Refractories in Reforming Waste Heat Boilers.

Refractories in Reforming Waste Heat Boilers.

This paper focuses on refractories in (WHB’s) Waste Heat Boilers, specifically in the Petrochemical Reforming (ATR) Auto Thermal Reformers, and (GHHER) Gas Heated Heat Exchange Reformers. SMR’s (Steam Methane … ) and to some extent SRU’s (Sulphur Recovery … ) and other related units may also apply.

The late and honestly great Dr. J.D Hancock wrote in his book: Practical Refractories; refractories are like “Black Magic”. This statement combined with some unique experiences led me totreat refractory applications as though it has its own unique “personality”. It allows me to engage with an open mind, as opposed to stop, and make or come to a conclusive decision when one finds some supposed generic failure or occurrence. “… oh it failed because of this or that and that’s it….” No, this is not always the case.

It’s typically that there could be more to a failed refractory than meets the eye. If we miss these “hidden” elements, then we miss an opportunity for progressive improvement. Only a few failure-, damage- & attack mechanisms are generic, and even then, they typically occur due to, or for very specific reason. Often failure is only unravelled after in-depth review, considering varying process conditions, upsets, refractory material qualities, workmanship, and a number of variables I address going forward.

“The less you see the more you believe (know)” This approach is dangerous because it could mean we ignored valuable history, trends, or information from various sources. Quote: Jacques Tourneur – Horror movie director.

But on the other hand. After a few investigation sessions I noticed that – “The more you see the less you know”. Yes, eventually a fair outcome is reached from the available information, but there comes a point in some inspection assessments that leaves me in wonder. I think this is where Refractories and “Black magic” meet. Yes, I quoted the Red-Hot Chilli Peppers.

Fact is there are always some elements or attributes involved that affect refractories and their ancillaries & auxiliaries uniquely. One should best not assume a result or cause without careful analysis of all information. It could pass as ignorance which could cost millions!

To conclude this section and move ahead while in a manner repeating myself: When focusing on refractories as a solution one should consider all aspects: From design to process, overall history, workmanship, and everything that has gone into the manufacturing, placement, and management of the correct lining. The one thing I want to add to this holistic approach is – the client. It’s not about anyone other than the client and the integrity of their refractory lined equipment. As a specialist I am are required to add value – I trust I am doing this to the best of my ability and believe herewith to encapsulate some principles in a condensed manner. In addition, I trust this may offer some guidance during the decision-making phases for the next refractory challenge you may face.

Content

Topic	Page Number
Introduction	3
Equipment	3
Process	4
Types of refractories, linings, and attributes	5
Installation & methods
Demolition and shell preparation	9
5.2 Anchorage	9
5.3 Monolithic (unshaped)
5.3.1 Gunning	11
5.3.2 Casting	12
5.4 Brick	15
5.5 Tube sheet & ferrules	17
5.6 Manway	19
Expansion	19
Dry out	19
Types of failures
8.1 General	21
8.2 non-process	22
8.3 Process conditions
8.3.1 Fouling	23
8.3.2 Metal dusting	23
8.3.3 Carbon	24
8.3.4 Silica leaching	25
8.4 Other	25
Maintenance issues	26
Research and development	27
References	28

1. Introduction:

It has always been my intention to help my clients with the most accurate degree of information and not let them have blind faith in what I do. To do this I rely on my sense of curiosity, my knowledge, my experience, but also the knowledge and experiences I had with peers, given I am in the fortunate position to be working with some of the best minds and hands in the industry. Today I am a freelance inspector and for some projects an inspection supervisor -which implies full time onsite oversight – ensuring projects run as smooth as possible, without compromising quality, and adhering to schedule. I have also given some basic refractory training, but we will see what the future holds in this regard.

I learned not to take uneducated & unwarranted “chances”, and not to run trials at someone’s expense. Without substantial proof that a recommendation will work for as long as it needs to within certain parameters one might be taking unwarranted chances. It’s important not to run with hypothetical assumptions, but instead actual relatable proof to support that which one may be doing to improve a situation, or to manage risks. As result, this is my attempt given my current level of knowledge of a factual summary – besides for a few hypothetical and falsifiable scenarios that I will point out.

I will also offer an overview illustrating that a general understanding of the equipment, its operation, the mechanical & chemical mechanisms, as well as an understanding of applicator and material constraints can helps one make the most informed decisions.

Among some of the points considered are materials, the various processes, and my favourite; man vs machine, or if you like; workmanship vs operation. I am referring to ops. in the widest possible scope considering installations and unit operation.

I trust it will add value to anyone aiming to learn or perceive the subject from a quality improvement point of view.

