Choosing a heat generator

The production of pellets, briquettes, wood-polymer composites and porous ceramic bricks needs the raw materials to be dried and chipped first. Most of the drying equipments require an exterior heating source. In this article we will try to find out which heating equipment is better to be used in the named production process.
1. Economy.
A bit of theory for the beginning. It is well known that to produce pellets and bricks the wooden raw material shall be of 10% moisture content whereas its starting moisture content is around 55% on average. Thus normally 2 tons of wet raw materials will produce 1 ton of dry product and 1 ton of water will be taken away. Normally to do this you need 1 MW of heating power. When you burn 1 kg of wood of 10% moisture content you get around 18 MJ of pure heating energy (the lowest calorific value). Hence, to get 1 MW within 1 hour you need to burn 200 kg of wood of 10% moisture content (1 MW х 3600 sec / 18 MJ = 200). Since the efficiency of the heat generator is around 90% in reality you will need 220 kg of wood per hour. Conclusion: to get 1 ton of ready products (pellets, briquettes etc.) in any case you need to burn at least 220 kg of dry wood shreddings (naturally, even more if the wood shreddings are wet). And now you have several variants. Let’s discuss them one by one.
Variant 1. We burn some part of ready dry chips, because the loss seems to be only 22%. At the same time you have an advantage: you don’t need an operator for this process because the flow to the heat generator can be organized automatically. Actually it is not totally like this. We have 1000 kg, we burn 220 kg, we get left 780 kg, but 220 from 780 is not 22, but 28%. In other words we economize on the cost of 1 worker but we loose around one third of the production.
Variant 2. We burn in the heat generator the wastes that can’t be used further: bark and slabwood, dirty and rotten raw materials, small logs (if you don’t have a chipping machine). In this case we need a worker but we keep the production on the same level.
What is more profitable? Let’s try to compare. We will discuss as an example production of wooden heating pellets. Let’s suppose that the wet raw materials are your own and hence you get it free of charge. After drying and chipping you need to press, sieve and pack it to get the ready product. To make our calculations easier let’s take a ready figure – not more than 900 RUR per ton (for more details please go to http://www.ecology-energy.ru/Files/Auto_Economist.xls). An average market price for pellets is 3200 RUR per ton (80 Euro per ton). Hence every lost pellet ton will cost you 2100 RUR. And this in case that the raw materials are free of charge. But if it is bought from somebody the difference will be even bigger. It is even more obvious if you produce briquettes of Pini-Key standard because the price for them is more than 4 200 RUR per ton. It becomes still even more obvious if you calculate the same figures per month. Within a working shift at the factory with 1 ton per hour capacity you can produce 8 tons of pellets. In one month within 21 shifts a factory can produce 168 tons of ready products. In case you have a restricted amount of raw materials and a part of it will be burned in the heat generator within one month you will loose 47 tons of ready products (28%) which makes 150 000 RUR of losses, in case of two shifts operation the loss will comprise 300 000 RUR. Economizing on heating workers salary will comprise only 15 and 30 thousand RUR correspondingly. It is obvious that the Variant 2 is more profitable. And sayings like «I’ve got lots of wood shreddings» from the economic point of view is naïve, since you never get them totally free of charge. In other words a cheapskate will pay not twice but for ever.
Conclusion 1: To become energy effective the heat generator shall be able to use any wood waste but not only particulate.
2. Quality (efficiency) of biomass combustion.
Again a bit of theory.
The following facts are widely proved:
- the most effective way of burning wood (and other biomass) is pyrolysis, i.e. if wood is first combusted at a high temperature with lack of oxygen into pyrogas and then this gas is burned with excessive oxygen – you’ll get more heat than burning wood in usual way;
- the pyrolysis process sows maximal efficiency at the temperature of 700 degrees Celsius and higher;
- to keep high temperature stable in the pyrolysis zone the air on the intake shall be hot;
Conclusion 2. To burn the biomass effectively the heat generator shall have at least 2 zones (pyrolysis and after-burner) as well as heated and distributed air intake.
3. Meeting the tasks set.
Generating heat for drying chipped biomass for producing biofuel and other above-mentioned products imposes special requirements:
- minimal spark formation and hence maximal fire safety;
- minimal ashes outtake and minimal ashes content in the product;
- maximal stability (stability of temperature and production).
The third requirement is met by many heat generators, but the first two are met only by vortex (cyclone) heat generators which have cylindrical form because the fire curved into a vortex makes much longer way around which provides for burning sparks in the heat generator itself. In other heat generators the sparks are just extinguished in various further cyclones, labyrinth spark-guards etc. which decreases the efficiency of the heat generator and increases the maintaining manpower effort. Speaking about heat generators with heat exchange units they for sure have minimal fire unsafety, but since a lot of heat is lost in the heat exchange unit these heat generators do not stand up to economic criticism.
Conclusion 3. The most effective unit to generate heat for drying the biomass is vortex (cyclone) heat generator of direct heating.
4. Quality of the unit and operating life.
One of the main quality features of a heat generator is inner lining. What can be tricky here? When heating and cooling the inner lining and the metal frame of the heat generator (especially when there’s no despotic cooling) suffer from very strong molecular forces of linear expansion (shrinkage). In rectangular heat generators these forces are summed which bring finally to consecutive destruction of the inner lining (sometimes it takes just several months). That is why such a form of generator is the most undesired for our purpose. Cylindrical inner lining is the most long living but at the same time the most difficult to make especially for the vortex heat generators since it shall also contain blast nozzles. It is very hard to produce such an inner lining at the desired level of quality even at special premises and virtually impossible at the working site of the Client. What will poor quality of the heat generator result in? It will result in often and long demurrages of the factory. It is even more harmful than the difference in the energy efficiency of the fuel which was described earlier.
Conclusion 4. The heat generator shall withstand multiple cycles of heating-cooling, hence it shall have cylindrical inner lining of very high quality which was produced at a specialized factory, it is also desirable that it has despotic cooling.
5. Possibility to dismantle and transport the equipment to a new site.
It is not uncommon that a pellet producing factory may find itself without any raw materials (e.g.: the owner of the raw materials has built his own pellet factory). What can be done in this case? It is necessary to move the factory to a new place. Moreover the possibility of such moves some investors take into account from the very beginning when they just choose the site for construction. Practically all the factory equipment can be dismantled and moved to a new place. However it is a bit different about the heat generator. Most heat generators can’t be moved without destruction of the inner lining and sometimes even the steel frame. Are there transportable heat generators? Yes, these are heat generators with cylindrical inner lining which is constructed around the horizontal axle so that neither brick is just capable to fall out from it. Such generator can be transported to any distance even without any supporting casing.
Final conclusion:
From our point of view the best heat generator to produce heat for biomass drying is the following:
Vortex pyrolysis heat generator of direct heating, horizontal, cylindrical, which can work on any solid fuel, with heated and distributed air intake, despotic cooling and which can be dismantled and transported to another place.
Naturally such a heat generator can’t be cheap. But let’s count. For a heat generator of 1 MW the price difference for different quality can be from 400 000 RUR up to 1,2 mln RUR. According to the part 1 where we calculated the economic efficiency it is possible to economize up to 300 000 RUR per month, in this case the higher price for a better generator will be paid back in 2-4 months, and this is without taking into consideration possible demurrages of the factory due to the low quality equipment. Now it is time for you to choose.