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Thursday, September 5, 2019

Pyrolysis

The world has a mountain of spent tires, plastic, (much of it dirty), wood wastes, tallow from abettors  used engine oil and even old clothes with no recycle value.  All these and probably many other waste streams can be pyrolyzed.  So what is pyrolysis.

Pyrolysis
Any carbon based material, if heated in a retort without oxygen, breaks down into a range of chemicals, many of them alkanes.  Classically, the gaseous part of the output is cycled back to the retort and burnt to provide the heat to power the process.  What a waste.  The gas is essentially LPG and can be compressed into tanks and used for domestic heating and cooking. The lightest fraction, methane, can be turned into methanol as we have done for years In New Zealand with methane from our petroleum industry.

Let's be more ambitious and set up a dedicated wind turbine to provide the energy for powering the pyrolysis  and use the lighter fractions. A bank of Zinc Bromide or liquid metal batteries can smooth out the power supply with any excess sent to the grid. As with any battery, both of these have their weak points but far outweighed by their strong points.  Both can be charged and discharged fully without any degradation of their capacity over time.

And where would we locate the wind turbine???  Anywhere with access to the grid and good wind.  That's why we have national grids; to transfer power from where it is generated to where it is needed.

If your chemistry is a little rusty, what are alkanes

Alkanes
Alkanes are chemicals with a chain of carbon atoms with hydrogen atoms on all the remaining bonds. Alkanes are saturated hydrocarbons meaning that there are no double bonds between the carbon atoms.  In ascending order of chain length alkanes are methane(CH4), Ethane (C2H6), Propane (C3H8) Butane (C4H10) and so on all the way up to very long chain tars. (or if you prefer, cooking gas, gasoline, diesel, jet fuel and so forth)

Alkenes are also be produced but are a pain in the kister.  With their double bonds, they can link up with other chemicals and form 'varnishes' and sludges that you don't really want in your carburetor.  More about them later.
 
                            Feed Stock
Tires
When tires are pyrolyzed, most of the resulting chemicals are the usual range of alkanes plus carbon and steel.  The steel is from the steel reinforcing in the tires.  Many manufacturing processes use carbon including the production of black plastic pipes, black paint, filters and new tires.  The steel can be accumulated in rail cars and shipped to a smelter.  All Steel mills use a proportion of waste steel along with the Iron ore they refine.

If the pyrolysis unit is located beside an oil refinery the mixture of alkanes can be fed right into the fractionation towers to separate out the various components.  Tires contain sulfur which is used for the vulcanization process and oil refineries are already set up to remove sulfur from 'sour' crude oils.

Sulfur is another valuable by-product and is a much use element in many industrial processes and particularly for the production of Sulfuric acid which is used in many chemical processes.

Oil refineries  use various methods to 'crack' long chain hydrocarbons to obtain a larger yields of the most needed fractions such as gasoline, diesel and aviation jet fuel and this can be similarly done for the output from a pyrolysis retort. Note that in Finland a plant to convert tires back into petrol has been set up and is reported to be pollution free.

Wood waste
The usual chemicals are produced plus wood char (charcoal) as a by product.  Wood char is a great soil additive which fills a similar function to humus in the soil. Charcoal has a very long life in the soil and hence results in a long term sequestering of carbon. At present, it is not economically worthwhile to produce char for farmers but with a pyrolysis unit producing a lot of this material as a by produce, the price of charcoal should come down.  The petroleum products produced are green and the charcoal sequesters carbon from the atmosphere.

Plastic
Plastics, long chain carbon compounds, are easily pyrolyzed.  The plastic doesn't have to be clean.  Food wastes, petroleum products and other contaminants will be pyrolyzed along with the plastic. This is a solution to our mountains of plastic and especially those which can't be recycled either because they are the wrong type of plastic or because it is not economic to clean them.  Note that it used to be worthwhile financially, to send these plastics to China before they refused to take any more.  Surly, then it is worthwhile to send them to a pyrolysis unit in your own country.

Tallow
Just recently (Sept 2019) some numpti of an abattoir poured  tons of hot liquid tallow into their drains.  The congealed fat totally disabled their municipal waste treatment (sewage) plant.  How much is that going to cost in clean up costs, fines and the ecological cost of raw sewage going straight into the environment until they get the pumps and pipes unclogged. How much better if they had accumulated  their tallow in a rail car and sent it off to the pyrolysis unit when the car was full.  Here we get into externalities (see below).  

So what else could we do in concert with a pyrolysis unit.

Side lines
With some of the electricity from our dedicated wind turbine or solar panels we could electrolyze water into Hydrogen and Oxygen and store it in large, low pressure tanks.  Why would we want to do this.  First the Hydrogen.


