Archive for category: Steam Accumulation Publications

ENERGY STORAGE

Steam Accumulation

*David Oakland describes an energy efficient solution to steam generation problems in paper and board mills
Energy storage has become the ‘hot topic’ of the 21st century for energy managers and operators of power plant. For electricity generation the main impetus is the balancing of intermittent between supply and demand, and, in the wake of 2015 climate change summit in Paris, increasing compliance with governmental legislation for reducing environmental pollution. For industry it is increased operating efficiency, achieving a smaller carbon footprint, lower energy consumption and securing the needs of production. ‘Steam accumulation’ is a long established energy storage technology for achieving these aims and can potentially be the right solution for many of the operational problems tolerated in the power plant and production processes of paper and board mills.

So what are these problems:

The manufacture of paper and board involves processes that require large quantities of steam and power. In a paper mill, one of the largest consumers of steam is the paper machine which, whilst variable between paper type and production speed, is normally steady and predictable. However, the ‘status-quo’ can be interrupted as a result of a downturn or a total cessation of steam supply from the steam generators. This may be loss of steam from base-load thermo-mechanical pulping resulting in a rapid loss of pressure in the steam supply network causing in turn a rising or sudden demand on the supplementary fired boilers. This in turn increases the risk of ‘priming or even ‘lock-out’ being induced when approaching limits of high and low water level in the boiler steam drums. If a pressure-critical back pressure steam turbine is also being used to feed the paper machine, this may further increase the risk of ‘paper breaks’ resulting in loss of production. Moreover, higher demand on the fired boilers can increase energy cost if a premium fuel (gas or oil) is used.

Steam turbo-generator performance can suffer if the back-pressure or pass-out steam rate is governed by an irregular process steam demand which if below maximum turbine speed, power generation and efficiency is reduced. If the steam demand exceeds maximum turbine speed, the shortfall has to be supplemented by fired boilers to ‘reach the peak’.

Paper breaks at the paper machine can occur at random and result in a sudden cessation of demand on the steam supply system necessitating a rapid ramp-down of the steam generators.

Whilst this alone can be difficult alone to manage, the paper machine, upon start-up, can re-impose a sudden demand of considerably higher magnitude than that of a normal running load and which is potentially incapable of being met by the steam generators due to insufficiently fast reaction, thus again risking deleterious steaming conditions in the steam drums.

Pulp digesters also consume large quantities of steam and this batch process additionally imposes a highly fluctuating and randomly varying demand on the steam generators requiring steaming capacity sufficient to reach the ‘peak’ whilst also subject to rapidly changing firing rates.

A steam accumulator can balance all these conditions, maintain steam supply, reduce supplementary firing, hold steam generation performance, maximise turbogenerator output and improve overall plant operating efficiency.

What is steam accumulation:

Put simply, it is the storage of steam heat (in a pressure vessel – a steam accumulator) produced in the boiler at times of low demand for subsequent release to supplement the output of the boilers at times of high demand.

The term ‘steam accumulator’ is a misnomer and a more apt description would be ‘heat accumulator’. This is because water (not steam vapour) is used as the storage medium in the storage vessel. The reason becomes evident when the greater heat storage capacity of water is compared with that of the same mass of steam vapour by volume at any given state of temperature and pressure. Most accumulators work on the ‘pressure drop’ or ‘sliding pressure’ principle whereby steam from the generator is charged at high pressure into the water – thus raising its temperature and pressure. Discharge takes place when the steam contained in the steam space of the vessel is released, the consequent reduction in pressure causing an imbalance of temperature conditions with that of the water resulting in ebullition at the water surface and the evaporation of more steam (flash vaporisation) as discharge continues and vessel pressure falls.

