Environment and engineering data for clean,

efficient combustion

single and dual fuel applications natural and biogases oils recovered fuels

www.dunphy.co.uk

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Dunphy Combustion Ltd designs and manufactures oil, gas and dual fuel burners and ancillary equipment for use in a wide range of industrial and commercial applications. Dunphy’s strategy of emissions reduction, without compromising energy efficiency, begins with the unique design of burners through the manufacture, quality control, installation and on-going maintenance of single and dual fuel burners.

Dunphy design engineers deploy three main approaches in the manufacture of heat and steam generation plant to reduce energy consumption and related costs:

• design burners on the unique axial air flow principle

• ensure conservation of energy by ensuring heat/steam/power is only generated when needed

• recovery, wherever possible, of heat and energy for re-use in the combustion process

Benefits of a high turndown ratio

4

Variable speed drive 5

Oxygen trim 6

Enhanced capital allowances

7

Energy Centre digital control

8

Case study—wastewater treatment applications

9

Measuring calorific value 10

Specifying boilers 12

Combustion—process and pollution

14

Air and the combustion process

15

Combustion elements and compounds

16

Calculating combustion efficiency

18

Calculating carbon emissions and savings

19

Minimising NOx 20

Controlling other combustion pollutants

22

Are you asking the right questions?

23

For additional

information about Dunphy product range and technical

documentation, visit our web site:

www.dunphy.co.uk

Case study 6

Case study –specifying for low CV gas

9

Contents

3

Energy efficiency—importance of axial air flow

The virtually perfect air distribution derived from the axial concept also ensures: • high combustion efficiency across a wide turndown range • lower fuel consumption • inherently low NOx

Fuel efficiencies are linked in large part to the fundamental process of combusting fuel in a way which consistently achieves the optimum burn ratio of approximately 1part fuel to 10 parts air. It is critical, therefore, that the air distribution is optimised.

The best way of ensuring optimisation is to specify an axial

airflow burner.

Axial air flow is a turbine design concept which operates using an axial fan. This turbine concept provides significant energy performance improvements when compared with other types of combustion equipment. The axial air flow produces uniform air distribution to the burner head at all levels of firing - including, in particular, low fire operation. Low fire status is very often ignored when measuring air flow distribution as most manufacturers state efficiency at maximum capacity rate (full firing rate).

Dunphy patented axial air flow burner

Burner designs There are 3 main types of burner on the market: gun-type, rotary cup and co-axial. The disadvantage of the old fashioned gun type or rotary cup burner designs is that of poor air distribution. The centrifugal forces created in gun type burners mean that the combustion air has to be straightened over the burner head in order to established satisfactory combustion conditions. The rotary cup burner also requires costly, energy absorbing splitters and diverter vanes to straighten air flow. A further drawback of those designs is that when the turndown ratio is altered, the air distribution is also affected. The laws of physics state that if the air distribution is not correct from its source, then no amount of costly and sophisticated equipment will rectify this basic design constraint.

Of the three main types of burner on the market, the most efficient design is the axial airflow burner.

The virtually perfect air distribution derived from the axial concept removes the need for costly energy absorbing splitters and vanes and, when used in conjunction with correctly dimensioned boilers, axial air flow burners are inherently low NOx emitters eliminating the need for investment in expensive, high maintenance flue gas recirculation systems. (More information on pages 20 and 21).

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Energy efficiency – benefits of high turndown ratio

1 2 3 4 5 6 7 8 9 10

Effect of turndown ratio on fuel costs Annual savings on fuel

£ 20,500

20,000

19,500

19,000

18,500

18,000

Turndown ratio of the burner

Note: Turndown may be limited by the ability of a particular boiler design to allow low flue gas temperatures.

Dunphy ‘T’ and ’TO’ series burners are designed to operate to a standard 10:1 turndown ratio. The smaller output pre-mix burner operates to 9:1 turndown.

The turndown ratio of a burner is the numeric ratio representing the highest and lowest effective system capacity. It is calculated by dividing the maximum system output by the minimum output at which safe, steady, controlled, efficient, pollution free combustion can be sustained. For example, a 10:1 turndown indicates that minimum efficient operating capacity is one tenth of the maximum capacity.

Burners with high turndown ratios can more readily respond to changes in load and are significantly less expensive to run. High turndown ratio burners: * save on fuel * reduce maintenance costs * reduce burner downtimes

‘TO’ series burners 12kW to 1MW

‘T’ series burners 160kW to 13MW

5

10 20 30 40 50 60 70 80 90 100

Variable speed drives (inverters) save energy and thus reduce costs by reducing the speed of the burner motor to the level optimally required to match air or fuel flows.

