Drying Wood Chips in Scandinavia

Sir

We are CDS Group in th UK. We have a system that dries wood chip. The process is very energy efficient and can be done continuosley if so desired.

Please contact me to discuss or email me.

Best Regards

Andrew Hall

CDS Group

+44 1782 336666

andy.hall@cds-uk.co.uk

SF are a Swedish company who make a flash dryer which should be suitable.

Since you are in a northern climate and you have a need large enough to justify some good research and development, please consider a freeze drying system.

Your climate will assist you much of the year which helps.

When you evaporate water the water absorbs energy to vaporize. Heat is the usual source. When you sublimate ice the transition still requires energy, but much less than heating to hot and then liquid to vapor. Since the sublimation of the ice still causes cooling there is less need to refrigerate in order to freeze in the first place and cooling systems are about twice as efficient as heating systems. The vacuum pumps will be less efficient before the mass is cooled enough to freeze, but they will never be less energy efficient than heating to boiling as the temperature difference between your material starting point and freezing will always be less than that starting temperature and boiling as long as there are no leaks.

Also, the vacuum pulls out the vapor where ever it is, so no additional aeration is required. The only difficulty is an air tight chamber of sufficient size and tightness. The sublimation pressure of water is about 1.2 psi absolute, which means a structural pressure of 11 psi or more.

If your process requires variable flow rates of chips you may find a very heavy flexible bladder will suit your needs best, if a relatively constant rate is expected a fixed structure like concrete over a dome may suit. You may wish to look at mounded earth technology for the form where a mound of earth is placed above a slab and inside a footing and the concrete with reinforcement for the dome is poured over and then the earth mound is washed or excavated out.

Sealing doors to such a structure would be problematic, If possible it would be preferable to have an automated auger or other type of material handling system in place. However, this system will be the source of fault with the design I am proposing, as faults in material handling systems are fairly common. Screw augers are sealable and highly reliable and energy efficient for short runs.

This type of design would open up the possibility of near continuous processing as a fresh load of chips were placed on top of the pile while a dry load exited the base. The packing of the material in the auger tube would keep leakage to a minimum while in operation, but would leak too severely to be open during drying. So, continuous batch processing would be the best description of this design.

The process is used effectively in food prep but under sterile conditions so most vendors of this type system would be a bit high price for drying wood chips.

One caution though. The system is completely dependant of the integrity of the vessel seal, so don't skimp there.

Very good filtration to remove fines from the air flow will be required to protect the vacuum pump and the heat source you mentioned can be used as heat is still required to drive evaporation, or sublimation. It will just need to be sealed, regulated, and near the bottom of the pile.

If a detailed study of the process indicates that pre cooling the chips will improve the results in summer, then cooling the chips in the intake auger with refrigeration coils would be the point in the system to do that.

If you are willing to look at a sustainable system of much greater capacity and slower cycle look at solar powered greenhouse drying. This is used extensively for wood drying in your country and several others. The process works a much larger volume of material in about a month to a month and a half for the spec's you mention but is about twice as energy efficient as any other method. The difficulty here is mostly in having a 480 tonne capacity facility with built in material handling system. If you are able to well use the hot air you mention then this maximum month and a half turn over rate could be greatly reduced which would decrease the size of the facility proportionally.

Best regards,

Mr. Gee

Appreciate your repsonse - thanks. I think the scale of what we have to do may rule out freeze drying because of the CAPEX needed to provide a sealed process capable of having a vacuum pulled on it. The process really needs to be a continuous one as opposed to batch ad therein lies the problem I think. At each site (typical) we will need to 'produce' every 24 hours about 60 tonnes of dried chips. However, we are looking into it - any idea who makes a proprietary freeze drier plant of this cale?

I am reasonably certain no one does, however, looking into grain dryer's I find;

http://www.mathewscompany.com/mathews_tower_vacuum.htm

these at least use vacuum cooling as part of the cycle - the capacity looks a little low for your application but they definitely have a systemic understanding of the issues and my be a qualified vendor to assist you in implimenting a solution.

same thing only different here.

http://www.deluxmfg.com/

One form of lime kiln introduces fresh material at the top and withdraws lime continuously from the bottom.

Combustion air also enters at the bottom, cooling the hot lime before being ignited about 1/3 to 1/2 way up.

These are very energy efficient, but usually built for much larger scale operation than you have.

Such an arrangement could be adapted for your purposes as wood chips don't pack badly to clog up the airways through the mass of material, unlike limestone.

They used to calcine limestone and recover so much from the exhaust gases that the exhaust would often fall below dew point. In addition, the calcine was quite cool when discharged as it had given up most of it's heat to the combustion air.

Unfortunately, I recently threw out all my old info dealing with this stuff ("Haven't used it for years, you'll never need it"), including the maker of this gear and the name of the equipment.

I think it is called a shaft kiln, but am not sure.

I think the German company KLM makes them, again I'm not sure of the name

Sorry I can't be more specific..

We will look into this - thanks very much. I have a shed full of the same files as you and keep managing to stop my good lady throwing them out completely - they were forcibly migrated out of the house and under the workshop bench where they hide quietly now.

Most wood chips dryer are rotary dryers. I was thinking about how to design high efficient wood chip dryer:

1, to crush it to less than 5mm, so it can be dried with superheated steam fluid bed dryer, which is high efficient in energy consumption. But I did not find out suitable wet wood chips crusher. Anybody knows wood crusher for my application?

2, to adopt tower dryer, same as suggested by Mr Gee, if it can move from top to bottom by gravity. This is lower in capital investment by modifying its design from grain tower dryer. My concern is if it is possible to move from top to bottom by gravity.

Long Huang

DryingTech Inc.

Toronto, ON, Canada

huanglong62@gmail.com

Why not a fluidized bed dryer?

Basically a cone with a grid part way up and chips on top of the grid.

Blow hot air in at the bottom at sufficient speed to fluidize the bed of chips.

As the chips dry, they get lighter and migrate to the top, eventually lifting off the bed and going out the exhaust, where simple cyclones are enough to separate them from the air.

Smaller chips automatically migrate faster but need less drying time.

Larger chips migrate slower, but also need the longer time in the bed.

The fluidization dramatically speeds up heat and mass transfer, which is just what is needed.

Fluid bed dryer is not suitable as its size and shape. That is why I am seeking for crusher. If we could find out suitable crusher, we can dry it with multi-effects superheated steam fluid bed dryer, which only needs 1/2 or 1/3, or even less energy comparing to hot air drying system. This is very important for thermal power plants.

I have the same exact application but at a lower throughput (about 10 ton/hr), i want to know what was the solution that you came up with for your application

Hi just a guest comment in passing. I think your application is important in the context of the question. for gasification you may not need to dry the chips, it will take place as part of the process. In fact the moisture will partly be the source for the hydrogen forming stage. any excess will be condensating out in the filtering/drying stage. Lx2u@hotmail.com

hi I am investigating wood chips drying options in the UK. I am interested in any information about viability of the wood chips drying from the economic point of view, but also companies that supply wood chips drying systems. I have residual heat 1 MW available that I would like to use for wood chips drying. Please contact me me to discuss details. woodchipsdrying@gmail.com