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Interim Report - Syngas GCV measurement and calculation methodology issues

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Final Report to Ofgem

April 2010

Final Report - Syngas GCV measurement and calculation methodology issues

i

DRAFT

Title Interim Report - Syngas GCV measurement and calculation methodology

issues

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Interim Report - Syngas GCV measurement and calculation methodology issues AEA

AEA ii

Contents

1 Introduction ........................................................................................... 1

1.1 Objective .............................................................................................................. 1

1.2 Background .......................................................................................................... 1

1.3 Description of Syngas .......................................................................................... 2

1.4 Context of Gasification & Pyrolysis Processes .................................................... 2

2 Options for determining syngas GCV .................................................. 3

2.1 On-line gas analysis ............................................................................................. 3

2.2 Off-line gas analysis ............................................................................................. 3

2.3 Efficiency factor calculations for small generating stations ................................. 3

3 Instrumentation ..................................................................................... 1

3.1 Overview of the main analytical techniques deployed in industry ....................... 1

3.2 Gas Chromatography (GC) .................................................................................. 1

(Measures GCV by Calculation) ...................................................................................... 1

3.3 Flame Ionisation Detection FID ........................................................................... 2

(Measures GCV directly) ................................................................................................. 2

3.4 Continuous combustion calorimeter .................................................................... 3

(Measures GCV directly) ................................................................................................. 3

3.5 IR Spectroscopy ................................................................................................... 5

(Measures GCV by Calculation) ...................................................................................... 5

3.6 Gas Analyser Summary table .............................................................................. 6

4 Measurement & calculation standards ................................................ 7

4.1 Monitoring Certification Scheme (MCerts) a candidate for a syngas gas analyser

standard. .......................................................................................................................... 7

4.2 Description of BS EN ISO 6976 Methodology for deriving the GCV ................... 7

4.3 Statement of Relevant Variables When Using the BS EN ISO 6976 Methodology

7

4.4 Likely Reference Values Table At 25 C & 101.3 kPa) ......................................... 8

4.5 Worked Example of GCV Fuel Gas Calculation Based On the BS EN ISO 6976

Methodology .................................................................................................................... 8

5 Gas mixing and meter positioning issues ......................................... 10

5.1 Characteristics of a Meter Positioning Strategy Necessary to ensure the

Extraction of a Representative Sample of Syngas for Analysis .................................... 10

5.2 A Brief Description of the Implications of Gas Mixing and Meter Positioning, ... 10

6 Designing a robust sampling plan when using batch sampling and

qualitative analysis of this approach ......................................................... 11

6.1 Qualitative Analysis of the Characteristics Needed For a Sampling Regime to

Produce an Accurate and Reliable Estimate of the Syngas GCV................................. 11


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6.2 The Likely Accuracy and Reliability of a Monthly Sampling Plan. ..................... 12

6.3 Qualitative Review of Monitoring Regimes Options .......................................... 12

6.4 A Potential option for small generating stations .. Error! Bookmark not defined.

7 Summary of Highlights ....................................................................... 14

8 Recommendations .............................................................................. 15


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1 Introduction

1.1 Objective

Ofgem have asked AEA to provide the necessary information and understanding of gas analysis instrumentation and calculation methodologies to meet their stated objective of being able to effectively assess fuel measurement and sampling procedures for generating stations using gasification/pyrolysis.

1.2 Background

Under the amended Renewables Obligation Order 2009, generating stations using gasification or pyrolysis to produce a gaseous fuel are obliged to measure the gross calorific value of this fuel so that Ofgem can place generation from a gasification / pyrolysis station within the appropriate band in a given month. This requirement is set out in Schedule 2 1 part 1 of the Order within the descriptions of the relevant bands as follows: “Advanced gasification” means electricity generated from a gaseous fuel which is produced from waste or biomass by means of gasification, and has a gross calorific value when measured at 25 degrees Celsius and 0.1 megapascals at the inlet to the generating station of at least 4 megajoules per metre cubed;

“Advanced pyrolysis” means electricity generated from a liquid or gaseous fuel which is produced from waste or biomass by means of pyrolysis, and

(a) in the case of a gaseous fuel, has a gross calorific value when measured at 25 degrees Celsius and 0.1 megapascals at the inlet to the generating station of at least 4 megajoules per metre cubed, and (b) In the case of a liquid fuel, has a gross calorific value when measured at 25 degrees Celsius and 0.1 megapascals at the inlet to the generating station of at least 10 megajoules per kilogram “Standard gasification” means electricity generated from a gaseous fuel which is produced from waste or biomass by means of gasification, and has a gross calorific value when measured at 25 degrees Celsius and 0.1 megapascals at the inlet to the generating station which is at least 2 megajoules per metre cubed but is less than 4 megajoules per metre cubed;