By reducing the speed of the motor, a variable speed drives ensures that only the energy absorbed by the motor is used -saving money AND reducing noise.

The graph below demonstrates the benefits of using a variable speed drive as compared with a fixed speed motor

Energy efficiency – benefit of fitting a variable speed drive

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Oxygen trim automatically and continuously compensates for some of the variables that affect combustion. This closed loop system (similar to lambda control on cars), is available for Dunphy burners and energy savings of around 1% can be achieved by fitting the system. Oxygen trim, per se, is not a fuel saving device. It is a system for maintaining good combustion efficiency. The system allows burners to compensate for changes in the calorific value of fuel, temperature and changes in barometric pressure. Without oxygen trim, Dunphy burners will operate at 1% oxygen without the creation of CO. Where oxygen trim is fitted to our burners, it provides a safety net to ensure continual and consistent operation at these low levels.

Oxygen trim

Case study—NHS steam plant incorporating oxygen trim

In 1988, Dunphy Installed their burners at the Airedale Hospital. In 2005, the company again won the tender to supply new burners for the three Spurr Inman (12,500pph) steam boilers. Three TD 415 dual fuel burners were supplied. Built to Dunphy’s patented axial air flow design, an optimum burn ratio of one part fuel to 10 parts air is consistently achieved-thus minimising fuel costs. This efficiency is enhanced by the oxygen trim system fitted to each burner and maximum efficiencies are achieved even during low fire operation-critical to the hospitals whose operating loads fluctuate significantly across 24/7, 365 day per year cycles. The burners provide full sequencing across the three boilers and each burner is additionally fitted with a Ratiotronic TM 3000 burner control system which manages the fuel and air trims as well as the operations sequencing for precise, real time control of the whole combustion process. A SCADA link is provided tol facilitate the remote application of intelligent links to the hospital building management software. The burners’ ultra low NOx heads maintain NOx emissions below 80mg/m3 on gas operations easily meeting the more stringent emissions targets due in 2010.

The Dunphy burner installation at Airedale Hospital includes variable speed drives which are managed by Ratiotronic TM 3000 digital burner system—with free

standing AOTC control consoles.

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To achieve listing on the approved Energy Technology List (ETL) all gas and dual fired burners rated above 400kW are required to have micro -processor based controls and a variable speed drive fitted to the forced draught fan. In addition, all such burner control elements (including combustion air dampers and fuel valves) must be fitted with a precision servomotor, which must be separately controlled and which must include positional or flow based feedback control. Procurement of ETL listed combustion equipment qualifies for enhanced capital allowance.

All Dunphy digitally controlled burners meet this new criteria which means our products are listed on the

Government approved Energy Technology List.

Pictured below is Dunphy’s Ratiotronic TM 3000 digital burner management system.

ETL and enhanced capital allowances

How to claim for ECAs Claims for ECAs are made in the same way as other capital allowances on the Corporation Tax Return for companies (the Income Tax Return for individuals and partnerships.) The Inland Revenue's guidance on the ECA scheme can be found

at www.inlandrevenue.gov.uk/capital_allowances/eca-guidance.htm. The Inland Revenue administers claims for ECAs. Claims must be based on the costs incurred. Where you have purchased a qualifying product that is not already incorporated into a larger item of plant and machinery you must use the price paid for the item as the base of your claim. If you have purchased a qualifying product which is incorporated into a larger piece of equipment, the eligible claim value can be found in the Claim Values section of the Inland Revenue’s ECA site. The remainder of the equipment can attract capital allowances at the normal (rather than the enhanced) rates.

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Energy Centre—digital control

A modulating burner with a high turndown, which can respond to rapid changes in steam pressure and which accurately maps the load, will improve efficiency. Every time a boiler switches itself off, self purges and switches back on there is a significant waste of energy and cost. For essential safety reasons, every time a burner re-starts, the boiler is purged with ambient air to ensure that no unburned fuel remains. This ‘purge’ of ambient air, which typically lasts about three minutes, cools the boiler and takes energy out of the system. The constant cooling and re-heating also continually expands, contracts (and thus weakens) the refractory and metal parts of the boiler and results in high maintenance and replacement costs as well as associated increases in boiler downtime. Add in to that hysteris effects (if mechanical modulation is still being used) and the waste of energy and of cash will be huge. A burner which can map the load to a modulation of a tenth or twelfth of maximum output, will reduce significantly the purging and cycling processes and thus reduce stressing of the boiler and components.

The burner is the ‘brain’ in the boiler house.