2. The Equipment:

WHB’s or heat recovery steam generators are widely used in processing industries to improve energy efficiency.

Essential components required for waste heat recovery are:

– A source of reliable and consistent heat.

– A recovery system to generate steam.

– End-use for the recovered (heat) steam for preheating, electricity supply, or process control in other areas of the plant.

Thanks to the unconventional energy source of WHB’s it has unique design features. This design is typically backed by high alumina refractories of which goals are to contain the heat, insulate the unit, contain the gas product, and optimize or lower maintenance cost ensuring minimal downtime and production loss.

In reforming the most common WHB design is a cylindrical steel shell with an interior of two opposing metal sheets each connected with a collection of horizontal tubes forming the boiler section which is filled to a specific level with the water required.

The cylindrical drum is divided into segments: manway or inspection door, dish end, barrel, the reforming to barrel interface or throat section, and the tube sheet.

3. Process:

The organic feedstock (e.g., natural gas) and steam (and sometimes carbon dioxide) are mixed directly with oxygen and air in the reformer. The reformer itself comprises of a refractory-lined vessel that contains the catalyst, together with an injector located at the top of the vessel. Partial oxidation reactions occur in a region of the reactor referred to as the combustion zone. It is the mixture from this zone that then flows through a catalyst bed where the actual reforming reactions occur. The heat generated in the combustion zone from partial oxidation reactions is utilized in the reforming zone so that in an ideal case the process can achieve thermal equilibrium.

Product gasses are then introduced into WHB (Figure 01 Configuration) system at around 750-1050°C from the ATR through a diffuser (Mushroom) & transfer line. In most ATR designs, heat is quenched with steam at the inlet because it exceeds safe gaseous temperatures with enhanced corrosive abilities concerning the boiler tube and other metallic surfaces eg: bypass valve brackets etc… In the event it flows to a connected GHHER the quench function is sometimes bypassed for sufficient energy for the endothermic reaction.

…

The hot equilibrated gaseous product enters through the hot tube sheet side during which it creates steam. Heat is transferred from one medium to another when natural water circulation occurs in the boiler with the differences in density of water heating up. The gas product exits at the cold side thanks to a pressure differential or mechanical draft @ approximately 350 to 480°C where it is directed for Syngas processing further downstream. A WHB in GHHER configurations is generally significantly smaller due to lower cooling duties required and limited steam which can be created. These WHB inlet temperatures are around 560 to 640°C.

In many designs, the tube sheet also consists of a bypass valve used to advance or retard flow in the event cold side temperatures rise to detrimental levels and vice versa.

A WHB’s practicality is measured by its steam to fuel efficiency: The ratio of the heat added to the boiler feedwater to produce the output steam to the amount of energy inputted with fuel.

Generally, the consumption of waste heat is considered a definite benefit as it recycles waste, is good for the environment & business both financially and pragmatically. It is however a given that for the by-products of upstream processes it is not the most efficient fuel source and WHB’s are less efficient than other types of boilers.

Steam is used for the transportation and provision of energy.

The benefits are that it is efficient and economically sound to generate, easy to distribute, easy to control, easily transferred to the processes and a steam plant is relatively easy to manage while the process is flexible. Process and in-service checks involve temperatures, pressures, boiler feed water quality & consumption, flow rates, hydrostatic testing, and ultrasonic testing.

To conclude with relation to the process, it is important to note that the relationship of steam to the pressure at which it is generated could be fairly simple in principle. But what is not as simple is to strike the perfect balance between pressure and temperature all in providing the best energy solution. These units operate under pressure but the higher the pressure of a boiler is the more heat must be applied to make steam. (Note the segment in Dry outs – Figure 11). High pressure – high temperature steam contains more energy per unit, which is known as Enthalpy.

Enthalpy: The thermodynamic potential made up of energy and pressure. More heat and pressure do not necessarily mean more energy, it is possible to get more steam per ratio at a slightly lower temperature than simply assuming more heat equals more steam energy.

4. Types of Refractories and Linings

The performance of refractories in a WHB are measured by the chemical, physical, mechanical, mineralogical, and thermal properties which all should be compatible with the application.

In further considering material types or expected quality characteristics one should look at process parameters, temperature profiles, mode of operation, chemical environment, operating pressures, atmospheres, quality of refractory, and workmanship.

Generally, the linings in reforming WHB’s are dual or three-layer linings (See Figure 02 Photo of multiple/three-layer lining), I have however come across single-layer linings as well. Lining thicknesses may vary from 150mm to 340mm in total. Factors such as water-cooled jackets could also affect the lining configuration.

Figure 02 internal
Hot face – High Density Brick Intermediate – Bubble Alumina Brick Cold face – cast Note the oil between each layer