In This Issue

Improving Food Waste is a Top Contender for Improving Sustainability

Floor Care – Green to Sustainable

Sustainability Principles Applied to Boiler Water Treatment: Part One

VOCs and Cleaning Products

Letter from the Editor

Sustainability Resources Sustainability Tips

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Volume 20   |   Issue 1   |   March 2018

Sustainability Principles Applied to Boiler Water Treatment: Part One

By John Chambers

A completely sustainable process or system is unusual, nature’s water cycle is the model of perfection.  Nature’s perfection will never be duplicated completely, but strides can be made toward more sustainable solutions every day.  Sustainability is a model for improvement that impacts the welfare of people, the health of the planet and the ability to provide for the needs of families-people, planet, profit.  With reflection on the principles of Sustainability, the efficiency of many systems can be increased.  The process of making steam is an interesting example.  With colder weather and shorter days, the seasonal demand for steam increases to provide heat and energy for comfort and industry.  The discussion will focus on concepts that can be used to make steam generation safer and more convenient, less harmful to our environment and less costly.  The first article in this series will explore the basic concepts of making steam and some improvements that make the model system more efficient.  In next quarter’s installment, the concept of returning heat and water to the system with recovering condensed steam (condensate) will be covered along with more sustainable chemical treatment options. 

Steam is generated to provide heat and do work.  When steam is used for heating, the thermal energy is extracted to provide warmth for schools, apartments, commercial buildings and hospitals.  At the other end of the spectrum, steam can be generated to provide mechanical energy where heat is a byproduct.  Using steam to spin a turbine to create electricity would be an example of the work steam model.  The only difference between heat and work steam is temperature and pressure; the more work that the steam needs to do, the higher the temperature and pressure. 

Modern steam boilers come in all different sizes, configurations and pressure ratings.  Some use exotic metallurgy, unique fuels and operate in supercritical pressure regions.  However, all steam boilers convert water to steam with heat from a fuel-pretty simple really.  For the sake of simplicity, the example of a kettle, used to make hot water for tea or coffee, can be used to describe some important boiler basics.   Boiling water in a kettle is as familiar to most as riding a bicycle and these concepts can be applied to steam making applications and sustainability improvements in a boiler with little modification.    

The first step in our simplified steam making process is adding the water to the kettle.  This water normally comes from the tap.  Most tap water comes from a reservoir (surface water) where it is filtered and chlorinated or from a well source (sometimes chlorinated, sometimes not).  The tap water contains dissolved minerals to varying degrees depending on the source and the geography.  Some areas like Boston or Seattle that rely mainly on excellent quality surface water will have low levels of dissolved solids.  Well sourced water in the Midwestern US could easily have 50 times the level of dissolved minerals.  When the tap water is heated, the dissolved minerals drop out and form a deposit in the kettle.  The deposit could appear as a dusty, tan or white powder on the inside of the kettle.  This is called inverse solubility and the concept is counter intuitive.  Usually when we cook or make candy on the stove, we continue to add a solid to a liquid and it continues to dissolve with more heat; not so with hardness salts like Calcium and Magnesium.  Now, imagine that the kettle was heated with new water, thousands of times every day.  Eventually, the deposits on the inside of the pot would insulate the water from the heat and it would take longer to bring the water to a boil.  This process would ruin the kettle and use a lot more energy to boil the water than necessary.  In sustainability terms, people are inconvenienced by the extra time necessary to boil the water and they are less safe due to the condition of the kettle.  The resources of the planet used to create heat energy are depleted more by a kettle in this condition due to the extra time needed for heating.  From a profit perspective, the extra cost of electricity or gas reduces the discretionary income of the kettle owner.

A steam boiler operates in much the same way as the kettle.  The difference is that water is continuously added to the boiler and the kettle is a batch process.  The scale of the operation and the cost of the steam generating equipment are enormous when compared to the kettle.  For example, an industrial fire tube boiler rated at 500 horsepower might cost one hundred and fifty thousand dollars or more and could use 46,900 gallons of water and 1,700 gallons of #2 fuel oil each day (The numbers above assume twenty-four-hour operation at full load and 10 cycles of concentration and no condensate return).  An insulating layer of deposition on the boiler tubes, coming from minerals in the water, could drive fuel consumption up by 70%.  Now it should be clear that big dollars are involved in the discussion, especially if fuel oil prices are close to the price of gasoline.  Additionally, valuable resources like water and fuel are consumed in large amounts and 17.2 tons of CO2 are released into the atmosphere.  A boiler running in the noted condition represents a grave safety risk as well as high replacement cost.  Boiler tubes could fail due to heat stress causing an explosion and the destruction of the boiler.  There are much safer, more practical and more profitable ways to improve the operation of this system.  Using sustainability as our guide, the first area for examination is make-up water quality.  Like the tap water used in the kettle, untreated make up water can create insulating deposits.  If the minerals are removed from the tap water or make up water prior to use, they cannot create deposits.

There are several methods for removal of unwanted minerals in make-up water for boilers.  The most common is called water softening.  This method involves passing the water through a vessel containing cationic resin that has been regenerated with a strong salt solution or brine.  As the water comes in close contact with the resin, it grabs hardness minerals like Calcium and Magnesium and releases Sodium.  The Sodium replaces the minerals, but does not contribute to deposition.  This is an inexpensive and effective process, but not very sustainable.  The salt solution used in regeneration ends up going to sewer and increasing the salinity of water downstream; not especially good for people or the planet.  The most sustainable way to remove minerals from water for use in a boiler or kettle is with membrane filtration.  The reverse osmosis process uses high pressure to force water through a filter with openings so small that minerals cannot pass through it.  The permeate stream (what passes through the filter) is similar in mineral content to distilled water.  The minerals are concentrated in what becomes the reject stream and this water can be used for process or utility purposes like rinsing, lawn watering or toilet flushing.  The permeate stream becomes boiler make up water and allows the boiler to concentrate water much further without insulating deposition.  In fact, with the use of reverse osmosis in the original scenario described above, the boiler uses 3,250 less gallons of water each day and 202 less gallons of fuel oil (no deposition scenario) to produce the same amount of steam.  The reason for the increase in efficiency is related to the increased concentration of the boiler water.  The increased concentration means the boiler loses less heat and water to drain (bleed).  With purified make up water we can concentrate the boilers water thirty times the original concentration as opposed to ten times in the first example. 

In next quarter’s installment, further efficiency gains making the model system more sustainable will be explored…