Compared with a new boiler, the burner fitted to the shell can be approximately half the capital cost.

Yet it’s the burner - not the boiler – that has the major impact on whole life costing, emissions and

efficiency.

Specifying burner requirements first, could save considerable project time and cost. And the improvement in energy performance and resultant cash savings over the whole life cycle of a heat or steam process will be substantial. The gas burner on the right is fitted with a Ratiotronic TM

3000 digital combustion control system. This manages the variable speed drive, fuel and air trims and operations sequencing for precise, real time fuel efficiency. It also facilitates remote energy centre management through SCADA links to all types of BMS. Digital control is available for all Dunphy single and dual fuel burners enabling consistent operation below 3% O

2 across all operating loads—and at less than 2% at

high fire.

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Case studies

The new Psyttalia wastewater biological treatment plant in Greece has installed Dunphy dual fuel biogas /oil burners because of their proven reliability in managing fuel and load variables over the essential 24/7 365 day process cycle - and because of their advanced technology for safely and efficiently burning sewerage gas and light fuel oil. Due to the biological process involved in producing sludge gas, the calorific value can vary by up to 20%. It is, therefore, essential that the burners are fitted with a fuel trim system with a quick response time. Integrated into each burner is a Ratiotronic™ 3000 digital control system with adaptive fuel trim which can respond within 15 seconds of a change. These units are specifically designed for either fuel or oxygen trim, responding within 15 seconds of any fluctuation from the set point. This enables the burners to maintain optimum fuel/air ratio levels over a turndown ratio of 1:10. The high turndown ratio over the burners enables the flow temperature to be maintained within 1oC irrespective of load requirements.

Psyttalia Wastewater biological treatment plant

The Ratiotronic™ 3000 manages operations with O2 levels of less than 2% and with NOx levels not exceeding 30mg/m3 corrected to 3% oxygen. The unit also provides remote real time data communication and plant operations logging. Biogas boosters and gas trains are manufactured with cast iron bodies and stainless steel fittings suitable for withstanding the corrosion properties of bio fuels.

NITRA district heating scheme Nitra is a busy commercial town north of Bratislava and energy officials there had come up with a unique plan to reduce the cost of their district heating scheme by switching gas supplies to an unused series of gas wells around 7 kilometres from the town. Local European burner manufacturers had been unable to solve the particular difficulties presented by both the “weak” gas (only 25% of natural gas calorific value) together with the problem that, when changing over from one well to another, there is a period of around 20 minutes when only CO2 (and no gas) is in the pipeline. Nitra had therefore been unable to take advantage of the virtually free local “weak” gas wells. Dunphy designed bespoke dual gas burners to deliver the required 19,000kW output. Each burner is controlled by a Ratiotronic system which delivers a controlled mix of weak and standard gas to optimum combustion efficiency levels.

Pictured below is one of the three Dunphy burners installed at the NITRA district heating scheme - now successfully up and running with substantial energy cost savings.

Specifying burners for low CV gas

Burner management in wastewater treatment applications

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Using gross calorific efficiency is a more meaningful input figure for calculating true boiler efficiency.

Boiler efficiency is, in general, indicated by thermal efficiency—ie energy output related to the energy input.

Boiler efficiency % = heat exported by water or steam divided by the heat provided by fuel x 100.

Calorific value is a key factor in boiler efficiency calculations. The heat or energy provided by fuel may be expressed in two ways—gross or net calorific value. Controversy has always existed about whether boiler efficiency should be expressed in terms of gross or net calorific value (CV). Gross calorific value is the theoretical total of the energy (heat) in the fuel. However, all common fuels contain hydrogen which burns with oxygen to form water—which is lost through the flue as steam. Gross CV, therefore, includes the energy used for evaporating the water in the combustion process. The flue gases from boilers in general are not condensed. The actual amount of heat available to the boiler is reduced. Net calorific value is the CV of the fuel excluding the energy in the water vapour discharged in the combustion process through the flue.

Example: the values for North Sea natural gas are:

Gross CV - 10.46 kW/m3 at 15oC

Net CV - 9.87 kW/m3 at15oC

Measuring calorific value—gross or net?

Boiler efficiency results which are calculated ignoring the latent heat (ie using the net CV ) will always be greater than one where the latent heat is included (ie using the gross CV.) Hence the popularity which exists among boiler manufacturers for quoting ‘net efficiency’. A higher efficiency always looks better! Nevertheless, boiler efficiency costs should be based on the total amount of energy (fuel) required—and gross CV measures are more accurate and realistic when comparing like for like estimates for burners and boiler plant.

