Le Roux Combustion

In my encounters with boiler operators, and even engineers, I still find a general lack of awareness among the operating and supervisory staff as to the significance of excess air. Considering that anything from 10% to 50% of fuel energy input may be wasted to atmosphere because of excess air, it is essential that everybody responsible for boiler performance has a solid understanding of its nature and impact.

In my vocabulary I like to refer to excess air (most unscientifically) as the necessary evil of combustion, or to put it more mildly, the air which hitches a ride through the combustion system without making a direct contribution to the combustion process. But unfortunately excess air is required just to make sure sufficient oxygen is going around to enable every carbon atom to find an oxygen mate (actually two of them!) to combine with and to produce heat and carbon dioxide (CO2) gas.

But maybe we should just refer back to Boiler Bits 5 to explain where we are heading.

The problem with excess air as an energy waster is of course aggravated when burning pea coal fuel. The more atomized the fuel is, the more intimately does it mix with the combustion air and the more rapidly combustion takes place. And the less excess air is required for complete combustion. For example, combustion in a petrol engine only requires some 5% excess air. And when firing a steam boiler with fuel oil or pulverized coal the excess air required is approximately 15%.

With burning of pea coal on a chain grate stoker the situation changes dramatically. The fuel consists of coarse material and the fire has to burn its way through the individual coal pieces for oxygen to reach the innermost fuel atoms, and even more so with less reactive coal. This process takes anything from 30 to 40 minutes at best before all fuel is completely burned out. And this also takes its toll in terms of excess air requirements – anything upwards from 50% to burn the coal to ashes. With less reactive coals excess air may be as high as 80% and more for complete combustion of the coal within a reasonable time on the grate.

Although high excess air levels promote complete combustion of the fuel, as well as cool stoker grates and (almost) no visible smoke from the stack, it wastes fuel energy and may add significantly to the fuel bill.

Let us consider how excess air wastes energy. Assume that 50% excess air is required for the complete combustion of coal on a stoker grate. As we have mentioned earlier the excess air does not take part in combustion and it does not give up any of its oxygen to combine with fuel atoms. It merely enters the furnace at ambient temperature, absorbs the heat liberated by the combustion process, its temperature rises to combustion temperature, it gives up some of the absorbed heat in a heat exchange process to produce steam and exits the boiler with the flue gas at flue gas temperature, which may be anything between 160 ⁰C and 260 ⁰C with a fire tube boiler. Thus there is a net energy loss to excess air which is a function of the excess air percentage and the temperature difference between flue gas and ambient air.

Stack energy loss (with coal fuel) normally constitutes between 50% and 70% of the total energy lost in the steam production process. The excess air contribution to this loss can be estimated if its percentage is known. With coal combustion it can be between 20% and 35% of the total energy loss. No wonder users of steam boilers spend millions on the optimization of excess air control in an endeavour to curb the fuel bill. Of course, and unfortunately so, excess air will always be part of the combustion of fuels and users of steam plant will just have to tolerate the energy loss that goes with it. But having said that, at the current cost of energy even one or two percent loss can run up significant costs over a period of months and years.

Minimizing excess air percentage (and flue gas stack temperature) addresses the single greatest energy loss from the steam boiler.

It is therefore vital that persons responsible for steam plant operations are well aware of the impact of excess air on the cost of steam generation, and are equally knowledgeable about its control and optimization. Achieving this is a topic for a next discussion.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about boilers and optimization of combustion? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

My first four Boiler Bits articles primarily focused on the potential of boiler automation technology to support Management in achieving their objectives and mission. From this issue of Boiler Bits onward I think it appropriate to shift our attention to the more technical side of steam plant operations. My passion is with optimized boiler performance and I would like to share some insights and experiences gained in this respect over many years. Unfortunately this requires some academic background, which should not be unfamiliar territory for combustion engineers. But just to make sure we all start off from the same page our readers will have to master some initial scientific principles pertaining to the art of combustion.

Maybe a word on “optimization” for starters. One of the objectives we strive for with a combustion control system is a consistently high efficiency of the steam raising plant. But because higher efficiency usually comes at a higher cost one has to strike a balance between the cost spent to improve efficiency, and the actual benefits derived from the higher efficiency. Striking this balance embodies the concept of optimization. It obviously does not always pay to invest vast amounts of capital in efficiency improving technology just to gain 1% or 2% in a reduced fuel bill, unless the quantity of fuel consumed warrants such capital expenditure.