“Standard pyrolysis” means electricity generated from a gaseous fuel which is produced from waste or biomass by means of pyrolysis, and has a gross calorific value when measured at 25 degrees Celsius and 0.1 megapascals at the inlet to the generating station which is at least 2 megajoules per metre cubed but is less than 4 megajoules per metre cubed;


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1.3 Description of Syngas

Syngas is a flammable mixture of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), methane (CH4) and other short chain hydrocarbon gases. This mixture is sometimes referred to as producer gas or town gas. Historically syngas has been generated using coal and then used as a chemical feedstock for many chemical processes and energy production. However it fell out of favour as mineral oil became more economic for transport and industrial process applications. Moreover, syngas is both flammable and toxic.

1.4 Context of Gasification & Pyrolysis Processes

Gasification / Pyrolysis processes have existed for hundreds of years. Examples of where man has used these processes include the production of charcoal, to smoke and preserve food, town’s gas production, for lighting and heating during the industrial revolution, and providing heat and chemical reagents for iron production. More recently these processes have been used in the commercial production of ammonia, methanol, synthetic petroleum products, synthetic natural gas and several other commercial processes. Today it is typical for these processes to occur at large petrochemical installations where the syngas produced is used as a chemical intermediate. These sites and processes traditionally require large capital investment, thorough technical design and must meet high safety standards.


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2 Options for determining syngas GCV

2.1 On-line gas analysis

This is where the gas analysis equipment is integrated into the gasifier or ducting for

generating scheme. Once the syngas has been extracted it is immediately analysed

providing a slightly delayed reading on the quality of the syngas. On-line analysis has

a higher capital cost due to the cost of purchasing the gas analysis equipment and

integrating it into the syngas generation process. However this approach has a lower

operating cost once installed and allows the detectors to operate unattended, in

potentially hazardous process areas, and can be integrated into the gasifier control

system.

Online gas analysers may operate in 2 modes:

Continuous operation where the gas analyser is providing the information

about the syngas in a constant stream and so there are no gaps in the

information about the syngas with time.

Batch / discrete operation where the gas analyser carries out an analysis

periodically, but there is no information on the syngas during the time period

when the analysis has not been carried out.

2.2 Off-line gas analysis

This is where samples of gas are extracted, collected and delivered to an analytical

lab for subsequent analysis. Off-line sampling has a lower capital cost because only

the gas extraction equipment is required; the gas analysis equipment is not installed.

However there are higher operating costs associated with transporting the gas to a

third party for analysis in a lab. The high sampling rate required to be confident of the

GCV average value would make off-line the cost sampling prohibitive.

Offline gas analysers can only operate in 1 mode:

Batch / discrete operation where the gas analyser carries out an analysis

periodically, but there is no information on the syngas during the time period

when the analysis has not been carried out.

2.3 Efficiency factor calculations for small generating stations

Although reviewing the option that could be appropriate for small generating stations is outside the scope of this report we will briefly outline how a small generating station under specific circumstances can use a less costly approach to calculating the syngas GCV.

For small operations using a reciprocating engine as a prime mover the GCV can be calculated by using the design efficiency of the engine. It can be assumed that


AEA 4

engines generate electricity with a constant efficiency. This efficiency is the design efficiency; and means that the electrical output is directly proportional to the fuel input.

Electrical output (kWh) = engine efficiency x fuel input (kWh).

In order to calculate the GCV of the syngas supplied to the engine, the following must be metered:

The syngas volumetric flow rate into the engine

The syngas temperature,

The syngas pressure and

The electrical output

These parameters must be metered either continuously or in accordance with the sampling frequency requirements set out in section 3.5. Once these parameters are known it is possible to calculate the energy delivered to the engine from the electrical output. Using this and the volumetric flow rate of gas supplier to the engine, corrected back to the reference conditions of 25°C and 0.1 MPa (absolute), allows the GCV of the syngas per unit volume to be calculated. This approach does have inherent uncertainties associated with it and the risk associated with these uncertainties will increase as the capacity of the generator increases. When defining a capacity threshold for small generators beyond which metering should be mandatory we suggest two approaches:

In terms of a payback period where the additional ROCs benefit would offset the cost of the additional metering over a reasonable period, for example the cost of a continuous gas calorimeter would be recouped by benefiting from the additional ROCs on 450MWh,

In terms of the fiscal value of the uncertainty inherent in assuming the design efficiency can be used to calculate the fuel input by metering the electrical output. This approach is similar to the example set out by the CHPQA programme, where the threshold has been set at an electrical capacity of 500 kWe or less.