How can a steam accumulator help:

A steam accumulator can provide a reserve of steam, instantly available, to maintain paper production in the event of a loss of boiler steam, can absorb excess steam during boiler ramp-down, can eliminate wide fluctuations of demand on the steam generators, improve the boiler load factor, reduce reliance on premium fuels, increase combustion efficiency and reduce fuel cost. If the power plant includes a steam turbo-generator, a steam accumulator would convert the variable process steam demand to a constant load thus enabling the electrical generator to be matched to the power demand and to run continuously for longer periods at higher output and at highest efficiency. And because of the ability of the accumulator to discharge steam instantaneously from ‘storage’ to meet sudden changes in demand, pressure in the steam distribution system is held constant and under a turbine plant trip, the standby boilers are given time to run up to full output and the interruptions that would otherwise occur need not be tolerated

What are the practical and economic benefits:

The benefits arising from balancing fluctuations of demand are based on improvements in steam generation performance and the potential for fuel savings. Operational savings can be evaluated financially with greater or less accuracy and relate to fixed capital, fuel and labour costs, and also to sundry costs associated with steam generation. However, other advantages are more difficult to assess. The reliability and simplification of the steam supply, the maintenance of steam conditions and the existence of a steam reserve are operational advantages that not only improve working conditions but secure the needs of production and improve product quality.

The most important energy savings obtainable from by steam accumulation derive from reducing and stabilising the necessary boiler output. Therefore the possibility of a steam accumulator meeting the steam demand with a smaller boiler plant is an important factor in investment decisions. Moreover, the improved thermal efficiency of steam generation achieved by balancing the load variations is also of considerable importance. The fuel losses due to irregular operation of the boilers vary greatly with the flexibility of the combustion equipment but doubtless, boiler efficiency is maximised when the boiler can be held at constant load within its rated output. The losses are due to part loading giving a low load factor, rapidly alternating loads and idle live capacity that increases standing heat and power losses.

Balancing of the boiler load also removes the adverse effects of load fluctuation on steam conditions, i.e., boiler pressure, steam temperature and steam dryness. Without steam storage, all load variations lead to some pressure change, whereas with an accumulator installed the pressure can be held constant. Also, the elimination of loss due to letting-off steam through safety valves has positive advantages for power generation and manufacturing processes. Moreover, the steam reserve in the accumulator is instantly available to secure the needs of production.

Account should also be taken of the greater strain on the boiler shell and pressure parts that is imposed by modern combustion equipment. Wide turndown response to the most demanding fluctuations leads to rapidly changing temperatures in the furnace and high thermal stressing of the metal. The longevity of boiler components and the minimisation of boiler maintenance costs are clearly allied to the stable load conditions that are enabled by the addition of steam accumulation.

Steam accumulation – its place in the modern boilerhouse:

Steam accumulation can be harnessed by designers of steam plant to secure higher energy efficiency, the creation of a steady boiler load and the provision of an immediate and unlimited supply of process steam while crucially assisting in the control of gaseous emissions and reducing energy consumption. The technology of steam accumulation should be viewed as a very useful and essential part of the modern boilerhouse and power plant.

*David Oakland is Principal Consultant at David Oakland Associates, a specialist UK ‘steam accumulation’ design provider

Download The PDF Here: DO562-1 Rev 1

David Oakland describes how steam accumulation can be harnessed to aid steam plant design and secure higher energy efficiency

Energy managers are constantly coerced, whether because of legislative environmental directives or simply self-imposed responsibility, to find and implement alternative energy-efficient solutions to reduce their greenhouse gas emissions. The spin-off benefits are lower energy bills and increased bottom line profitability. Compliance with regulation and achieving energy cost reductions have become necessary bed-fellows.

A major concern for the steam plant designer is, in many cases, the fluctuating nature of the process steam demand to be met. The demand for steam from many industrial processes is erratic and step changes in load can have a detrimental effect on steam production and efficient power generation in the case of combined heat and power plant. However, there is a solution to this problem that not only removes the effect of transient steam loading completely from the steam raising or power plant but also improves energy efficiency and reduces operating costs. It is the incorporation of a ‘steam accumulator’ into the steam supply system. Steam accumulation is a long established but too often forgotten technology that, if correctly applied, can continue to be of great benefit to enhancing the design and performance of steam plant and cogeneration in the modern boilerhouse.

What can steam accumulation achieve?

Imagine a boiler able to reach a ‘peak’ demand ten times higher than its rated output. Consider that two out of a bank of three boilers might be eliminated and still allow the remaining boiler to reach the same ‘peak’. Contemplate maintaining a low-pressure steam supply while diverting boiler output to critical high-pressure consumers. These are just three examples of the many application possibilities for steam accumulation.