Everything you ever wanted to know about combustion rests on three unshakable pillars of physics, namely fuel, oxygen and heat. Yes, I know we were taught the elements of fire in primary school, but if managing combustion under controlled conditions becomes one’s occupation, your approach to the matter will invariably take on a new dimension. 

So let us look at combustion a bit closer. A very basic definition of combustion may be something like this: it is the process of burning something (super basic!). Or a more complex and detailed approach: combustion is any process in which a substance (fuel) reacts with oxygen to produce heat and light.

Keeping in mind also that heat (a source of ignition) is required to start and sustain the combustion process. Typically the spark plug in an internal combustion engine or the ignition arch of a coal fired boiler serves this purpose. In many instances the heat liberated by the combustion process is sufficient to sustain it.

Any fireman knows that removing only one of the three pillars of combustion will cause the process to collapse and the fire to be extinguished. Thus all fire fighting practices are based on removing either the oxygen from the fire (spray foam or inert gas), or by removing heat from the fire (spray water), or by removing the fuel (isolate the fuel source). Similarly, interfering with any one of these pillars with your boiler in operation will cause inferior combustion performance, or even entire loss of combustion.

Another aspect of combustion is that the generated heat invariably causes an increase in temperature. It is this high temperature (resulting in a temperature difference between combustion gases and heat exchange surfaces) that causes heat to flow and to perform useful functions and work, such as producing steam.

The chemical nature of combustion also produces by-products of combustion, typically CO and CO2 if the fuel contains carbon. If the fuel contains hydrogen (H2), water (H2O) may be produced as a by-product; sulphur in the fuel will produce SO2 gas, etc. 

But let us get back to the basics of combustion. Due to the chemical nature of combustion a certain amount of oxygen will always combine with a specific amount of a combustible substance during “perfect” combustion of that substance. For instance, 12 kg of carbon requires 32 kg of oxygen for its complete combustion and produces 44 kg of CO2 and some 390 MJ of heat (energy). Unfortunately these numbers are only achievable under conditions of perfect combustion where each atom of carbon finds exactly two atoms of oxygen to combine with, and at the end of the process there is no unburned carbon or oxygen left. This process is also known as stoichiometric combustion.

In the real world we find however that for complete combustion of the fuel more than the stoichiometric quantity of oxygen is required. Furthermore, because of inevitable imperfections in the combustion process (like lack of turbulence and intimate mixing of air and fuel particles) more than the stoichiometric oxygen requirements must be provided to make sure every atom of fuel finds the correct number atoms of oxygen for its complete combustion. This “more than the stoichiometric air requirement” is appropriately called “excess air” and is normally expressed as a percentage of the stoichiometric air requirement.

Because air is normally the carrier of the oxygen we must keep in mind that for every kg of oxygen a total of 4,32 kg of dry air needs to be delivered to the combustion process, consisting of 1 kg of oxygen and some 3,32 kg of nitrogen.

The excess air requirement depends totally on the fuel burned and the combustion environment. Typical excess air requirements are 50% to 80% for pea coal, 5% to 10% for gas and 10% to 20% for fuel oil. 

But know for certain that excess air plays a major role in the combustion process, its efficiency and its control. In our next edition of Boiler Bits I will discuss the significance of excess air on the combustion process in more detail.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about boilers and optimization of combustion? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

Many years ago I was employed by a fairly large company with many and diverse operations to keep the gears turning towards increasing value for shareholders. It was during those years that I realized the value of setting clear objectives that will create focus on desired performance and outcomes in a complex organization. And if every division did its bit of setting, managing and achieving objectives, we should arrive at an accomplished mission for the greater organization. Not that we always succeeded in every respect, but the company’s performance was definitely above average and today is a JSE listed company of note. 

Applying this principle to the micro business environment takes us to the boiler house with a burning question: is there a proper mission for steam plant operations (SPO)? And can it be broken down into key performance areas (KPA’s) and key performance indicators (KPI’s)? I am convinced it can be done, provided we ask the right questions and adopt a proper approach.

The first question is to identify who the “Client” of steam plant operations is, as the client will definitely have a significant say in the objectives of SPO. Normally it is Production, or Operations, or the entity using the steam to create value elsewhere.

