Gas Data Ltd

Flying High

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Operating a biogas plant without gas quality and flow instrumentation is like trying to fly a plane blindfolded - an experienced pilot may be able to keep the plane in the air for a while through shear skill, feel and a few helpful hints from the control tower but landing safely on the right airfield will be more likely a miracle. Where is your biogas plant going to land?

 

The technique of using bacteria under anaerobic conditions to convert a wide variety of carbon based bio-materials into methane gas has been used for decades yet its unappealing association with the treatment of sewage and animal slurry has left it largely ignored as a solution for clean, green energy generation. But that is changing dramatically on a global scale.

 

Almost daily, news about the need for secure and environmentally acceptable energy sources at competitive prices is to be found in the national and political press. In 2010 the change of UK government brought a fresh emphasis on this technology and the race is on to catch the world leaders who are to be found predominantly in Germany. The aspirations of the Anaerobic Digestion and Biogas Association are for 2000 new biogas plants to be constructed in the UK by 2015.

 

Modern biogas plants require significant capital expenditure and are carefully planned around expected efficiencies and tarriffs to predict both the return of capital and on going operating profits. It is typical for plants to target payback of capital within a three to five year period.

 

The attitude that energy from waste is energy for free is no longer the norm. Feedstocks all come at a price and operational costs need to be controlled. The prime energy gas from the anaerobic digestion process is methane. Depending upon the plant size, increasing the the methane quality by just 1% may easily equate to hundreds or thousands of pounds of extra revenue.

 

The majority of biogas plants have two stages of digestion (fermentation) to make the best use of feedstock. From the digesters the generated gas will be stored in a gas storage bag ready for consumption by the combined heat and power or gas upgrade unit. As both stages are sensitive to feedstock, feed rates, temperatures and stirring rates, it makes sense that the gas output at each stage be measured.

 

 

Parameter

Why measure

Where to measure

Common Technology

Methane

(CH4)

This is the energy source within the gas

Each digester stage output

Gas storage bag output

Infra-red gas analyser

Carbon Dioxide

(CO2)

It is an additional indication of digester activity

Each digester stage output

Gas storage bag output

Infra-red gas analyser

Oxygen

(O2)

Oxygen levels will typically be close to nil yet controlled to reduce formation of hydrogen sulphide

A sudden increase in the level of oxygen may indicate a plant leakage or seal failure

Each digester stage output

Gas storage bag output

Electrochemical sensor

Paramagnetic analyser

Hydrogen Sulphide

(H2S)

Corrosive and forms corrosive gas after combustion leading to damage of metalic components in plant and engine

Each digester stage output

Gas storage bag output

Electrochemical sensor

 

Gas Flow rate

To quantify rate of feedstock to gas conversion and rate of energy generation

Each digester stage output

Gas storage bag output

Thermal dispersion

Orifice plate

Pitot tube

Turbine meter

Pressure

Required for calculation of gas density and therefore mas flow rate if orifice plates or turbine meters used

Each digester stage output

Gas storage bag output

Resistive bride or strain gauge on diaphragm

Temperature

Required for calculation of gas density and therefore mas flow rate if orifice plates or turbine meters used

Each digester stage output

Gas storage bag output

K type Thermo-couple

PT100 Device

Secondary channel on thermal dispersion flow meter

 

Technology for flow measurements is most often in the form of thermal dispersion devices. These have be used extensively in the more advanced markets and are to be found in countries such as Germany where companies like Binder Engineering and others have worked in conjunction with biogas plant manufacturers and operators over many years to provide flow measurement instruments that are easily installed, maintained and above all, deliver long term accurate results. These devices are characterised to give true normalised outputs over all ranges of biogas concentrations temperatures, pressures and moisture contents. Thermal dispersion transducers do have both ‘pros and cons’ yet their speed, eases of installation and long term stability for negligible maintenance input are difficult to beat.

 

Other flow measurement devices can be used yet each have a weakness that must be carefully understood before doing so, for example, orifice plates, pitot tubes, and turbine meters all require accurate measurement of absolute pressure and temperature parameters which must then be fed into a flow calculation module before a normalised result is available. This is not especially difficult but is another system to be set up and checked.

 

Turbine meters have been used extensively and can yield good results as long as the compensation for pressure and temperature is done and also, most importantly, that the flow is steady. Turbine meters have a spinning element that does so at a rate proportional to the velocity of the gas but the kinetic accumulated by the turbine must be dissipated by the gas as the flow decreases. This is a relatively slow process so turbine meters tend to ‘over run’ in this situation and can report over optimistic flow values which when integrated to calculate total volume can be a very significant error.