The purpose of a steam accumulator is to provide the means to convert an irregular process steam demand into a steady load. The potential benefits are numerous, the primary one being significantly lower energy costs through increased operating efficiency. Additionally, it can provide immediate response to the steam demand, a secure steam supply at constant pressure, temperature and dryness, and will eliminate boiler priming high and low water lock-outs due to wide load swings. It can also assure product quality and – where new steam or power plant is required – result in lower capital costs. Crucially, it can achieve all this while reducing environmental pollution.

So what exactly is steam accumulation? In short, it is the storage (in a pressure vessel) of surplus steam produced at times of low demand for subsequent release to supplement the output of the boiler at times of high demand. Any industrial manufacturing process having a variable demand for steam and where an effective differential exists between boiler and process pressures can benefit from this energy efficient technology.

How do accumulators work?

Water is used as the heat storage medium. The reason becomes evident when the greater heat storage capacity of water is compared with that of the same volume of steam vapour at any given state of temperature and pressure. Most accumulators work on the ‘pressure-drop’ principle, whereby steam from the boiler or waste heat steam generator is charged at high pressure into the water causing a rise in temperature (and pressure) and steam for process is discharged from the water (as ‘flash’) at low pressure to meet the demand.

The size of the storage vessel depends on the difference in pressure raised in the boiler and that of the pressure required by the process, and the amount of storage required. Storage vessels can range in size from 1m3 to 150m3 in volume (and more), or be a multiple of vessels to achieve the required storage volume.

What industries can benefit?

Steam is an essential commodity for many industrial processes. The technology can be successfully applied to any manufacturing process where steam is used for batch processing or at highly fluctuating rates. These conditions can be found in food production, canning, distilling and brewing, chemical manufacture, textiles, rubber, paper and board, plastics, pharmaceuticals, steel making, laundries, ammunitions, brick and concrete curing, turbine testing, tyre manufacture, combined heat and power. The list is endless!

When it comes to the energy saving economics, the most important energy cost savings to be gained from steam accumulation derive from a reduction in the boiler output in combination with load stabilisation. In all cases, there are prospects for energy savings emanating from: the elimination of excess steam generating capacity and part-loading that causes a low load factor, the removal of a rapidly fluctuating demand and idle live capacity that increases standing heat and power losses. Therefore the possibility of a steam accumulator meeting a fluctuating steam demand with a smaller boiler operated at a higher load factor and at constant output with higher efficiency is real.

Expectations on energy savings will vary depending upon individual circumstances and as such, predictions on these and operational gains can only be generalised. However, a conservative expectation of improved boiler combustion efficiency for a modern boiler and burner equipment using the latest accumulator control methods could be between 2 to 4%, and potentially higher for older boiler plant. The necessary data from which to obtain the required information will likely be readily obtainable from the existing energy management system but with older plants where scant information is available, the addition of the necessary temporary or permanent instrumentation could easily be acquired.

Standing heat and power losses resulting from excess online steam generating capacity could equate to a further 1 to 3% of maximum boiler rating again depending on the type and age of the boiler.  Where a boiler is of large refractory construction, savings on maintenance could have an even greater impact on the case for investment.

Operational gains

Fewer boilers online-operated under steady load conditions would lead to less wear and tear and as a result, lower maintenance costs. The potential for labour and material cost savings may be easily examined but it’s safe to say that a reduction in operating costs would contribute greatly to the decision to invest in the technology of steam accumulation.

David Oakland is principal consultant at David Oakland Associates, a specialist steam accumulation design provider.

Read More Here: http://www.engineerlive.com/content/steam-accumulating-interest

“One year to save the planet from climate change disaster.” Those words were uttered by Ed Davey, UK Government’s secretary of state for Energy and Climate Change, ahead of the United Nation (UN) Climate Change Conference held in Lima, Peru, in December 2014.

Energy managers are often coerced, whether because of legislative environmental directives or self-imposed responsibility, to find and implement alternative energy efficient solutions to reduce their greenhouse gas emissions. The spin-off benefits are lower energy bills and increased bottom-line profitability. Compliance with regulation and achieving energy cost reductions have become the necessary bedfellows.