This would mean that small operators do not face the cost barrier imposed by options 1 & 2 as outlined in the introduction.


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3 Instrumentation

3.1 Overview of the main analytical techniques deployed in industry

There are several analytical techniques used for gas analysis. However, discussions

with suppliers have shown that industry tends to favour a few particular technologies.

In the case of fuel gas detection an important consideration is the risk of the gas

analysis technology creating a hazard. Technologies that require combustion, or are

destructive, may provide a source of ignition in areas where there is a high probability

that the surrounding atmosphere would be flammable. As a result non-destructive

gas analysis technologies such as gas chromatography or infrared spectroscopy are

preferred over other techniques. It is also common in industry for several

technologies to be coupled together in an array. Below is a review of the common

gas analysis technologies that are used for analysis of fuel gases similar to syngas,

which can be used to measure the GCV of the syngas being delivered to an

electricity generator.

3.2 Gas Chromatography (GC)

(Measures GCV by Calculation)

A gas chromatograph is a gas analysis instrument which analyses a sample of the

extracted gas taken, this analysis takes several seconds and therefore a sample of

the gas must be extracted periodically.

The sample gas is mixed with a carrier gas and then allowed to diffuse along a

narrow media filled tube. A detector is then fitted at the tube exit. The various

components in the fuel gas mixture diffuse along the tube at different rates

depending on their density, their respective chemical and physical properties and

their interaction with the tube media. As the chemicals exit the end of the column,

they are detected and identified electronically. A gas chromatograph can be used in a

standalone basis however it is unsuitable for analysing some gases, most notably

hydrogen (H2) which may contribute significantly to the syngas GCV. A number of

other detectors can used in conjunction with gas chromatography to monitor other

gases or parameters which are not easily identified using gas chromatography. It is

common for a gas chromatograph to be paired with a Flame Ionization Detector (FID)

which monitors the total hydrocarbons present or a Thermal Conductivity Detector

(TCD) which monitors the hydrogen (H2) concentration.

The advantages of this technology are:

It is a non destructive detector so that other sensors can be connected to the

gas chromatograph (GC) in series.

It provides a breakdown of mixture components.

It has a high accuracy (uncertainty of 0.5% for component gases)


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The detection process does not involve combustion and so there is no

potential explosive hazard presented.

The disadvantages of this detector are that:

The detection is a discrete (non continuous) process. This means that

samples of the gas are taken and processed at regular intervals. These

intervals are user regulated. Samples can be taken at a rate of up to one

sample per minute.

The GC detector has a limited operating temperature range and will only

receive samples at temperatures less than 100 °C and greater than -50°C.

The GC detector will require periodic calibration and maintenance depending

on the model selected.

3.3 Flame Ionisation Detection FID

(Measures GCV directly)

This is a continuous gas analysis instrument which directly measures the syngas

GCV by combusting the sample gas between two electrically charged plates. The

combustion produces chemical ions that transfer charge across the space between

the plates. This allows an analysis of changes in the charged plates to provide a

summary of the total hydrocarbons present. Typically an FID would be used in series

with other instruments after a gas chromatography analysis. The illustration in the

following section shows the operation of a flame ionisation detector (FID).

The advantages of this technology are that:

The fuel gas is continuously analysed.

The detector will operate at elevated temperatures up to 350 °C

The disadvantages of this detector are:

It destroys the sample in the detection process.

It requires a naked flame and this poses a hazard of explosion when placed in

the vicinity of other flammable gases, as would be the case on the exit of a

gasifier.

The detector senses all hydrocarbons including those that are not gases and

those that would be considered condensable vapours and tars. This would

introduce a level of bias to the calorimeter if the metering position is not

appropriate as directed in section 5.2. This uncertainty could be significant in

low temperature fast pyrolysis processes. I would recommend that this

instrument is preceded by a gas cleaning step ensure that the impact of tars

and vapours is minimised.


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Flame Ionisation Detector

Courtesy of Euro FID Total Hydrocarbon Analyser product information.