The composition of gas is the main influence on the level of NOx emissions achieved in this type of combustion. CV varies enormously with the origin of natural gas used. All sources of European natural gases differ significantly in their CV and, in Dunphy’s experience, natural gases in mainland Europe tend to contain more inert gases which ultimately result in lower NOx levels. British natural gas has historically had a higher CV – and hence higher NOx levels. Dunphy has an advantage in that our equipment is widely used across Europe where increasingly stringent regulations are being put in place. The most stringent NOx compliance levels are in Holland and we readily achieve these in boilers or furnaces with the rIght geometry. We also supply burners fuelled with biogas to sites in Britain which are achieving NOx emissions as low as 20mg/m3.

Impact of calorific value (CV) on NOx

Britain is now moving towards becoming a net importer of natural gas and the gas supply companies are free to vary their sources of supply to end users in relation to their own procurement contracts. In other words, it is impossible to guarantee a consistent source of natural gas supply in this country. No burner or boiler manufacturer is, therefore, in a position to offer a firm guarantee on the NOx levels which will be achieved by their equipment.

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Dunphy was called in by their customer United Utilities to solve a case of out of control energy and maintenance costs at their Hatton wastewater treatment plant near Arbroath. Existing biogas burners imported in four years ago from a German company were failing to cope with a range of unusual on site operating parameters.

Case study

The utility is designed to run on biogas, but there are three unusual variables. Firstly, the supply of biogas is variable and natural gas is having to be piped in to supplement supplies. Secondly, the CV of the biogas is variable which means that control of output was proving difficult. And thirdly, the operating load of the utility fluctuates around the clock. Following a site audit, Dunphy drew up a bespoke design for a dual gas burner. The burner is fuelled by biogas and natural gas simultaneously and consistently operating at 3% O

2 across all

loads.

This high level of efficiency is achieved by two separate combustion chambers at each burner head (one for biogas and one for natural gas). The burner is controlled by a Ratiotronic TM

3900 which consistently measures the values of the combination fuels and automatically makes mix corrections to ensure optimum efficiency at low, medium and high fire. Energy and running costs have tumbled, efficiency has escalated and payback per burner is expected to be easily achieved within 6 months. An added benefit was that installation and commissioning by Dunphy was completed in just 4 days.

Specifying burners for low CV gas

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• specify large heat transfer surfaces to ensure the lowest flue gas temperatures possible. • a low surface heat release rate is

essential to achieve low NOx emissions (typically below 1.5MW/m3 with a three pass

design). • a generous furnace diameter will

allow for internal flue gas recirculation to take place thus reducing NOx production. • check the gas side resistance.

The higher the resistance, the more fan power is required by the burner—thus increasing costs.

• specify economisers. By making

the most of the heat put into the system - and thus reducing flue gas temperature to the bare minimum - large savings can

be achieved. • high density insulation must be

used on ALL parts of the boiler, not just the basic shell.

Significant energy savings can be achieved by ensuring all smoke boxes and manhole covers are insulated. • pn steam boilers, check that the condensate temperatures will allow appropriate temperature increases from the economiser as potential fuel savings of

around 6% can be gained.

Three pass boiler designs

Reverse flame boiler design

Specifying boilers for maximum efficiency

It is important to evaluate the operational outcomes of

the different designs of boiler available in the market place.

3 pass boilers will typically give the

best performance in terms of emissions and turn down and will deliver better overall efficiency

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Dunphy burners fit all makes and types of boiler, oven or furnace

(Above) Astellas Pharma one of two steam boilers (Below) Dual fuel burner fitted to a Yorkshireman steam boiler

(Above) Calder High School (Below) A Dunphy burner fitted into the side of a kiln at Caledonian Brewery

(Below) Delivery of one of five 12,500kg/hr dual fuel burners being installed at Manchester’s central PFI hospital site

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Combustion—process and pollution

Minimising the creation of any form of pollution is a priority at Dunphy and the company prides itself on being a world leader in clean combustion technology. Reducing pollution depends on a number of variables including: • type of fuel used • local conditions • design of the combustion chamber • heat release rate • the design of the burner and how that

complements the geometry of the boiler Although some methods claim to greatly reduce emissions, they may actually cause a reduction in efficiency, reduced heat outputs or an increase in carbon monoxide emissions. This next section describes how to measure and control harmful emissions from the combustion process.