When longer molecules are cracked to make more of the short chain molecules,,,, everywhere a chain breaks, there is a free carbon bond that needs filling.  If it is not filled, alkenes will result and as we saw these are not desirable in an engine.  If Hydrogen is introduced into the retort at the correct temperature and with the correct catalyst, the hydrogen saturates these bonds converting the alkenes to alkanes.  So how about the Oxygen.

If tires are the main feed stock, this will result in considerable amounts of sulfur being produced.  If it is burnt in air it produced Sulfur dioxide.  However if burnt at high temperature in an oxygen rich environment, Sulfur trioxide is produced.  Add this to water and you produce Sulfuric acid which is a much used chemical in industry, not to mention in lead acid batteries.

When sulfur is oxidized (burnt) heat is produced.  It might be possible to capture this heat for use in the processes.

So already were are producing better fuel by saturating broken bonds with hydrogen and  we have a sulfuric acid producing plant on site.

                          Economics
Esternalities
When calculating the economic feasibility of pyrolysis, externalities must be included.  Externalities are:
 " a consequence of an industrial or commercial activity which affects other parties without this being reflected in market prices" (wikipedia dictionary)

For instance:
*the cost, economic and ecological, of having a mountain of tires leaching poison into the environment and providing water pools for mosquito breeding;
*the cost of a mountain of plastic sent to land fill or entering the environment and eventually  the sea;
*the price to store mountains of materials that could be used;
*the price to our environment of having to mine more hydrocarbons instead of using the ones we already have above ground and
*the hydrocarbons we can produce from renewable sources such as wood waste, thus further reducing the amount of crude that must be extracted.

All these sort of costs should be credited to the plant that is operating the Pyrolysis plant.

                          The Future

Let's look to the future; of what else we could pyrolyze.  

Electronic Equipment
Old computers, radios, communication equipment and so forth are largely plastic these days.  But they also contain many metals.  If they are pyrolyzed, the plastic is converted to the usual gaseous and liquid hydrocarbons but the ash left over contains copper, gold, lead, tin and other metals.  This residue can be sent to a smelter to be separated.  It might be worthwhile to first chop up the material and apply a magnetic and then an eddy current separation first This will separate out some of the elemental ferrous and not ferrous metals.

Treated timber 
At present there is no place to get rid of treated timber.  Tantalized timber contains copper, arsenic and chromium.  If it is burnt and the ash applied to the soil in the mis-belief that you are adding valuable wood ash to your soil, you will have caused serious contamination.  None of these metals are one's you want in your vegi garden.  All the scrap treated wood from construction could be pyrolyzed as well but one would have to make sure that these chemicals, especially arsenic, were recovered from the output of the process. At the end of a batch-pyrolysis, oxygen should probably be introduced to get rid of any remaining carbon, turning the residue into ash which is now further concentrated and can be sent to a refinery.   

Nappies 
A mountain of nappies, both for infants and even more from the aged, are created every year.  They can also be pyloyzed.  Since they will have a high water content, it would probably be necessary to bring the pyrolysis retort up to, say 1100C and hold it there until water vapor ceased to be expelled from the retort.  Some other feed materials might benefit from a similar treatment. A benefit of an initial phase of heating to just above 100degrees C is that you expel all the air in the retort before starting the pyrolysis cycle.  Remember, all this is done using renewable energy from our dedicated wind turbine or solar panels.

Old waste dumps
Recently a waste dump close to the sea was breached by a storm, augmented by our steadily rising sea level.  Old dumps which must be removed for whatever reason could also be pyrolized.  In the anaerobic environment in a dump, much of this material is unchanged.  We could clean up the sins of our grandfathers.

Almost none of the materials which can be pyrolized are effected by being stored in a rail car until it is full.  The rail car can then be sent to the pyrolysis unit, which, hopefully, is located within the grounds of a standard petro-chemical refinery.  These cars can be tacked on to existing trains when they are heading in the right direction.  Thus the cost of shipping should be 'reasonable'.  But we must never ignore the cost of externalities as we so often do.  Pyrolysis should be credited with the cost to us, both long and short term, of doing nothing.

                  Other Benefits of Pyrolysis  
  
Balance of Payments
Whatever materials we product by pyrolysis reduces the amount of Crude we have to import from overseas, improving our balance of payments. The demand for oil is already decreasing as electric cars gain a greater share of the market.  Oil from pyrolysis would become a greater and greater proportion of the oil we use as this process continues.

Waste Dumps
Pyrolizing as much of the feed to waste dumps as is possible will lengthen their life and ultimately, as we make use of other materials going to waste dumps, could eliminate them. Eventually old waste dumps could be mined and eliminated.

Our Green Image
New Zealand depends to some extent on her green reputation for the marketing of her products and especially her agriculture products.  Reducing our use of 'mined' petroleum fits within our story.  

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