3.4 Continuous combustion calorimeter

(Measures GCV directly)

This is a continuous gas analysis instrument which directly measures the syngas

GCV by combusting the syngas in a flame that heats a bar wedged between a wall

and a load sensor. As the fuel gas varies its calorific value the flame temperature

varies in a direct relationship the syngas calorific value. As the temperature of the bar

increases the bar expands directly in proportion to the increase in temperature. This

expansion applies a force to the load sensor that is measured and used to work out

the flame temperature, the energy released during combustion and therefore the

syngas calorific value (GCV).

The advantages of this technology are:

The syngas GCV is continuously monitored.


The detector provides the GCV with no further processing necessary.

The disadvantages of this detector are:

It destroys the sample in the detection process preventing the gas sample

being used for further analysis. Further analysis may be necessary for quality,

control or safety reasons.


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It requires a naked flame that poses a hazard of explosion when placed in the

vicinity of other flammable gases, as would be the case at the exit of a

gasifier.

The GCV calorimeter would require periodic calibration and maintenance

depending on the model selected although the maintenance intervals may be

long due to the relatively simple and robust designs.

The detector senses all hydrocarbons including those that are not gases and

those that would be considered condensable vapours and tars. This would

introduce a level of bias to the calorimeter if the metering position is not

appropriate as directed in section 5.2. This uncertainty could be significant in

low temperature fast pyrolysis processes. I would recommend that this

instrument is preceded by a gas cleaning step ensure that the impact of tars

and vapours is minimised.

The illustration in the following section graphically shows the operation of a continuous combustion calorimeter.

Continuous Combustion Calorimeter

Courtesy of Flo-Cal High-Speed Calorimeter Thermo Electron Corporation


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3.5 IR Spectroscopy

(Measures GCV by Calculation)

Infrared spectroscopy covers a range of techniques, the most common being

absorption spectroscopy. This is a continuous gas analysis instrument where a

sensor measures the absorption spectra of the gas mixture. The component gases

can be identified because each chemical compound has a different absorption

spectrum and overall spectra of the gas mixture will be a combination of the

absorption spectra of the syngas component gases. IR is an established method for

measuring CO, CO2 and CH4 in gas mixtures.

The advantages of this technology are that:

The fuel gas is continuously metered.


The technique does not require a sample to be extracted from the main

syngas flow.

No moving parts or parts exposed to the syngas flow making this a low

maintenance design.

The disadvantages of this detector are that:

This detection methodology is sensitive to the presence of water vapour in the

syngas.

The detector is not sensitive to the presence of hydrogen in the syngas and

therefore would have to be used in conjunction with a analyser that can detect

hydrogen.

The detector has a low sensitivity to longer chains hydrocarbons.

The gas analyser will require periodic calibration.


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3.6 Gas Analyser Summary table

AEA have reviewed the online detector market for process gases, refinery gas, blast furnace gas and natural gases. The following

table contains a sample of the gas analysis systems and calorimeters available. A scheme would probably use a standalone

installation or a combination these types of detectors depending on the parameters they are interested in for quality (GCV), control or

safety reasons.

Table 2: Analytical Methods for Major Gaseous Components and Hydrocarbons

Technology Suppliers Model Uncertainty Operating Temperature

Example of Use Budget Price

Gas Chromatography

ABB NGC 8206 0.5% 50 – minus 18°C BP Stanlow refinery £20K – £25K

Flame Ionisation Detector

SICK EuroFID < 3% Up to 750°C unavailable £12K – £22K

Servomex Servopro FID unavailable Unavailable unavailable unavailable

Continuous Calorimeter

Fluid Data Flo-Cal 1% 40 - minus 5°C North Sea gas network US$60K – US$75K*

Union CWD 2005 0.5% 40 – minus 5°C National Grid £17K

Orbital UK CalorVal 3% 40 – minus 65°C unavailable £14.5K

Infrared Spectroscopy

SICK FTIR 0.5% 40 - 0°C unavailable £75K – £85K

Gasmet

(Quantitech UK)

CX-4000 2% 40 - 0°C Tetronics (advanced plasma power)

£84K

(unreliable information*)

Interim Report - Syngas GCV measurement AEA and calculation methodology issues

7 AEA

4 Measurement & calculation standards

4.1 Monitoring Certification Scheme (MCerts) a candidate for a syngas gas analyser standard.

The MCerts performance standard for the chemical testing of soil is an application of ISO

17025:2000 (General requirements for the competence of testing and calibration

laboratories). MCerts covers the selection and validation of test methods, sampling pre-

treatment and preparation, the estimation of measurement uncertainty; participation in

proficiency testing schemes, and the reporting of results and information.