Combined heat and power (CHP), otherwise known as ‘cogeneration’, is a long-established and well-advanced technology for lowering operational costs. However, its relevance to Ed Davey’s comment is now even more important in the context of reducing energy consumption.

In the wake of the Lima conference and in advance of the forthcoming UN summit in Paris in December 2015 to achieve legally binding and universal agreement on future climate change action, limiting and reducing harmful man-made changes to the ecological state of the Earth’s atmosphere is at the top of the agenda for the 196 participating countries.

CHP Defined

CHP is the simultaneous generation of electrical power and heat for process use achieved with higher levels of energy efficiency and lower emissions than possible as separate entities. The reduction in fuel used, while displacing fuel consumed by central generation stations helps achieve environmental policy objectives. Gas turbines and gas engines (the prime movers for cogeneration) are among the cleanest fossil-fueled power generation equipment commercially available.

The main advantage of CHP installations over conventional arrangements—where power is imported from the grid and heat for process use is raised on-site—is that, the heat contained in the exhaust gasses is produced at negligible cost as a by-product of electricity generation.

The most common use for this exhaust gas energy is the recovery of the heat for conversion into useful thermal energy. This is usually in the form of steam or hot water in a heat recovery heat exchanger or as a source of direct energy for process fluid heaters, or for pre-heating of combustion air for fired boilers. The steam produced may also be used to drive a steam turbine in a combined-cycle plant.

A major concern for the CHP designer often is the fluctuating nature of the process steam demand to be met. Many systems use supplementary firing in or before the heat recovery steam generator as means of increasing system flexibility to compensate for variances in the power load or process steam demands. However, control of steam production cannot be fully accounted for in the design of most CHP plant without additional measures.

The demand for steam from industrial processes is invariably erratic and step changes in load can have a detrimental effect on efficient power and steam production. The electricity generator in many cogeneration applications is sized to run most economically at full output. Therefore, when less than full power is required due to a drop in steam requirements, the efficiency of the turbine or engine reduces and emissions increase, especially at half load and below.

So it is almost always the case that standby boilers and supporting auxiliary services are also required in the CHP system to supplement process steam supplies when required, or at maximum demand rate in the event of a CHP plant trip. In such circumstances, even with auxiliary back-up, it is almost inevitable that the production of process steam falls and distribution pressure (and temperature) will be lost because of the inherent lag in auxiliary system response.

However, a solution to these problems exists that not only removes the effect of transient steam loading completely from the CHP plant, but also reduces or eliminates the need for auxiliary steam plant with all the attendant fuel and operating costs. It is the incorporation of a steam accumulator into the steam supply system. Steam accumulation is a long-established but often forgotten technology that if correctly applied can continue to enhance the design and performance of CHP plant and in the modern boiler house.

The purpose of a steam accumulator in CHP applications is to provide the means to convert an irregular process steam demand into a steady load. This enables the sizing of the electrical generator to be matched to the power demand and for it to run continuously for longer periods at full output and maximum efficiency. In view of the ability of the accumulator to discharge steam instantaneously from ‘storage’ to meet sudden changes in demand, pressure in the steam distribution system is held constant. Under a CHP plant trip, the standby boilers are given time to run up to full output and the interruptions that would otherwise occur need not be tolerated.

Additional Applications

Imagine a boiler that can reach a peak demand 10 times higher than its rated output. Consider that two out of a bank of three boilers might be eliminated and still allow the remaining boiler to reach the same peak. Contemplate maintaining a low-pressure steam supply while diverting the boiler output to critical high-pressure consumers. These are just three examples of the many applications possibilities for steam accumulation. Steam is an essential commodity for many industrial processes. The technology can be successfully applied to any manufacturing process where steam is used for batch processing or at highly fluctuating rates.

These conditions can be found in food production, canning, distilling and brewing, chemical manufacture, textiles, rubber, paper and board, plastics, pharmaceuticals, steel making, laundries, ammunitions, brick and concrete curing, turbine testing, tire manufacture and CHP.