The second question follows naturally from the first, namely what exactly does the client/user require from SPO in terms of steam supply?

And lastly SPO has to consider the requirements of the larger organization and how it expects SPO to contribute to its mission. This includes departments such as finance, occupational health and safety and the environment, quality assurance, engineering, etc.

It cannot be too difficult to find the answers. My personal attempt is listed below:

1. Consistent steam pressure (steam supply) usually tops the chart. No steam means no production. Even low steam pressure may affect production throughput and product quality, or can ruin product in process.
2. The cost of steam production draws an equal amount of attention, especially from the accountants’ side. This requirement boils down to maximizing boiler efficiency and minimizing the cost per ton of steam produced.
3. Asset preservation. Boilers operate at high pressure and temperature which can severely damage steam production assets if proper care is not exercised.
4. Boiler safety. Nobody wants to have a boiler explode on site, or personnel being injured by unsafe plant or unsafe acts of operators.
5. Pollution of the environment has gained greatly in importance over the past number of years. New legislation on emissions standards and control are promulgated every so often.

My proposed Mission for SPO then: To continuously supply steam to Operations at the required steam pressure at the lowest total cost per unit, whilst preserving steam plant assets and not compromising the health and safety of persons, or the environment. (Or any derivative of the aforementioned.)

How is SPO to respond to this proposed mission? In a nutshell:

1. Properly maintain assets. Focus on preventive measures and inspections. Utilize your boiler control system to assist with aspects of predictive maintenance, and in identifying operator malpractices and damaging operating conditions.
2. Diligently manage water treatment! It affects all aspects of boiler operation, including boiler capacity, efficiency, safety and preservation.
3. Train and manage boiler operators.
4. Manage coal procurement. Coal is part of the combustion system and coal management and procurement should fall under SPO if they are to be held accountable for specific boiler performance.
5. Use a proper control technology to improve boiler efficiency and to eliminate human intervention as far as possible. Technology drives productivity improvement!
6. Ensure boilers are adequately protected against safety incidents and that appropriate alarms are activated in case of deviations. Test safety devices on a regular basis.
7. Install smoke suppression equipment to reduce smoke emissions if necessary. Otherwise monitor and treat emissions to comply with legislative standards.
8. Identify and manage KPI’s. A well designed boiler control system can go a long way towards calculating and processing boiler performance indicators and other management information.

Unfortunately there is no shortcut to excellence. It always requires hard work, sound knowledge and lots of dedication to accomplish.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about steam plant management? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

Looking back over the past 25 years it is amazing to see the technological advances that have been made in the area of boiler control. And technology, being one of the main drivers of productivity improvement, has also managed to do its thing in this specific field.

When I started off with boilers, combustion control was still carried out with what we called electro-mechanical control systems, utilizing dampers, links and electrical and mechanical devices to automatically regulate the combustion process in accordance with steam demand. This was a most basic automated control system, which one can call Level 1 automation, i.e. one notch up from manual operation.

With the advent of frequency inverters (VSD’s) and more advanced electronics the electro-mechanical controls were gradually being phased out and replaced with more advanced control systems, providing for more accurate control of air-fuel ratio and subsequent higher efficiencies and fuel savings, generally ranging between 8% and 15%. Here too one can distinguish two levels of automation.

1. At the bottom end one finds a control system consisting of VSD’s, a few electronic transducers (e.g. steam pressure), controllers and typically a pressure differential switch to control the FD fan (furnace pressure). The air-fuel ratio is adjusted by means of a potentiometer which adds gain to the speed command of the stoker. Very basic, but effective and generally improving the efficiency of steam production. With a well trained operator and good supervision these systems are simple to operate and able to produce fairly good efficiencies. One can refer to these as Level 2 automation systems.
2. At the top end one typically finds a control system with more advanced electronics, more instrumentation, a plc and HMI, and most probably oxygen trim control to absolutely fine tune air-fuel ratio on a continuous basis under all demand conditions. These systems are typically 2% to 3% more efficient than Level 2 systems, and often provide the ability of computation, control of complex processes, data capture and processing, information management and live boiler monitoring. We refer to these as Level 3 automation systems. And typically they would come with a higher price tag than Level 2 systems.