The MCerts certification scheme is used as a standard for the testing methodology and

equipment used when monitoring emissions from landfill sites. The MCerts standards

required for equipment used for emissions monitoring on these sites may provide guidance

for setting up a metering standard for metering syngas. We reason that the monitoring units

used on landfill sites detect the same gases that would be metered from the output of a

gasifier / pyrolyser. The only difference would be found in the higher concentrations of the

gases of interest when compared to landfill gas.

4.2 Description of BS EN ISO 6976 Methodology for deriving the GCV

We recommend that the BS EN ISO 6976 methodology is used for calculating the syngas

Gross Calorific Value (GCV). BS EN ISO 6976:2005 is a standard methodology for

establishing the Gross Calorific Value of a fuel gas (typically natural gas) from an analysis of

its constituent gases at defined temperatures and pressures. This methodology provides

defined reference values for the following properties of common component gases. These

physical properties include:

Gas molecular weights

Gas compressibility

Gross Calorific Value (GCV)

Net calorific value (NCV) and

Molar volume at 0°C, 25°C & 15°C.

4.3 Statement of Relevant Variables When Using the BS EN ISO 6976 Methodology

The BS EN ISO 6976 methodology is based on the volumes of each component gas. This

means that the volume fraction or molar fraction of each component gas is required to

calculate the GCV. The advantage of this approach is that the methodology is independent of

temperature and pressure.


AEA 8

4.4 Likely Reference Values Table At 25 C & 101.3 kPa) 1

The following tables show reference values used by AEA since the last revision of BS EN

ISO6976 in 1999. However the International Organisation for Standardisation periodically

revises the reference values for BS EN ISO 6976. Revisions that are made to the reference

values are relatively small and therefore AEA believe the values provided in the following

tables are close to the likely reference values that would be used for calculating the GCV of

syngas (Up to date reference values can be purchased from the International Organization

for Standardization (ISO).

Table 2

Component gas CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16

Enthaplystandard

KJ/Nm3 39724 69595 99048 128363 157768 187160 216546

Enthaplyhigh

KJ/Nm3 35796 63704 91195 118546 145987 173413 200835

Table 2 continued

Component

gas C8H18 C6H12 C7H14 C6H6 C7H8 H2 CO H2S

Enthaplystandard

KJ/Nm3 245909 176362 205261 147299 176140 12753 12626 25098

Enthaplyhigh

KJ/Nm3 228235 164582 191517 141408 168285 10789 12626 23134

4.5 Worked Example of GCV Fuel Gas Calculation Based On the BS EN ISO 6976 Methodology

Take the example of a syngas composed of:

60% CO (vol.)

25% N2 (vol.)

5% H2 (vol.)

10% H20 (vol.)

The gas composition is determined using the technologies described in section 2.1. The

compositions are given as volume fractions that allow the GCV to be calculated without the

need for temperature and pressure correction.

The GCV would be calculated as follows:

Taking CO, the energy released by burning 1 m3 is 12626 kJ. This multiplied by the

volume fraction of CO which is 60%.

0.6 x 12626 kJ = 7575.6 kJ

N2 & H20 are non combustible gases and therefore make no energy contribution.

1Robert Stewart AEA ISO 6976 Fuel analysis spreadsheet 2002 AEA have not used the ISO standard tables as these have to be purchased for

use.


9 AEA

H2, the energy released by burning 1 Nm3 is 12753 kJ. This multiplied by the volume

fraction of H2, is 5%.

0.05 x 12753 kJ = 637.65 kJ

The calorific value of the gas mixture is the sum of the energy release burning the

component gases and is therefore:

GCV = 7575.6 kJ + 637.65 kJ = 8213.25 kJ = 8.21325 MJ/m3


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5 Gas mixing and meter positioning issues

5.1 Characteristics of a Meter Positioning Strategy Necessary to ensure the Extraction of a Representative Sample of Syngas for Analysis

The Renewables Obligation 2009 states that syngas GCV must be known at the generating

station “inlet”. This statement requires that a boundary be defined around the generating

station. We suggest that syngas entering this boundary is only acceptable if it meets the

following criteria.

1. The gas must be homogenous and therefore the fluid flow conditions around the

sampling probe must be well mixed and turbulent.

2. The sampling probe must be in a position that is demonstrably unobstructed and

where stagnant conditions cannot occur under normal operation.

3. The gas being sampled can no longer be participating in any chemical reactions.

We believe the onus should be placed on generating stations to prove to Ofgem that the

location of the sample extraction meets the previously mentioned criteria.