Variable steam demand profile showing the load balancing effect of a steam accumulatorThe potential benefits can significantly lower energy costs through increased operating efficiency, a secure and immediate response to the steam demand, the assurance of constant product quality and where new boiler plant is required, lower capital cost and crucially achieving all this whilst reducing environmental pollution.

Steam accumulation is the storage (in a pressure vessel) of surplus steam produced at times of low demand for subsequent release to supplement the output of the boiler at times of high demand. Any industrial manufacturing process having a variable demand for steam and where an effective differential exists between boiler and process pressures can benefit from this energy efficient technology.

How Do Accumulators Work?

The term ‘steam accumulator’ is a misnomer and a more apt description would be ‘heat accumulator’. This is because water (not steam) is used as the heat storage medium. The reason becomes evident when the greater heat storage capacity of water is compared with that of the same volume of steam vapour at any given state of temperature and pressure. Most accumulators work on the ‘pressure-drop’ or ‘sliding pressure’ principle whereby steam from the boiler (or HRSG in the case of CHP) is charged at high pressure into the water and steam for process is discharged from the water (flashed) at low pressure to meet the demand.

The size of the storage vessel depends on the difference in pressure raised in the boiler and that of the pressure required by the process, and the amount of storage required. Storage vessels can range in size from 1m³ to 150m³in volume (and more), or be a multiple of vessels to achieve the required storage volume.

How Is the Water Heated?

Charging of an accumulator takes place when ‘surplus’ or ‘excess’ steam from the boiler is condensed in the water space of the accumulator. This is achieved by directly injecting the steam into the water by means of special charging nozzles. The total heat of the steam is given up to the water resulting in a continuous rise in the temperature of the water. This process continues until the corresponding pressure in the vessel has reached that of the charging steam.

What Happens When Steam Is Released?

Discharging takes place when the steam in the head space above the water is released (by opening the outlet valve), which causes a momentary reduction of pressure leading to an imbalance of that lower pressure with the saturation temperature of the storage water, whereupon the surface water starts to boil and more steam is released as ‘flash evaporation’ to replace the released water.

The evaporating steam carries away with it the latent heat of vaporisation and this process may continue for as long as steam is required to be discharged. Eventually the accumulator pressure will fall to the minimum required by the process being supplied and at that point the accumulator is effectively exhausted, requiring to be recharged.

Accumulator Control

The application of the technology has changed markedly since the era of cheap energy. At that time, storage vessels were overly large, coal burner response was slow and control was relatively uncomplicated by today’s standards. The main purpose of accumulator control was to enable the steam supply plant to reach an otherwise unreachable peak, but did not reflect the need to maximise boiler efficiency.

Currently, the picture in the modern boiler house is different and the need for industry to continuously reduce its energy bill through improved operating efficiency is a priority. The latest accumulator control methods are a far cry from those of the old days and are now designed to optimize boiler efficiency in conjunction with today’s modern burners.

Matching the amount of steam stored to modern burner performance, connecting the storage to the steam supply system in the most effective way, and customizing the microprocessor accumulator controls are the keys to a successful installation. Control systems that continuously balance the boiler load to a varying ‘average’ rate, thus allowing the installed storage capacity to be minimized, and others that utilize the modulating capability of modern burners, but at a gradual rate of change in response to a sudden peak demand are typical examples.

A reduction in on-line generation capacity leads to practical improvements in boiler operation. Balancing the boiler load removes the adverse effects of load fluctuations on steam conditions, which are boiler pressure, steam temperature and dryness. Without the storage of steam, all load variations lead to some pressure change, whereas with an accumulator interposed between the boiler and the process, the pressure can be held constant. Also, the elimination of high and low boiler water lock-outs due to wide load swings has positive advantages, not only for manufacturing processes, but also for power generation. The reliability and simplification of the steam supply, the maintenance of steam conditions and the existence of a steam ‘reservoir’ are operational advantages that improve working conditions. Being capable of release at relatively high rate and in large quantities, the steam held in storage in the accumulator also secures the needs of production and the conditions to maintain product quality.