Combustion by-products include (but are not limited to) NOx, SOx, CO and CO2, volatile

organic compounds (VOCs) and particulate matter. All vary in relation to the fuel being burned and can be harmful to the environment. Dunphy’s strategy of emissions reduction- without loss of energy efficiency - begins with the unique design of our burners through the manufacture, quality control, installation and on-going maintenance of our products. Unique axial air flow design The patented axial air flow design of Dunphy burners with its precise mixing of air and fuel ensures that all fuel is completely burned and the by-products of combustion properly exhausted providing the ideal conditions for clean combustion.

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Combustion can only be regarded as perfect when the process is completed without the presence of excess oxygen in combustion by-products. An example of combustion that is neither complete nor perfect is that of the transition of carbon to carbon monoxide. Oxygen is the sole supporter of combustion and the source of oxygen is air. The proportionate parts by weight of atmospheric oxygen and nitrogen are: • 23.15% oxygen • 76.85% nitrogen

To supply one kilogram of oxygen for combustion it is necessary to supply 1/0.2315 = 4.320 kgs of air.

For each kg of oxygen introduced into the furnace for combustion, 3.320kgs of nitrogen accompanies it. It serves no useful function and is a main source of heat loss in combustion.

With insufficient oxygen, carbon will combust not to carbon dioxide but to carbon monoxide. Insufficient oxygen also results in incomplete combustion of the fuel. Although some fuel will be completely burned to CO

2 some will be partially burned

to CO. The heat loss due to this incomplete combustion is very high.

Air and the combustion process

The selection of a correctly designed burner and boiler package is critical for the optimisation of combustion efficiency which ensures a reduction in carbon emissions. Broadly speaking, the process of minimising carbon emissions is by: • controlling excess air/oxygen without increasing CO whilst ensuring complete and high efficiency combustion • using high combustion efficiency to deliver

lower fuel use which in turn will reduce carbon dioxide emissions – ultimately delivering low carbon emissions

Nitrogen is introduced into the boiler furnace along with the oxygen required from air for the atmospheric combustion of any fuel. It is an inert gas which performs no function in the combustion process. It passes through the furnace and boiler without change (except in temperature and volume). It dilutes the air, absorbs heat and reduces the temperature of the products of combustion. For this reason, and because of the high proportion of nitrogen to oxygen in atmospheric air, it is the principal source of heat loss in combustion. Where nitrogen is present in quantities more fuel is required for combustion and hence more carbon is generated.

Excess air

Any oxygen supplied to the furnace in excess of that required for combustion results in the same losses as in nitrogen. Furthermore, this excess oxygen is accompanied by additional nitrogen which accentuates the combustion losses. In cases when there is insufficient oxygen for complete combustion, the nitrogen losses become inappreciable when compared to losses caused by the incomplete combustion of the carbon fuel.

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Substance Molecular Symbol

Atomic Weight

Molecular Weight Form

Carbon C 12 * Solid Hydrogen H

2 1 2 Gas

Oxygen O2 16 32 Gas

Carbon Monoxide CO * 28 Gas

Carbon Dioxide CO

2 * 44 Gas

Methane CH4 * 16 Gas

Combustible Substance Reaction Carbon to (CO) 2C+O2 = 2CO Carbon to (CO2) C +O2 = 2CO2

Carbon Monoxide 2CO + O2 = 2CO2 Hydrogen 2H2 +O2 = 2H2O

Sulphur to (SO2) S + O2 = SO2 Sulphur to (SO3) 2S + 3O2 = 2SO3

Methane CH4 + 2O2 = CO2 + 2H2O

Combustion elements and compounds

1 Calculation of products resulting from complete combustion of 1kg of carbon From the chemical reactions table, it can be seen that C + O

2 = CO

2. .

From the atomic weights 12+(2x16)=44. This means that in burning one kg of carbon to carbon dioxide, 12 parts of carbon combine with 32 by weight of oxygen to form 44 parts by weight of carbon dioxide. Any weight, therefore, of carbon dioxide must be composed of 27.27% (12/44) by weight of carbon—and 72.73% (32/44) by weight of oxygen. That is: one kg of CO

2 = 0.2727C+0.7273 O

2.

Since the ratio of carbon to oxygen in carbon dioxide is 1:2.667(0.7273/0.2727) then in burning one kg of carbon to carbon dioxide, 2.667 kgs of oxygen will be required. If one kg of oxygen is contained in 4.32kgs of atmospheric air (1/0.2315), then to completely burn one kg of carbon to carbon dioxide will require 11.52kgs air (2.667x4.32). Since each kg of oxygen is accompanied by 3.32kgs of nitrogen, then the total amount of nitrogen passing through with the oxygen will be 8.885kgs (2.667x3.32). In the complete combustion of one kg of carbon to carbon monoxide, the resulting products will be:

1kg C + 2.667O2 = 3.667kgs CO

2 and 2.667x 3.32kgs N

2 = 8.885kgs of N

2

The chemical reactions of combustion

From the atomic weights of the elements involved in combustion, it is relatively simple to calculate the proportionate parts by weight of the elements entering into the resulting compounds and the weights of the products of combustion. With the amount of oxygen required for combustion thus

determined, the amount of air required is directly indicated by the oxygen-nitrogen ratio of atmospheric air. This important consideration is demonstrated by the 3 examples which follow:

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The example to

the right sets

out the

calculation of

products

resulting from

complete

combustion of

one kg of

hydrogen

2 Hydrogen The chemical reactions table shows that two atoms of hydrogen will combine with one atom of oxygen to form water vapour - 2H+O=H

2O.

From the atomic weights (2x1) +16 = 18. That is, in burning one kg of hydrogen to water vapour, one part of weight by hydrogen will combine with eight parts of weight of water to form nine parts of weight of water vapour. Hence in one kg of water vapour, there will be 0.111 kgs of hydrogen (2/18) and 0.889 kgs of O

2 (16/18).

One kg of H

2O = 0.111 kgs H + 0.889 kgs O

2

Since the ratio of hydrogen to oxygen in water vapour is 1:8 (0.889/0.111) it will require eight kgs of oxygen for the complete combustion of hydrogen –ie (8x4.32) = 26.56 kgs of N

2

The products of combustion for one kg of hydrogen are therefore:

1kg H2 + 8 kgs O

2 = 9kgs of H

2O and8x 3.32kgs of N

2 = 26.52 kgs of N

2

CO2 kgs H2O kgs N2 kgs

0.75(C) + 2(O) kgs

2.75 * *

0.143 (H2) +

1.144O kgs * 1.287 *

13.28*(air) x 0.768 (N)

* * 10.41

Typical

combustible

fuels are

hydrocarbon

compounds.

This example

shows the

products of

combustion for

one kg of

methane

The atomic weights on page 16 show that CH

4 =C+4H equals 16 (12+4)

Thus one kg of methane is composed of 0.75(12/16) kgs of carbon plus 0.143 (4/28) kgs of hydrogen. To burn 0.75 kgs of carbon to carbon dioxide will require (0.75 x 2.667) = 2.00 kgs of oxygen. To burn 0.143 kgs of hydrogen to water vapour will require (O.143 X 8) = 1.144 kgs of oxygen. The total oxygen required will be ( 2 + 1.1444) = 3.1444kgs. The total air required will be (3.1444 x 4.32) = 13.28gs. The products of combustion for one kg of methane =

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%O2 by volume 0 2 4 6 8 10 12 14 16

% excess air level 0 10 23 36 55 80 122 186 268 (approx)

Natural gas combustion efficiency

How to read the graph Start with the flue gas temperature line on the right hand side and use the excess air/% oxygen line to find where they intersect. Then read across to the left hand scale to identify percentage efficiency. Eg: at 250oC and a percentage oxygen by volume of 2%, combustion efficiency is around 89%. At the same temperature and with a percentage oxygen by volume reading of 8% (fairly typical in buildings with old plant) then efficiency drops to only 84% This 5% drop in efficiency is expensive in fuel and will escalate significantly if inefficient modulation is causing hysterisis and low turndown rates.

Calculating combustion efficiency

The aim of an efficient burner is to achieve combustion conditions as near as possible to the stoichiometric parameter of the fuel. (The term stoichiometric is used to denote a condition in which the equivalent weights of substances in a chemical reaction have been precisely determined.) At near stoichiometric level, the oxygen content of flue gas is at the lowest percentage and CO

2 is at the

highest. The more efficient the burner, the lower the amount of excess air used in combustion resulting in the least volumetric emission of CO

2 in flue gases. Perfect combustion involves using exact weights of

air and fuel—but, in practice, this perfection is just not possible. Even the most superior burners will use a certain amount of excess air.

Dunphy burners operate at levels well below 3% excess oxygen.

Measuring efficiency from flue gas loss Boiler efficiency can be measured by comparing percentage oxygen levels taken from flue gas readings. Most boilers will emit flue gas at approximately 250oC. On comparative boilers, the difference in flue gas temperature may differ by (at most) 15oC. This will affect efficiency by less than 0.5%. But the difference between excess oxygen levels can affect efficiency by as much as 10% and this does have a major impact on efficiency and the cost of fuel.