5.2 A Brief Description of the Implications of Gas Mixing and Meter Positioning,

It is not possible to generalise about where a sample of gas can be extracted from within a

gasification or pyrolysis reactor for several reasons. The gas flow regime between

gasification or pyrolysis systems will vary significantly. Each design will be different and

changes that occur during operation will mean that the flow regime will change continuously

as well. As a result there may be areas of stagnation or highly turbulent areas within the

reactor. The chemical reactions that occur in gasification and pyrolysis require heat to drive

them and continue to occur until such a point that energy is no longer available or the

temperature has fallen sufficiently to slow the reactions to a halt.

It should also be considered that operationally any sampling probe would be vulnerable to

being struck by solid matter with the reactor and probably would not be reliable.

We suggest that where possible probes inserted into the syngas flow is inserted in a zone

after the reactor. For designs where the gasification and oxidation processes occur in the

reaction chamber it should be up to the operators to prove that their probe placement is

appropriate and any samples taken are representative of the actually syngas stream.


11 AEA

6 Designing a robust sampling plan when using batch sampling and qualitative analysis of this approach

6.1 Qualitative Analysis of the Characteristics Needed For a Sampling Regime to Produce an Accurate and Reliable Estimate of the Syngas GCV

Gasification and pyrolysis are processes that are dependent on the chemical composition

and consistency of the feed material and on the operating temperature. Periodic changes

within the reaction chamber will change the syngas composition significantly. For example a

process called bridging could occur periodically where the feedstock forms a crust which

builds up and then fails. This process will create significant changes to the temperature and

pressure profiles and the balance of syngas composition. The capacity and design of the

gasification / pyrolysis scheme may have a lesser impact on the variation of the syngas GCV.

For example, a design with a large capacity and a long residence time would produce syngas

with a lower rate of variation in syngas GCV than a design with a short residence time.

Additionally a design that promotes good mixing such as a fluidised bed design would have a

lower rate of syngas GCV variation than a design that operates with little mixing such as a

moving grate design.

AEA believe that implementing a sampling regime that will provide a sound methodology for

determining an average syngas GCV will require a very large number of samples to be taken

over a month This is because the only rigorous basis for determining a sample rate links the

syngas GCV to the rate of the feedstock material that was processed to generate the

syngas.. We strongly recommend using a continuous GCV calorimeter to measure the

syngas GCV; this is the simplest and most cost effective approach. In situations where the

syngas cannot be cleaned (steam cycle and fired expander designs) it would be necessary to

calculate the syngas GCV from the gas composition as is explained in section 4.2.

If a generator are insistent on applying a sampling regime we suggest that the syngas output

is sampled with a time interval that is a third (⅓) of the mean time feed material stays within

the reactor as this feed material is converted from solid material to syngas (mean residence

time). Taking samples at one third of the mean residence time will be sufficient for the GCV

variation profile to be reconstructed mathematically and pick up significant variations in the

feed material. This approach is based on the Nyquist sampling theorem that requires the

sampling rate to be at least twice the highest frequency of interest. In the case of syngas

GCV this rate is the inverse of the mean residence time (taverage-1). This approach is well

understood and used for digital signal processing where signals must be digitised by

sampling them. We believe this will provide a good indication of the average syngas GCV

over a month.2

Practically gasification and pyrolysis processes optimised for syngas production will tend to

have relatively long residence times (several minutes) and we would anticipate that the

2 Nyquist–Shannon sampling theorem


AEA 12

required sampling frequency would be achievable by the online sampling meters currently

available.

Downdraft gasifier designs using a homogenous feedstock such as wood or waste wood, it is

possible to monitor the carbon monoxide percentage continuously and calculate the syngas

composition and therefore the GCV using a numerical model. However the model would

need to be verified on a regular (daily) basis by carrying out a complete gas analysis. This

approach can only be applied to the downdraft gasifier designs which are the simplest to

model.

6.2 The Likely Accuracy and Reliability of a Monthly Sampling Plan.

The uncertainty related to deviating from the suggested sample frequency can be expressed

as a confidence level. That is a 5% uncertainty would be a 95% confidence that the samples

reflect reality. We suggest that Ofgem take a pessimistic view of processes that are unable to

monitor the gas composition or GCV continuously or at the required sample frequency. For

simplicity we have built in some over sampling into the formula for the recommended

sampling regime (it is simpler to calculate ⅓ tmean than 1/2.1 tmean ) therefore there would be not

reduction is accuracy for schemes provided they sample the syngas at intervals that are less

than twice the mean residence time (tmean). Gasification and pyrolysis process with short

residence times are likely to have a higher proportion of tar and vapours which should not be

