PRODUCT : PRE-COMBUSTOR, GASIFIER

 

The biomass is loaded into the pre-combustor or gasifier through its upper opening and moves downward, while the air is injected trough the bottom and flows upwards. There are four layers along the biomass fuel path (see figure):

Pré-combusteur

  • In the first (topmost) stage, the biomass fuel is dried by the hot ascending current of combustion gases.
  • As the biomass fuel moves downward, its volatile compounds become vaporized in the second stage.
  • In the third stage, the carbon dioxide in the gas is reduced to carbon monoxide by reaction with a part of the fixed carbon of the biomass fuel (Bouduard reaction).
  • Finally, in the bottom stage, the remaining fixed carbon burns in fresh air thus providing the carbon dioxide for the third stage.
    Ashes are removed periodically from the bottom of the gasifier. Ashes come out cold, dry and with low carbon content (<2%).

Since the main reaction in the gasifier is slightly exothermic, its walls are cooled by water tubes connected to the boiler’s risers and downcomers. This way, all the thermal energy is carried into the boiler either by the hot gases or through the water circuit. The result is a high-temperature (ca. 500ºC), low calorific value mixture of volatile organic compounds, carbon monoxide, steam and nitrogen. This gas is then sent to the boiler’s burner: the cyclonic chamber. In this technology, since the final fuel is a gas, a high degree of completion of the combustion reaction is achieved with low air excess. Additionally, the flame can be controlled as in any regular gas combustion, with a high speed response.

The technology of updraft gasifiers is well proven and has been around for over five decades. Its major advantages are well known:

  • insensitive to the size of the biomass fuel, from a few centimeters to over one meter
  • complete reaction
  • the gas obtained has a low content of particles, since the upper layers of fuels act as a filter.

On the other hand, the main disadvantage of updraft gasifiers with respect to other gasification technologies is the high content of tars in the produced gas. Thus, if the intention is to fire an engine or turbine, the gas will have to be cleaned in a complex, expensive and heavy maintenance process of cooling and chemical purification system. In the present technology, we overcome this problem by directly burning the gas into a cyclonic chamber (optionally, together with solid small size biomass fuels). Since the gas never cools down below 500ºC, the tars remain gaseous and do not deposit on the walls of the equipment.
No refractory is needed in this technology, so the load change flexibility is increased and immediate (range 40 to 100%). In addition the maintenance and the operation costs are drastically reduced.

The gases leave the chamber at a temperature of about 400 °C to be burnt in the cyclonic chamber.
The pre-chamber is the vertical type, the walls are constructed of tubes of water to reduce radiation losses. Collectors tops and bottoms are connected to the boiler to ensure a natural circulation of water.
The pre-combustion air is fed through a fan via the base of each chamber.

This technology allows:

  • The use of wet biomass (50%);
  • Flexibility of the particle size of the incoming feedstock (from a few centimeters to one meter);
  • Rapid fluctuations of the boiler capacity;
  • 20% in 3 minutes or 2 minutes if small boiler (10 TPH);
  • A wide operating range of the boiler (40 to 105%);
  • Low maintenance;

Dozens of references are in operation worldwide.


 

Lorient,17 rue Henri Estier - F-56100 LORIENT - France - Tél. +33(0)2 97 37 10 44- Fax. +33(0)2 97 37 34 43
Paris,149 rue de Saint-Honoré - F-75001 PARIS - France - Tél. +33(0)1 44 76 02 95 - Fax. +33(0)1 44 76 03 11