Energy Saving Economics

The most important energy cost savings to be gained from steam accumulation derive from a reduction in the boiler output in combination with load stabilization. Fuel consumption arising from a highly fluctuating boiler load will vary with the degree of control sophistication of the combustion equipment used. However, in almost all cases, there are prospects for energy savings emanating from the elimination of excess steam generating capacity and part-loading that causes a low load factor; and the removal of a rapidly fluctuating demand and idle live capacity that increases standing heat and power losses. Therefore, the possibility of a steam accumulator meeting a fluctuating steam demand with a smaller boiler operated at a higher load factor and at constant output with higher efficiency is an alternative energy efficient solution that should not be ignored.

The economic case for steam accumulation will vary depending upon individual circumstances and as such predictions on energy savings and operational gains can only be generalized. However, a conservative expectation of improved boiler combustion efficiency for modern burner equipment using the latest accumulator control methods could be between 2% to 4%. For older boiler plant, the saving could be more, perhaps 3% to 5%, depending on the type of boiler, state of maintenance and conditions under which the boiler is operated.

The necessary data, where modern boiler and burner equipment are installed, from which to derive the required information will likely be readily available from the existing energy management system, but with older plant where scant information is available the addition of the necessary temporary or permanent instrumentation could be easily provided.

Standing heat and power losses resulting from excess on-line boiler generating capacity could equate to 1% to 3% of fuel consumed relative to the maximum combustion rating of the boiler.

Where a boiler is of large refractory construction, savings on maintenance could have an even greater impact on the case for investment.

Operational Gains

Fewer boilers on-line operated under steady load conditions would lead to less ‘wear and tear’ and as a result lower maintenance costs. The potential for labour and material cost savings may be easily examined, but safe to say a reduction in operating costs would contribute significantly to enhancing the case for steam accumulation.

Power Generation Storage

Energy storage for power generation has entered a significant stage of development. Establishing a low carbon economy requires the fluctuating nature of the power supply generated from renewable energy sources to be managed. There are now various grid-scale and R&D developments for electrical power storage being investigated—some dependent on government subsidies, others funded by private and academic initiatives. These include flow batteries, compressed (liquefied) air and lithium cell technologies.

Electrical power produced from concentrated solar power plant, which raises steam to drive a turbo-generator cannot be maintained during the hours of darkness or diminution of sunlight without some form of energy storage and this can impact detrimentally on the feasibility of new build projects.

Steam accumulation can provide large-scale indirect storage of electrical power by accumulating excess steam produced by the steam generator for later release to drive the turbo-generator. Its purpose can be to maintain power output when demand exceeds supply or to balance a variable load. Disparity between power demand and supply is at the heart of the economic and operational problems to be solved by the solar power plant designer and steam accumulation is one solution that can provide the answers. Industrial size accumulators can be very large, but there is no practical limit to size, storage capacity or operating pressure of the storage vessels to balance demand variations or maintain electrical power output at the levels required.

The commercial and economic arguments for the use of steam accumulation in the power generation storage mix have to be made for each individual circumstance. However, technological viability need not be seen as a barrier to further investigation of the potential for steam accumulation in this important emerging sector.

David Oakland is principal consultant at David Oakland Associates, a specialist steam accumulation design provider. enquiry@steamacc.co.uk

Read More Here: http://insights.globalspec.com/article/1396/steam-accumulation-an-energy-efficient-technology

BOSTON, MA, Dec. 1, 2004 (Viewpoint) – The global effects of sustained atmospheric pollution are forcing governments worldwide to introduce measures to limit and ultimately reduce the emission of ‘greenhouse’ gases, particularly carbon dioxide (CO₂).

Reaching target CO₂ reductions will call for contributions from all the major sectors of energy use. Attaining higher efficiency in the use of energy at lower operating cost is a compelling enough preoccupation, but for some government legislation, such as the ‘climate change levy’ and investment tax incentives introduced in the UK in 2001, will be the driving force. As rising fuel prices continue to add to the effect on competitiveness, environmental issues and the acquisition of energy efficient technologies already sit high on the boardroom agenda.

