfer link. The component will then convert the results from mass% to mol% for presentation as well as calculating the flue gas molar mass and water dew point temperature. In cases where hydrogen sulfide is present in the fuel gas, the SOx acid dew point temperature is also calculated. The SOx dew point temperature depends on an assumed SO₃ to H₂SO₄ conversion rate which may be specified in the component. The default value of 5% is frequently assumed in the gas industry. For the example at hand, the results compare as follows:

Table 6: Combustion Performance and Flue Gas Analysis

The HMB model does not perform any psychrometric calculations, hence the combustion air composition was simply copied from the Flownex^® calculations. Furthermore, the Winburn and HMB models do not calculate the production of CO and HMB does not account for the production of Argon or Hydrogen. The Flownex combustion model also produces very small amounts of other combustion products including NO, O and OH which are not included in the gas definition, and these have been added to Argon for simplicity. This explains why the amount of Argon has increased and is higher than what WinBurn calculates.

The dew point calculation is checked against ES Flue Gas which is a dedicated software to calculate flue gas properties.

Table 7: Dew Point Calculations

1.   ES_FlueGas by ENGSoftware.

It is shown that accurate dew point temperatures are predicted by the NG Flue Gas Analysis compound component and that a 5% SO₃ conversion rate is a conservative estimate.

SUMMERY

Using a few simple compound components, the capabilities of Flownex® has been extended easily to include natural gas combustion processes. It has been shown that the results obtained are accurate and in close agreement with other available software. This extension enables Flownex® to be utilised as a complete heat and mass balance tool whilst simultaneously performing as a fluid dynamics, thermodynamics and heat transfer tool in this industry. The ability to extend the capabilities of Flownex® for any particular task through simple compound components sets Flownex® apart from other tools in the industry.

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