Always check efficiency measurements at low fire levels. Low fire status is very often overlooked when air flow distribution is measured. Many burner manufacturers state efficiency at maximum capacity rate (full firing rate). In fact, most specifications tend to over provide capacity and across a full year cycle the most common mode will be medium - low fire with full firing accounting for less than 2% of operational time. Electronically modulated Dunphy gas burners are designed to operate at less than 2% excess air at high fire and at 3-3.5% at low fire.

Net

perc

en

tag

e e

ffic

ien

cy

19

This chart shows typical carbon dioxide and oxygen analyses of flue gas when combustion is carried out at different levels of excess air. The concentration of CO

2 falls

and that of O2 rises as more

excess air is used for combustion. Note: the mass of C0

2 will not

change in the flue gas but its percentage will decrease with increasing levels of excess oxygen.

Using the graph opposite it is possible to calculate carbon emissions

Example A A Dunphy burner operating at an average of 2% oxygen on a good quality boiler rated at 5,275 kW at 50% load with flue gas temperature of 180◦C. Converting efficiency to mass:

5,275/2 *24*360/0.845/10.4 = 2.59 million m3/year 1m3 = 0.304kgs, gross CV =10.4kW/m3

It follows, therefore, that 25.9 million m3/year of gas

=788 tonnes/year = 2,168 tonnes of CO

2 emissions per year

Example B A less efficient boiler with a non Dunphy burner will operate at an average of 4% oxygen with a flue gas temperature of approximately 205oC

5275/2*24*360*/0.815/10.4 =2.68 million m3/year = 2,247 tonnes of CO

2 emissions per year

Burner ‘A’ delivers a saving of 79 tonnes of CO

2 per year

Note: this saving is based on a specific point in the turndown range and does not take into account any additional efficiency gains and fuel savings through improvements in turndown.

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Minimising NOx - burner design, air and fuel staging

In addition, Dunphy burners can be fitted with low NOx combustion heads which ensure ultra low NOx gas and oil configuration which will meet all national and international standards without the need for expensive, energy consuming flue gas recirculation systems.

Clean, simple and effective, Dunphy axial air

flow burners can be retrofitted on existing plant and are the most preferred and approved method of

reducing NOx.

Burner design is critical in ensuring maximum efficiency and low NOx. Part of Dunphy’s international success over forty years has been as a result of our technological innovation minimising the creation of NOx. Crucial to the consistent reduction of NOx is the principle of axial air flow which, when used in conjunction with a correctly dimensioned boiler, enables burners to be inherently low NOx emitters. Air/fuel staging is one of the most effective methods of reducing NOx and is achieved by separating the air and fuel into areas of sub stoichiometric and over stoichiometric conditions. The main aim of this technique is to divide the air into three distinct and separate patterns: primary, secondary and tertiary. Each of these patterns is controllable in its own right. The geometry of the boiler or furnace must be such that hotspots are eliminated; eddying currents are created to facilitate internal flue gas circulation - and there is a low and smooth transfer of heat. And because NOX is a thermally produced gas, its reduction is also dependent on the calorific value of gas. Without firm data on air flow, boiler geometry and the CV of gas, boiler and burner suppliers cannot guarantee consistent levels of NOx emissions.

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It is generally accepted that UK regulations governing NOx

emissions will, from 2010, move closer to the more stringent levels already in

force across much of Europe. The chart alongside gives examples of current European NOx standards for

gas and 35 seconds oil (all based at 3% O

2).

Country

Gas Oil (35 seconds)

Switzerland < 80mg/m3 < 150mg/m3 Holland < 60mg/m3 < 120mg/m3

Austria < 80mg/m3 < 150mg/m3

Germany < 100mg/m3 < 200mg/m3

Minimising NOx—other types of solution

External flue gas recirculation

External flue gas recirculation (FGR) recovers a percentage of exhaust gas and feeds it back through a fan into an annulus (the shape made by two concentric circles). This inert gas is re-introduced to the combustion air and passes through to the hottest part of the flame—the core. The state and composition of the flue gas not only cools the flame but also creates sub stoichiometric conditions. This, in turn, reduces the creation of thermal NOx. Advantages of FGR FGR is a popular and proven technique for reducing NOx levels -particularly thermal NOx. Disadvantages of FGR Reductions of NOx levels can be relatively modest and installing FGR does create major restrictions in boiler design. For example, if the quantity of re-circulated gas is 20%, the capacity within the boiler will need to be increased—thus increasing its size. This means that FGR cannot be easily retrofitted without the boilers being down-rated. FGR also requires sophisticated control, extensive and cumbersome external pipework and in some instances, blowers—all of which have to operate at exhaust gas temperatures. As a result, first costs and maintenance costs increase and boiler reliability diminishes.