BOSTON, MA, Dec. 1, 2004 (Viewpoint) – The global effects of sustained atmospheric pollution are forcing governments worldwide to introduce measures to limit and ultimately reduce the emission of ‘greenhouse’ gases, particularly carbon dioxide (CO₂).

Reaching target CO₂ reductions will call for contributions from all the major sectors of energy use. Attaining higher efficiency in the use of energy at lower operating cost is a compelling enough preoccupation, but for some government legislation, such as the ‘climate change levy’ and investment tax incentives introduced in the UK in 2001, will be the driving force. As rising fuel prices continue to add to the effect on competitiveness, environmental issues and the acquisition of energy efficient technologies already sit high on the boardroom agenda.

One such technology that continues to play an important role in the modern boilerhouse is steam accumulation. While it can often be justified on the grounds of practical advantage alone, its potential to reduce fuel consumption is invariably the carrot that consolidates the case for investment by providing the vehicle for payback. This and legislative measures that limit ‘greenhouse gas’ pollution will now turn the industry’s attention to looking in more detail at ways to achieve sustainable energy prices and emissions reductions.

Steam accumulation: the basics

The basic purpose of steam accumulation is to eliminate boiler peak loading by providing a reserve of steam to meet intermittent demands. In the charged condition, the accumulator holds a quantity of steam produced as “excess” steam in the boiler at times of low load. When the demand rises, the accumulator discharges and the steam peak is met from storage rather than from the boiler.

Lower consumption of carbon-based fuel will result in lower emissions of noxious gases in equal proportion. By means of load stabilization, steam accumulation enables higher combustion efficiency in the boiler resulting in less fuel being burned.

Efficiency improvements can be as high as 5% and modern methods of control figure prominently in achieving this. In many applications, an accumulator would permit the selection of smaller boilers than would otherwise be the case. Capital costs would be lower, standing heat and power losses would be correspondingly lower and further reductions in atmospheric pollution would result — adding further to the environmental and economic benefits.

Incorporating steam accumulation into CHP

Combined heat and power generation (CHP) is the simultaneous generation of heat (usually recovered as steam) and electricity achieved at much higher levels of energy efficiency than is possible as separate entities. The reduction in fuel consumed enhances the economic case for new build developments while at the same time cutting the potential for environmentally damaging emissions.

However, those concerned with the design of CHP plants do not always consider the benefits of incorporating steam accumulation into the steam supply system. The purpose of ‘steam storage’ is to provide the means to convert an erratic process steam demand into a steady load. Fluctuating steam demand forces transient operation of the steam generator. This is not conducive to operating at the highest fuel efficiency. It also detracts from optimum sizing of the turbine or engine, which is required for electricity generation. Steam accumulation can provide the answer to marrying these two ideals.

The effect of a steam accumulator is to balance load variations, thus reducing or even eliminating the amount of supplementary steam generating capacity that might otherwise be required. The benefits are maximization of boiler load factor, optimization of combustion efficiency, and lower fuel consumption and, therefore, reduced emission of atmospheric pollutants. Moreover, the electrical generator can be sized with the benefit of a steadier heat load and can be run continuously for longer periods at full output.

CHP plant ‘trips’ can be the result of sudden load variations imposed by production requirements or due to a malfunction of any of the major power plant components. In the latter case, the cause may be in the prime mover (the gas turbine or engine) or the heat recovery steam generator, or could be due to failure of the gas compressor, loss of fuel supply or a problem with the steam turbine. Whatever the reason, the production of process steam falls rapidly, requiring the standby boilers to fire or, in the case of a stoppage of the steam turbine, its by-pass PRV station to open.

It is invariably the case that the standby boilers, supporting auxiliary systems and bypass facilities must be capable of maintaining process steam supplies at maximum demand rate in the event of a CHP plant trip. However, it is also inevitable that steam distribution pressure (and temperature) will be lost because of the inherent lag in auxiliary system response. A steam accumulator can largely remove this problem and practically eliminate it altogether because of its ability to discharge steam instantaneously from the reserve held in storage. Pressure in the steam distribution system is held constant while the standby boilers are given time to run up to full output and the interruptions that would otherwise occur need not be tolerated.