 

Whatever your final choice of flow measurements device, flow measurements should be reported as ‘dry’ because water vapour is inert and contributes nothing to the calorific value of the gas. Water vapour content is highly dependent upon temperature and pressure. In practice, changes due to pressure are quite small as most plants operate at near constant pressures very close to atmospheric but process temperatures can vary tremendously with ambient temperature.

 

The error can be very significant because the biogas in the head space of a digester running at 35oC will be diluted approximately 8% by volume of water vapour and digesters running in the thermophilic mode at 55oC and will suffer with a water vapour content in excess of 17%. (Pressures near atmospheric are assumed for this approximation).

 

This physical phenomena is often the cause of a significant misunderstanding when calculations of expected energy output are made.

 

The readout of methane content from the gas analyser is unlikely to be the biogas from the head space of a digester at 35oC will carry with it approximately 8% water vapour. A gas analysis taken under these conditions would be likely to show that the methane content of 55%, and carbon dioxide content of 37%.

 

The same rigorous approach should be applied to the gas quality analysis too by measuring the gas concentrations at each stage and under known conditions of pressure and water content. Almost all systems currently on the market will analyse methane and carbon dioxide using infra- red adsorption analysis combined with electrochemical based sensors for oxygen, hydrogen sulphide and occasionally ammonia. Some systems exist that are ‘in pipe’ or ‘in vessel’ which have the great advantage of avoiding the necessity to pump small samples of the biogas to a central analyser. However, the hostile environment inside the biogas process makes this method extremely difficult so this type of analyser is rare and are yet to achieve an acceptable track record in the industry.

 

Extracted sample systems are very much the norm. Small bore (3mm to 8mm) pipes are used to carry biogas samples on demand to an analyser system. A single analyser can be time shared between as many sample points as necessary making this a highly cost effective method in terms of capital outlay and running costs. Sampling systems are almost always flexible in the way that they are programmed giving options for sample rates and sequences. They can be optimised from one to ten or more sample points scanning each in a programmable order typically completing a single cycle of all points in fifteen to sixty minutes depending on the size of the plant.

 

A major advantage of using a single analyser fed sequentially from the different locations on the plant is that any error due to analyser calibration drift will be equal across all measurements so when subtracting one reading from another to make comparisons between stages the error is cancelled. This means small differences between them can be more easily resolved. For a system using multiple analysers this advantage is lost. A further advantage of sequential sampling systems is for one of the sample points to be connected to a cylinder of accurate calibration gas. This gas can then be used each cycle or each day to verify the calibration status of the analyser automatically.

 

Measurement of gas quality and quantity not only measures the efficiency of feedstock consumption but also allows energy production efficiency to be calculated. Only by knowing the exact quantity of methane being consumed by a generator can its conversion efficiency be calculated accurately.

 

Biogas will almost always contain sulphur in the form of hydrogen sulphide gas. This gas is odorous, poisonous and is the source of metal corrosion when present in both its raw form and when combusted in the engine to form sulphur dioxide (SO2). The need to monitor this gas and control its generation is most important. All engine manufacturers will provide guide lines that sate the additional maintenance requirements that must be put in place for any given level of hydrogen sulphide in the gas. Without accurate knowledge of the levels then there is a real risk maintenance will either be carried out unnecessarily or even worse be left undone. Both outcomes can be very expensive.

 

The addition of instrumentation to a biogas plant does require careful planning if accurate results are to be obtained but it need not be onerous. Practical and theoretical limitations of the instruments must be taken into account such as temperature and pressure ranges, expected gas concentrations ranges (particularly that of hydrogen sulphide), lengths of sample pipes etc.

Additionaly standards and work practices that are appropriate should not be ignored. In the UK the Health and Safety Executive (HSE) publish documentation called the Dangerous Substances and Explosive Atmosphere Regulations (DSEAR) which provides a framework for the structure of plant and in its turn refers to the need for suitable equipment ratings such as ATEX.

 

With many educational establishments such as agricultural college Reaseheath, Nantwich, UK providing courses for the necessary skills of running biogas plants efficiently and safely, the monitoring of biogas is proving to be an integral part of biogas plant operation.

 

Happy landings!

 

Chris Dakin

 

Chris Dakin is the Managing Director of Gas Data Ltd and has built this successful business based on gas analysis instrumentation for environmental, industrial and green energy applications. In recent years he has been a strong advocate of the virtues of anaerobic digestion as an important solution to the current global demand for green, carbon neutral, sustainable energy. Gas Data Ltd is a member of the Anaerobic Digestion and Biogas Association.

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