The challenge facing many users of steam boilers is to decide on the level of automation they should opt for. Naturally everybody would like to have a Level 3 control system for the sake of improved boiler operation and higher efficiency; however there are a number of factors to consider when making an educated decision:

1. With business conditions becoming tougher and more competitive every day, there is increased pressure on management to employ technology at their disposal to the utmost, including the recording and processing of information, and even to monitor plant in operation. It saves time and improves the integrity of information and decision making, and these features alone can offset the premium of owning a Level 3 system by far. However, if a business is not committed to continuous improvement, or actively managing steam plant performance, they may be wasting good money on a Level 3 control system.
2. Level 3 control systems may create the impression that they are complex to set up, to adjust and to operate. This is specifically true where operators are not adequately equipped to understand the requirements for efficient boiler operation. On many occasions however, I find the problem is with supervisory and managerial personnel that are at a loss as to the basic principles of combustion and boiler control, and the potential value the control system can add in terms of processed data and efficiency related information.
3. And lastly of course the more expensive system must be financially justifiable. A large steam user may find that the additional fuel cost savings brought about by a Level 3 control system pay back the additional cost of the more advanced technology within single months. With lower steam usage it may be financially more attractive to employ a Level 2 system, unless the boiler management features offered by the Level 3 system takes preference. 

Thus the question of the correct automation technology level hinges on four factors:

• The budget
• The technological and operational knowledge base
• The management culture
• The return on investment.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about boiler control systems, or boiler optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

I guess I am in a privileged position to speak to many engineers and users of steam boilers in the course of conducting business. I get exposed to their challenges and have the opportunity to experience how they go about operating and managing their steam plant. Quite often the conversation turns to boiler control and automation, specifically the desirable base line features of a “good” automated boiler control system. Some engineers have to decide on acquiring a new technology control system, others realise that their current control system is not up to standard and are investigating an upgrade. Whatever the reason, the cost of the new or upgraded control system must typically fall within an already stretched budget and must offer a justifiable return on investment, if not in monetary terms, then at least in line with company objectives or policies.

My advice to anyone who has to acquire new or upgraded control technology is to at least make sure that the following features and functionalities are properly covered:

1. The control system must be able to operate the boiler at highest efficiency within the constraints of fuel properties and the configuration of the boiler as designed and installed. There are still boilers out there which are manually operated, and we know that with modern control technology substantial efficiency improvements can be achieved with typically between 8% and 15% fuel savings. Unfortunately energy losses per se do not reflect as an entry on the balance sheet, but the cost of capital expenditure does, and often it is a matter of what cannot be seen, cannot cause harm. And so energy wasting practices continue unchecked.  
2. Manual and auto control of the boiler should be positively separated, which means manual operation should run independent of auto operation. The loss of a plc or HMI must not render the boiler inoperative; selecting manual operation must allow the operator to carry on producing steam, using the analogue instruments at his disposal to maintain steam pressure, furnace pressure and the water level.
3. If the user is serious about remote boiler monitoring and information management a plc is a must. It has computational and data processing abilities. Modern plc’s come with on-board web servers and boiler monitoring and processed data can be accessed on any internet enabled device, such as a smart phone, tablet or computer. Ensure that quality information is captured, such as efficiencies, boiler outputs, consumptions, alarm conditions, safety and functional tests, diagnostics, etc.
4. Eliminate human intervention in boiler operation as far as possible, but also make it as easy as possible for the operator to use and adjust the control system. Even with the highest level of automation operators are still needed to observe and adjust the combustion process, and this necessitates higher levels of understanding and skill from operating staff to meet performance expectations. Combustion conditions must be interpreted intelligently and responses and adjustments must be logical and accurate.
5. Protect engineering settings behind a password, and assign different passwords to different levels of authority in the organization.
6. Consider how alarms are being raised and for what conditions sirens are being sounded. A siren that sounds for every minor problem becomes a pain in the butt and quite often the operating staff will subtly sabotage or silence it. Rather make use of flashing lights or LED’s on the panel to draw the attention of the operator.
7. Limit the steam output of the boiler to its maximum continuous rating. It helps in preserving a very valuable and costly asset.
8. On a practical level: install the panel out of the way of movement of persons and other traffic, but make it easily accessible to the operator. Also consider exposure to high temperatures (radiant heat), water (steam) and dust.
9. Please note that I have deliberately avoided the thorny issue of automation technology platforms, their cost, support, etc. in our discussion above. Most companies adhere to specific policies in this respect.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about boiler control systems, or boiler optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.