Water and steam injection

Theoretically, this method operates on the same principle as combustion gas recirculation. Water or steam is introduced to the combustion zone. This cools the flame and creates sub-stoichiometric conditions. Advantages

The injection of water and steam offer the possibility of reducing NOx levels—but it is questionable whether emissions are truly reduced. Disadvantages

Water injection reduces boiler efficiency because the added water has to be evaporated. If fueled by residual fuel oils, then sulphur in the flame makes more likely the creation in the stack of SO

2 and SO

3.

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Sulphur oxides - checking for SOx levels in fuel The formation of sulphur oxides is a result of the sulphur contained in fuel being emitted through the combustion process. SOx levels are insignificant in natural gas, but must be considered and controlled when oil, coal or other types of sulphur heavy fuels are used. SOx mixes with water vapour to form sulphuric acid in the atmosphere which is corrosive and dangerous to the environment. Specifying low sulphur fuels is the most cost effective method of SOx reduction. Within the EU the sulphur content of gas oil and fuel is regulated. The chart below shows the maximum level of sulphur permitted as a percentage of mass.

VOCs –volatile organic compounds contain combinations of carbon, hydrogen and sometimes of oxygen. VOCs vapourise easily when emitted into the air and can form ground level ozone. Through careful air management, Dunphy burners minimise VOC emissions across the whole firing range - a critical factor in most drying applications. Proper maintenance of the burner is important. Improperly maintained equipment can result in VOC levels over 100 times higher than normal.

Controlling other combustion pollutants

Fuel type Current permitted amount Amount permitted from 2008

Gas oil

0.2% by mass 0.1% by mass

Fuel oil

0.1% by mass (or permit which confirms compliance with SOx emission limit of 1700mg/Nm3)

Noise is also a pollutant.

ADE noise regulations state that room sound levels from single burner operations must not exceed 74dB(A) at one metre at maximum firing range.

To ensure ‘whisper quiet’ operations, Dunphy burners are designed: • on the axial air flow principle which prevents sinusoidal movement so the burner is highly efficient and inherently quiet. • with the motor and fan fully enclosed in an aluminium

body • with 100% of combustion air passed over the forced

drive motor - this provides 100% heat recovery eliminating the need for an additional noisy (and energy absorbing) fan.

If the burner specification is right, noise standards

can be achieved without the need for expensive and space consuming acoustic screens.

Dunphy burners typically operate at

approximately 65dBA at one metre at mid fire without any separate

noise attenuation equipment

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• Is boiler efficiency quoted as net or gross? (Net efficiency is, on average, about 10% higher than true gross efficiency). • Is the burner designed with axial air flow—or is it an old style pressure jet or rotary cup model? • Is the boiler a ‘true’ three pass design or a two or three pass, reverse flame design? • At what point in the turndown range is efficiency being quoted? (Probably at maximum firing ! ) What turndown will be realistically achieved? • What excess oxygen levels are being considered to determine the efficiency? (Excess oxygen levels are always at the lowest level at maximum output, but a boiler will typically spend 97% of its time at less than high fire therefore oxygen levels at less than high fire are critical). • What NOx levels (when referenced to 3% oxygen) will be achieved? (3% oxygen is the industry benchmark) • Is digital modulation incorporated to ensure tight fuel control? • Is the risk of disruption to natural gas supplies being minimised through dual fuel burners?

If it’s YOUR budget that pays for fuel costs,

are you asking your engineers the right questions?

For further information related to specific applications and sites—or for a FREE energy audit—contact Dunphy Combustion by e-mailing

sharon.kuligowski@dunphy.co.uk

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Our burners: • fit all makes and types of boiler, oven, and

heat treatment plant • are fully tested and pre-fired ready for

speedy and efficient on site installation, commissioning

• are supported by our nationwide maintenance and spares service • are available from12kW to 30MW output

Dunphy contacts: main switchboard 01706 649217 Burner/boiler specifications, technical data, quotations:

Sharon Kuligowski sharon.kuligowski@dunphy.co.uk

Maintenance and technical service: Jonathan Rigby service@dunphy.co.uk

Spare parts service:

Phil Sherratt spares@dunphy.co.uk

Dunphy Combustion Ltd. Queensway, Rochdale OL11 2SL www.dunphy.co.uk

Dunphy burners are designed for safe and clean use with all types of single or dual fuels. For example: • natural gas of all calorific values, biogas, coke

oven gas, coal and biomass sources • recoverable solvents and waste, light and heavy

oils, ethanol, tallow oil and rapeseed oil

Reduce the risk of natural gas supply shortages by specifying dual fuel burners.

Dunphy—products and services