~20 °C ≈ 1 pp efficiency
Stack-temperature rule
Boiler · Pressure parts & heat recovery
Boiler performance retrofit: the boiler as a system
A boiler loses efficiency in known, measurable places: hot flue gas up the stack, surplus combustion air, fouled heat-transfer surfaces. Every ~20 °C of stack-temperature reduction recovers about 1 percentage point; an economizer retrofit typically returns 3.1–4.0 pp; trimming excess air to λ ≈ 1.15 adds 0.7–0.8 pp. This page maps the losses to the equipment that recovers them.
+3.1–4.0 pp
Economizer retrofit, typical
+0.7–0.8 pp
Excess-air trim to λ ≈ 1.15
~50 %
Bagasse moisture as fired
01 — The system
Combustion, heat transfer, recovery, cleaning — one balance
Efficiency is not a component property. It is what the whole gas path leaves behind.
A solid-fuel boiler is four coupled subsystems: combustion (grate, air supply, fuel handling), the pressure parts that absorb furnace heat (water walls, superheaters), the back-end heat recovery that strips what the furnace could not (economizer, air preheater), and the cleaning systems that keep all of those surfaces working. Arrow Energy Co., Ltd. designs and fabricates across all four, which matters because a boiler performance retrofit in one subsystem always moves the others: lower stack temperature changes the ESP operating point, air trim changes furnace heat flux, and a fouled bank quietly undoes both.
Where does a bagasse boiler lose efficiency?
Three recoverable places: sensible heat in flue gas (each ~20 °C of stack temperature ≈ 1 percentage point), surplus combustion air (trimming to λ ≈ 1.15 is worth 0.7–0.8 pp on units running high excess air), and fouled heat-transfer surfaces, which raise stack temperature until cleaned. The largest single loss — evaporating the fuel's ~50 % moisture — is fixed by the fuel, not the boiler.
That is why the delivery model is measurement first: a boiler energy audit establishes the loss map by the indirect method before any steel is proposed. The arithmetic behind it is published on the engineering calculators page.
02 — The losses map
Recoverable losses and the equipment that recovers them
Indicative figures for biomass units — design values, not guarantees.
| Loss | Mechanism | Indicative recovery | Recovered by |
|---|---|---|---|
| Dry flue-gas (stack) loss | Sensible heat leaving above ambient | ~1 pp per 20 °C stack reduction | Economizer, air preheater |
| Economizer retrofit, combined | Stack heat into feedwater | +3.1–4.0 pp typical | Economizer |
| Excess-air loss | Surplus air heated ambient → stack for nothing | +0.7–0.8 pp trimming to λ ≈ 1.15 | Combustion instruments, O₂ trim |
| Surface fouling | Ash blankets tube banks; stack temperature creeps up | Restores design heat absorption | Soot blowers |
| Moisture-in-fuel loss | Evaporating ~50 % moisture (bagasse, LHV 7.2–7.5 MJ/kg) | Fixed by fuel — not a retrofit target | — |
| Availability losses | Tube failures, attemperation faults, ash bottlenecks | Lost generation, not lost efficiency | Pressure parts, PRDS, material handling |
The map is deliberately honest about what a retrofit cannot touch: on a 50 %-moisture fuel the latent-heat loss dominates the balance and no back-end surface recovers it. What retrofits do recover — stack heat and excess air — is measured in percentage points that repeat every operating hour, which is why the payback arithmetic on the economizer page is stated in fuel displaced, not adjectives.
03 — The range
Boiler products, by subsystem
Pressure parts to ASME/JIS practice; code-stamp scope stated per project.
Heat recovery
Economizer
The highest-leverage retrofit on most solid-fuel units: +3.1–4.0 pp typical, sized against the acid-dewpoint margin with the fouling and erosion allowances biomass ash demands.
Economizer design and retrofitHeat recovery
Air preheater
Preheated combustion air dries and ignites high-moisture fuel earlier — the recovery path chosen when combustion stability, not feedwater, is the constraint.
Air preheater designPressure parts
Superheater
Coil design, material selection and replacement banks for the hottest, most failure-prone surface in the gas path.
Superheater coils and banksPressure parts
Water wall
Membrane-wall panels fabricated to ASME/JIS practice — furnace enclosure repair and re-tubing without redesigning the boiler around them.
Water wall panelsFeedwater
Deaerator
Dissolved-oxygen removal ahead of the feed pumps — the cheapest corrosion protection the pressure parts will ever get.
Deaerator systemsSteam conditioning
PRDS
Pressure-reducing and desuperheating stations that deliver process steam at the pressure and temperature the process actually needs.
PRDS — pressure reducing and desuperheatingFabrication
Pressure parts
Headers, coils, panels and bends fabricated to ASME/JIS practice at the Samut Sakhon factory; code-stamp scope stated per project.
Boiler pressure parts fabricationAvailability
Spares
Boiler and ESP spares held against the components that actually fail — supplied under the ISO 9001:2015 (TÜV Rheinland) scope for spare parts, installation and maintenance services.
Boiler and ESP sparesMeasurement
Instruments
O₂, temperature, pressure and level instrumentation — the measurements the λ-trim and efficiency arithmetic depend on.
Boiler instrumentsFuel & ash
Material handling
Fuel feeding and ash extraction sized for high-silica, abrasive biomass ash — the systems that decide whether the boiler runs at rated load.
Fuel and ash material handlingSurface cleaning
Soot blower
Steam and rake soot blowers that hold tube banks at design heat absorption — the pillar page covers types, sequencing and the stack-temperature evidence of fouling.
Soot blower systemsFAQ
Engineering questions, answered
How much boiler efficiency does an economizer retrofit recover?
Typically 3.1–4.0 percentage points on biomass units, by dropping stack temperature toward the acid-dewpoint margin — each ~20 °C of reduction is worth about 1 percentage point. The recovered heat goes into feedwater, so it displaces fuel directly. Guaranteed figures are stated per project after the technical assessment, on a stated basis.
What is the right excess air for a bagasse-fired boiler?
Biomass units commonly run high excess air; trimming toward λ ≈ 1.15 is worth 0.7–0.8 percentage points of efficiency, because every kilogram of surplus air is heated from ambient to stack temperature for nothing. The trim needs reliable O₂ measurement and combustion instruments, then verification that CO and unburned carbon stay in bounds.
How low can stack temperature safely go?
Down to the acid-dewpoint margin, not through it. Sulphuric-acid dewpoints run 115–140 °C for sulphur-bearing fuels and lower for low-sulphur biomass; cold-end metal must stay above the dewpoint at all loads, including start-up. The design basis works a 203 °C stack downward with that margin held — the last 20 °C is never worth a corroded economizer.
Related engineering pages
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