included in the syngas GCV because they would be stripped out of the syngas stream when

the syngas is cleaned. In cases where a generating station could not achieve sampling at

intervals shorter than half the mean residence time the site should declare their site

uncertainty and statistically justify the confidence level they have that sample GCV reflects

reality. The statistical justification should be validated by Ofgem to show it has a rigorous

statistical basis and sound engineering assumptions. The confidence level would then be

converted to an uncertainty and the GCV would then be reduced by the calculated

uncertainty. This would mean that a 95% confidence level in the sampling regime would

result in the calculated average GCV being reduced by 5%. For cases where the sampling

regime meets the sampling frequency requirements it is reasonable to accept the average

monthly GCV as calculated from sampling the syngas output.

6.3 Qualitative Review of Monitoring Regimes Options

Front end monitoring is reasonable in combustion / oxidation processes because an excess of highly reactive oxygen heavily outweighs the influence of other factors. In the case of gasification and pyrolysis processes the presence of oxygen, and therefore its influence, is limited and therefore other factors become more important. These factors include:

1. The heterogeneous nature of the feedstock. Even virgin wood is composed of several

chemically distinct and unrelated compounds.

2. The chemical composition of the feedstock will vary continuously particularly in the

case of solid recovered fuels (SRF).


13 AEA

3. Gasification / pyrolysis processes involve many equilibrium reactions and in most

reactor designs (all designs except downdraft gasifier designs) the chemical reactions

are kinetically stabilised, that is the syngas temperature falls rapidly before chemical

equilibrium is reached effectively freezing the chemical reactions before they are

complete. This means that modelling techniques, which must assume that equilibrium

conditions have been reached, never describe the real situation, where equilibrium is

never reached.

4. The presence of common elements such as alkali, alkali earth and transition metals

will catalyse or inhibit some reactions. Under equilibrium conditions catalysis should

make no difference, but in non equilibrium conditions accelerated reactions, through

catalysis, will further distort the gas partial pressures, the reactor temperature profiles

and any adjacent or subsequent reactions.

5. It is not possible to quantify the presence or position of catalysts or inhibitors in the

reaction chamber (due to their insertion as part of the feedstock and removal as ash),

6. It is not possible to quantify how these catalytic species will interact chemically with

the highly heterogeneous environment in the reaction chamber and how they will

affect other reactants present.

All of these factors significantly increase the uncertainty associated with the syngas

composition estimated on the basis of an empirical relationship. For such an approach to be

statistically rigorous a process would have to spend sufficient time in each mode of operation

to capture and quantify the influence of the inherent variation in the heterogeneous feedstock

for each mode of operation. This would be an extremely laborious and difficult task, which

would almost certainly cost more than installing continuous monitoring. It is worth noting that

many of the gases present in syngas would probably have to be monitored continuously for

health and safety, and process control reasons.


AEA 14

7 Summary of Highlights

AEA would like to draw to Ofgem’s attention the following points related to metering of

syngas in order to establish its GCV:

The metering requirements required to fulfil Option one & Option two of this report

may act as a barrier to small generating stations

There is another potential option for small generating stations to be considered. This

option carries a higher uncertainty than Option one or Option two but it does not carry

the metering and capital costs of these options. The risk associated with this

alternative option may be acceptable for smaller generating stations.

The BS EN ISO 6976 methodology should be used for calculating GCV from metering

the component gases.

The MCerts standard may form the basis of a standard for equipment and

methodologies when carrying out gas analysis to meter the GCV of syngas.

The syngas output should be sampled with a time interval that is not more than a third

(⅓) of the mean time feed material stays within the reactor as it is converted from

solid material into syngas (mean residence time).

For generating stations that choose to sample with a time interval greater than a third

of the mean residence time should state and statistically justify the confidence level

with which the sample of gas accurately reflects the GCV of the rest of the gas. In

this situation, the GCV of the Syngas should be reduced by the confidence level

expressed as an uncertainty.

It is not possible to calculate or model the syngas composition on the basis of the

process design, because the factors that most influence the gas composition are

related to the feedstock composition, which may meet specified requirements (such

as the SRF standard requirements) but will show significant chemical variation.

The Renewables Obligations Order 2009 legislation requires that a boundary is

defined around the generating station. Syngas entering the boundary around the

prime mover is only acceptable if it meets the following criteria.

1. The gas is homogenous and the fluid flow conditions around the sampling probe

are well mixed and turbulent.

2. The sampling probe is in a position that is demonstrably unobstructed and where

stagnant flow conditions cannot occur under normal operation.

