How does the wood gasification boiler work?

Wood gasification

Before wood can burn, it must be converted into gas by applying heat. At 100°C the water contained in the wood escapes. At 200°C the material decomposes into 20% charcoal and 80% wood gas, a mixture of carbon dioxide, carbon monoxide, hydrogen, methane, methanol, various phenols, acetone and acetic acid. 400°C suffice for outgassing, but at least 900°C – better 1,100°C – are required to completely break up the phenols (wood tar) into combustible carbon, carbon monoxide and hydrogen. In addition to a high temperature, complex compounds take time to break up, which explains the long gas flame of a wood fire.

Large woodpile in fuel chamber

At the foot of the woodpile in the fuel chamber a small gasification fire is maintained with controlled air intake (primary air). The boiler electronics regulate the gasification rate via the primary air intake The wood gas is extracted downwards into a hot combustion chamber. This prevents the woodpile in the fuel chamber from outgassing in an uncontrolled manner and igniting. A boiler with a large wood capacity, slow burn-up and long burning period is thus possible.

Mixing jet and complete swirl

A mixing jet is located between the fuel chamber and combustion chamber. At this point pre-heated combustion air (secondary air) is added to the wood gas. The flame from the mixing jet is ejected at high speed onto the hot floor of the combustion chamber and is once again whirled around (4) to ensure that every combustible gas particle encounters sufficient oxygen for complete combustion.

Complete combustion in the glow zone

To achieve an uncooled combustion at high temperatures, the patented glow zone combustion chamber is made of high refractory bricks and is additionally thermally insulated. In this glow zone the flame has sufficient time until the last stubborn carbon rings (phenols) break out of the lignin of the wood and burn at temperatures of between 900 and 1,200°C. ETA have thus succeeded in remaining below the magical limit of 100 mg/MJ carbon monoxide emissions in the flue gas of a wood boiler.

TECHNICAL DESCRIPTION

The ETA SH Wood Gasification boiler has an induced draught ventilator for efficient firing of the fuel, a large chamber (150 litres for SH 20/30 and 225 litres for SH 40/50/60 models) so that refueling is reduced to once or twice a day. Complete with a control panel including a timer for two heating circuits. Approved by TÜV in Germany where the ETA SH has performed top of the class in its field in relation to lowest emissions and highest degree of efficiency.

1.LARGE FUEL FILLING SPACE: fuel capacity of 150 litres in the SH 20/30 models and 225 litres in the SH 40/50/60 models means that refueling intervals kept to a maximum.

2. ZONE ISOLATED AND ANNEALED COMBUSTION CHAMBER: The combustion chamber is lined with a high temperature ceramic Fire Brick that is light and prevents ash build up.

3. LAMBDA SENSOR: The SH boiler uses a lambda probe to adapt the firing process to suit differing types of wood, it identifies the energy density of the timber through the oxygen content of exhaust fumes and then adjusts the burning process to suit.

4. EASY REGULATION: The control panel allows for easy, and accurate, regulation of the boiler, buffers, water cylinders and circulating pumps.

5. RE-LIGHT WITHOUT MATCHES: When you refill the chamber with new fuel you don’t have to go messing about with

6. GASIFICATION: Secondary burning of carbonised gas from the wood fuel (normally lost to the atmosphere in the exhaust gas) reduces emissions and ensures optimal usage of all wood fuel burned in the combustion chamber.Temperatures in the combustion chamber can reach up to 1200 °C as a result.

7. PRIMARY AND SECONDARY AIR ACTUATORS: The primary and secondary air valves have a regulated position feedback link to the main control panel; this allows accurate and subtle control of primary and secondary combustion whenever required.

8. SILENT COMPRESSOR: The boiler has a barely audible, step-less, regulated induction fan with speed feedback to the control panel. Max electrical power consumption of the motor is 76W.

9. EXHAUST GASES: Seamless replacement of previous boilers due to monitored exhaust gas temperature, feedback loop to control panel.

10. EASE OF REGULAR MAINTENANCE: Regular maintenance is easily done through the main front door, there is no need for extra side doors and hard to reach hatches.

11. EASY CLEANING: the heat exchanger is the easiest of tasks, just pull the shaker handle on the outside

BETTER PERFORMANCE

For the first time ever it is now possible to burn all forms of wood in the same boiler, it doesn’t matter if it Ash, Spruce, Beech, Wood Briquettes or even waste carpentry timber. The ETA SH boiler can burn all of these woods efficiently while at the same time keeping it’s emissions with in the boundaries of the magical carbon monoxide (CO) limit of 100mg/MJ. This magical figure is achieved through the use of the ETA Lambda probe; the probe measures the remaining oxygen particles in the boiler exhaust gases continuously and then regulates the primary and secondary air induction to match so that the boiler is always operating at the optimum level of efficiency. In addition this system also makes sure that not too much air enters the burning process, surplus air would draw heat from the boiler out in the exhaust and thus reduce efficiency

SIMPLE OPERATION AND NO NEED FOR MATCHES

To refill the heating chamber you just open the door and load it with fresh timber or wood briquette fuel. Due to the high load capacity of the chamber you only have to refuel once or twice a day depending on outdoor temperature and house size. Once the door is closed the firing stage begins using the remaining embers to light the new fuel can be ignited automatically – you light the fire manually the first time and the boiler will keep enough embers to relight the fire for 24 hours (This only applies if you are burning hardwood such as Ash, Beech, Oak, etc).

The lambda probe checks the oxygen content of the exhaust and adapts the firing process to suit the fuel used, it identifies the energy density of the timber in this manner adjusts the primary and secondary draughts to suit varying levels of moisture content – thereby always giving the fuel optimal burning conditions.

Before leaving the boiler room you just have to pull the lever on the side of the unit to clean the heat exchanger in the boiler and you are done – this ensures a high degree of efficiency at all times.

BETTER THINKING

High efficiency is the sum of innumerable details and ETA have gone into all the details with this unit - for example, air that is used for combustion is drawn along the gap between the boiler panels and the internal boiler so as to reduce the amount of heat lost from the boiler through radiation resulting in an extremely low 0.78% of heat lost through radiation including floor loss.

BETTER HEAT

Since combustion air is tightly controlled, there is no excess air to cool the fire, as a result the ETA combustion chamber burns between 1000 and 1200° C.  Combustion chambers in conventional wood burners do not have the durability to withstand such heat, whereas the ETA multipart combustion chamber is made from a stable high temperature ceramic material with built in expansion joints.