Biomass burners for steam boilers — fired on shell and crop residue
The burner changes. The boiler does not have to.
A biomass burner fires a steam boiler by replacing the oil or gas burner at the furnace tube, feeding the same flame path with cashew shell or crop residue instead. Capacities run 200,000 to 900,000 kcal/h, and a 1,000 kg/h boiler at 10 bar needs roughly 150 kg/h of shell.
Indicative price of a biomass burner, Ex-Works (EXW): USD 1,050–12,750
Shipping, customs clearance and local duties are extra. The final price depends on the configuration and is fixed in the written quotation.
What we supply, and what we do not
GreenMach supplies the burner and the firing system: the fuel store, the conveyor, the surge hopper, the metering screw, the pyrolytic combustion chamber and the flue-gas train. The boiler is yours — existing, or separately procured — and so is the water side, the feed pump, the blowdown and the safety valves.
The line between the two is exact, and it is worth stating in the words the engineering narration uses: the burner’s job ends here, the boiler’s begins. Everything upstream of the furnace tube is ours; everything downstream of it is the boiler’s and its own maker’s.
How the flame reaches the water
Fuel enters a sub-stoichiometric chamber where it pyrolyses rather than simply burning: the primary air is held at a lambda of about 0.4, which is deliberately short of the oxygen combustion needs. What leaves that chamber is combustible gas, not flame.
That gas is ignited at the throat with secondary air, above 900 °C and now with excess oxygen. The flame passes through a refractory quarl into the furnace tube of a three-pass wet-back fire-tube boiler, crosses the tube nest twice more, and leaves through the flue. Heat moves into the water by nucleate boiling on the outside of the tubes, and what comes off the surface is dry saturated steam.
Gas temperature entering the boiler is around 1,050 °C and around 200 °C leaving it. That drop is the boiler doing its job; a leaving temperature that climbs over a season is the first sign the tube nest needs cleaning.
The worked case
A 1,000 kg/h boiler producing saturated steam at 10 bar(g) from feedwater at 80 °C. Every figure below is design basis rather than a measured result, and each one can be checked against the next.
| Parameter | Value |
|---|---|
| Steam output | 1,000 kg/h saturated at 10 bar(g) |
| Feedwater temperature | 80 °C |
| Enthalpy rise | 2,446 kJ/kg |
| Useful heat to steam | 679 kW |
| Boiler efficiency, design basis | 80 % on GCV — 78–82 % typical for a 3-pass wet-back |
| Burner heat input | ≈ 850 kW |
| Cashew shell fired | ≈ 150 kg/h at 4,800 kcal/kg |
| Sensitivity across the efficiency band | 149 kg/h at 82 % · 157 kg/h at 78 % |
| LPG displaced | ≈ 60 kg/h |
| Fossil CO₂ avoided | ≈ 175 kg/h |
| Saturation temperature at 10 bar(g) | 184 °C |
Combustion control
The chamber runs four pyrolysis zones in sequence, with primary air held sub-stoichiometric at a lambda near 0.4 so the fuel gasifies rather than burning where it sits. Secondary air meets that gas at the throat, above 900 °C and with oxygen in excess, which is where the flame actually forms.
A choked surge hopper acts as a gas seal, so nothing burns back up the feed. The PLC trims screw speed and damper positions against a steam-pressure transmitter on the boiler, which means the firing rate follows demand rather than a fixed setting — the difference between a boiler that holds pressure through a load swing and one that cycles.
- Shell as received. 4–8 % moisture w.b., 1–3 % ash, 11–14 % fixed carbon and 80–83 % volatile matter on a dry basis. The high volatile fraction is why staged pyrolysis suits it.
- CNSL. Cashew nut shell liquid carries about 11,380 kcal/kg and is the reason an ordinary firebox tars up on this fuel. The staged chamber is built around that fraction specifically.
- Biochar. What leaves the grate is 70–75 % carbon at 25–28 MJ/kg — a saleable product rather than a disposal problem, where a buyer exists.
Flue gas and draught
A cyclone takes the particulate, and an induced-draught fan holds the combustion chamber slightly below atmospheric pressure so nothing leaks into the boiler house. Steam pressure runs 8 to 10 bar across the normal range.
The reference installation
A 900,000 kcal/h cashew shell burner fires the steam boiler at Premium Cashew Commodities Limited in The Gambia, on the empty shell the factory produces itself. It is the company’s one commissioned installation, and it is a steam application rather than a drying one — no drying line has been built anywhere yet.
We publish the configuration because every part of it is checkable. We do not publish efficiency, steam output or fuel consumption from that site: those figures belong to the operator, and we will publish them only with their agreement rather than estimate them here.
The site has a page of its own, with two films: the PCCL installation in The Gambia.
Specifications
| Parameter | Value |
|---|---|
| Burner range | 200,000–900,000 kcal/h |
| Steam pressure range | 8–10 bar |
| Boiler type suited | 3-pass wet-back fire-tube, existing or separately procured |
| Primary combustion | Sub-stoichiometric, lambda ≈ 0.4, four pyrolysis zones |
| Secondary combustion | Above 900 °C with excess oxygen, at the throat |
| Boiler gas temperature | ≈ 1,050 °C entering, ≈ 200 °C leaving |
| Design fuel | Empty cashew shell, 4,800 kcal/kg |
| Control | PLC trimming screw speed and dampers from a steam-pressure transmitter |
| Warranty | 12 months from date of shipment |
| Manufacturing lead time | 10–18 weeks from confirmed order and receipt of advance payment |
The same combustion core drives the cashew shell burner and the biomass hot air generator for drying duty. For a cashew factory in particular, see steam for cashew processing; for what else can be burned, the biomass fuel envelope.
Where this equipment is used
The same equipment is specified across several product classes. These are the ones it is configured for most often:
tree nut and oilseed drying, tea, coffee and cocoa drying and fruit and vegetable drying.
Each of those pages carries the drying parameters, cycle times and quality targets for that product class.
Frequently asked questions
Do you supply the boiler as well as the burner?
No. We supply the burner and the firing system, anywhere from 200,000 to 900,000 kcal/h — fuel store, conveyor, surge hopper, metering screw, combustion chamber and flue-gas train. The boiler, the water side and the safety fittings are yours, existing or separately procured. The burner’s job ends at the furnace tube and the boiler’s begins there.
How much cashew shell does a 1,000 kg/h boiler burn?
About 150 kg/h at 10 bar with feedwater at 80 °C, on shell rated 4,800 kcal/kg. The honest form is a band rather than a number: 149 kg/h if the boiler achieves 82 % efficiency and 157 kg/h at 78 %, which is the normal spread for a 3-pass wet-back.
Can an existing oil or gas boiler be converted?
Usually, and it is the cheaper route. A 3-pass wet-back fire-tube is the normal candidate. What has to be checked first is furnace geometry, refractory condition and whether the shell was ever rated for solid fuel — a biomass flame is longer than an oil flame, and a furnace tube sized for the latter can impinge.
What does the burner displace in fuel terms?
On the worked case, roughly 60 kg/h of LPG and about 175 kg/h of fossil CO₂, for 150 kg/h of shell the factory already produces and would otherwise pay to remove. Whether that is worth doing depends on your LPG price and your disposal cost, which are yours rather than ours.
Discuss a boiler firing system
Tell us your steam duty, your boiler’s make and pass arrangement, and the residue you have on site, and we will tell you what the firing system looks like.
Request a proposal