3. The gas being sampled can no longer be participating in any chemical reactions.

4. The gas entering the inlet of the generator has a GCV equal to or greater than the

threshold of 4 MJ / m3, as expressed in the legislation


15 AEA

8 Recommendations

It is our opinion that a generating station would most easily and cost effectively meter the

syngas GCV using a continuous calorimeter directly measuring the gross calorific value

(GCV) as suggested by option one. Metering the GCV continuously and directly has two

main advantages: simplicity because continuous meters make mean that it is unnecessary

to develop a sampling regime is unnecessary and a gas calorimeter will provide the GCV will

no further processing and cost because the continuous gas calorimeter systems available

are significantly cheaper than alternative designs. The approach proposed in option two

would be considerably more expensive and may require a sampling regime to be developed.

In the case of option two we recommend that the MCerts methodology and gas analyser

standard are used as the basis for creating a syngas metering standard. AEA also

recommend that the BS EN ISO 6976 methodology is used for calculating the GCV from a

known syngas composition. We advise against off-line sampling because the number of

samples required to be assured of the average GCV on a monthly basis would make this

approach prohibitively expensive and difficult logistically.


Glossary

BS ISO EN 6976 – The standardised methodology for measuring the gross calorific value of

fuel gases.

Continuous meter- This is a design of meter that constantly processes the syngas output

and provides a continuous real-time reading.

Confidence Level – the confidence that a measured value is in fact the true value.

Destructive gas analysis – gas analysis that destroys sample of gas in the process of

conducting the analysis.

Discrete meter - This is a design of gas analyser that takes samples of the syngas output at

defined time intervals and provides an almost instantaneous snapshot of the gas

composition. If a sufficient number of snapshots are taken over a monthly period the average

GCV can be calculated with low levels of uncertainty.

Extraction detectors – these detectors draw (extract) a sample of syngas from the main

flow and then test the sample of gas extracted.

Feed material – the material either biomass or a waste based fuel that is inserted into the

gasifier / pyrolyser where it is processed to generate syngas.

Gasification – this is the partial oxidation of feed material to produce syngas

Gross Calorific Value – Also known as the higher heating value, this is the energy release

including latent heat from water vapour present by the combustion of one unit of fuel at

standard conditions (1 atm & 25°C)

Inline gas analysers – these systems have detectors which protrude into the main syngas

flow.

Mean residence time – The average time period feed material stays inside the gasifier /

pyrolyser.

MCerts – This is a standard for landfill emissions monitoring equipment and methodologies.

It covers the selection and validation of test methods; sampling pre-treatment and

preparation; the estimation of measurement uncertainty; participation in proficiency testing

schemes and the reporting of results and information.

Meter / Metering – The process of determining the GCV of a gas under consideration on a

ongoing basis.

Metering Class – The level of uncertainty associated with a metering design. For example

Class 1 has an uncertainty of 1%.

Molar Volume – This is the volume taken up by one mole of gas at standard conditions.


Net Calorific Value - Also known as the lower heating value, this is the energy release

excluding latent heat from water vapour present by the combustion of one unit of fuel at

standard conditions (1 atm & 25°C)

Non destructive gas analysis - gas analysis that does not destroy sample of gas in the

process of conducting the analysis.

Off-line gas analysis – This is where syngas is periodically extracted from the main flow

contained in bottles or bags for later analysis.

On-line gas analysis – this is where a detection meter is installed onto the process plant

allowing real time or slightly delayed readings.

Partial Pressure – the pressure a gas in a gas mixture exerts on its surroundings.

Prime mover – This is a machine that transforms energy from thermal or pressure form to a

mechanical form - typically an engine or turbine.

Pyrolysis – the thermal breakdown of feed material in the absence of oxygen

Reciprocating engine – this is internal combustion engine used to generate electricity from

burning syngas as a fuel.

Sample frequency – The frequency with which samples of the syngas are analysed when

using a discrete gas analysis method.

Steady state – this is the mode of normal operation for a generating station when it is not

starting up, shutting down, commissioning, or conducting trials.

Syngas – This is the gas mixture produced as a result of gasification / pyrolysis. This gas

mixture usually contains carbon monoxide (CO), carbon dioxide (CO2), steam (H2O),

methane (CH4), Nitrogen (N2), condensable vapours (VOCs) and tars.

Uncertainty – the probable margin of error associated with a measurement.

Volume fraction – the fraction of the total gas volume taken by the particular gas of interest.


AEA group 329 Harwell Didcot Oxfordshire OX11 0QJ

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