A steam accumulator can enable load balancing in paper mills where paper breakages are commonplace. The accumulator provides the means to dump (store) excess steam as the demand plummets, thereby enabling the steam and power generators to be efficiently ‘ramped’ down. The losses associated with steam blow-off to atmosphere or the costs associated with dump condensing are therefore avoided. On start-up, the demand is sudden and the magnitude of peak loads can be far in excess of the normal production demand. These are met initially from the reserve of steam held in ‘storage’ while the generators are gradually run up to match the load.

Steam accumulator installed at a paper mill

Case study

In one paper mill, a steam accumulator provided the solution to CHP ‘trips’, overcoming all the problems the mill had previously experienced in maintaining power and steam production. The accumulator forms part of a CHP plant that comprises a gas turbine exhausting into a water-tube type waste heat boiler. This supplies steam to the accumulator under ‘demand trend regulation’ (DTRS) control, which automatically and continuously regulates the flow of steam into the accumulator (and hence the demand on the boilers) at a constant rate corresponding to the consumption of the paper machine.

In addition to stabilizing the load according to production requirements, the control PLC is also configured to provide the means to absorb excess steam generated at times of sudden load cessation.

This potential loss of steam to waste is avoided and the stored steam is subsequently used to meet the instantaneous start-up conditions of the paper machine which occur at steam flow rates well in excess of power related generating capacity.

Furthermore, the regular occurrence of high and low water ‘lock-out’ of the waste heat boiler under load swings has been eliminated. Previously, these had been the cause of the total loss of boiler steam to all consumers in the mill.

The inclusion of the steam accumulator into the steam supply system at the mill was not only a sound engineering solution to the boiler steaming problems, but it also subsequently provided a useful reduction in capital cost associated with the selection and installation of a new smaller waste heat boiler than would otherwise have been required.

These benefits are in addition to the other gains associated with eliminating the variable and transient loading such as those associated with papermaking where response time, process temperatures, and steam quality are paramount.

The benefits of steam accumulation

The viability of the case for steam accumulation relies largely on the magnitude of the pressure differential that, together with the required storage capacity, governs the physical size of the storage vessel. Steam storage systems are usually operated on the ‘pressure drop’ principle. Generally, a ratio in excess of 2:1 between charge and discharge pressures would offer the basis for favorable practical and economic arguments to adopt the technology but every application has to be judged on its own merits.

The advantages of steam accumulation in power generation applications are that it:

• Gives instantaneous response to sudden load changes
• Maximizes power generation and boiler load factor
• Eliminates boiler carry-over and power plant ‘trips’
• Maintains steam quality and safeguards production processes
• Delivers dry-saturated steam. Holds boiler and process pressures constant
• Enables optimum sizing of the electrical power generator. Allows the turbine to be run continuously at full output
• Minimizes the requirement for supplementary ‘peak load’ boiler capacity
• Improves plant efficiency, reduces fuel consumption and the emission of pollutant gases
• Automates operation and lowers plant maintenance
• Pays for itself within two to three years on average.

Savings in fuel and power consumption will depend on the age and condition of the boiler plant, the fuel used and the nature of the load applied. However, improvements in combustion efficiency of 2-5% are attainable and modern methods of control (such as DTRS) could achieve more. As increases in boiler efficiency directly relate to the cost of fuel purchased, then typically an improvement of 5% on an annual fuel bill of $1 million would increase profitability by $50,000 and potentially more.

Moreover, a reduction in standing heat and power losses would contribute a further 1-3% depending upon the downsizing of a new boiler plant, or the number of existing boilers closed down, thus adding to the operational savings to be made. Overall annual savings relating to fuel alone might be expected to approach 30% of the capital cost of the installation.

Another advantage is that mills would need smaller and/or fewer boilers leading to a reduction in maintenance costs, while less tangible gains such as security of steam supply, stable pressures and temperatures, improved production and better product quality all add to the case for investment.

Steam accumulation should be recognized as an energy efficient and widely adaptive technology. It is a real and effective tool for achieving higher operating efficiency, lower energy consumption and reductions in boiler emissions.

Read more Here: http://www.risiinfo.com/magazines/December/2004/PPI/The-role-of-steam-accumulation.html