What is the inorganic content, “ash forming matter”, of biomass and wastes?

All solid fuels contain an inorganic, not combustible fraction, constituted by inorganic elements such as: Cl, S, K, Na, Mn, Cd, Cr, Zn, Si, Mg, etc. Those elements are present in the fuel in different phases and minerals (e.g. silicates, oxides/hydroxides, sulphates, phosphates, carbonates, chlorides, etc.). During thermal conversion (combustion, gasification, etc.), the inorganic matter present in the fuel undergoes chemico-physical transformations, changing its association, form, phase and forming an etherogeneous (solid, liquid or gaseous) matter called generally “ash”.

What is ash?

Ash, the transformed inorganic matter, stays in the plant in solid/liquid phase or is released to the environment in gaseous phase. Ash can often be splitted in bottom ash (or grate ash) and fly ash (or filter ash). Fly ash is composed by a fine (or aerosols) and a coarse fractions. The split bottom/fly ash is dependent on the fuel type, technology and plant operating conditions.

What are bed materials and combustion additives?

Bed materials are inert (at specific conditions) which are used to increase the inertia of the bed, in fluidized bed combustion, allowing to convert the fuel at lower temperatures (850°C), increasing the thermal efficiency and reducing the emissions (e.g. NOx) of the thermal system. Example of bed materials are: sand, feldspar, dolomite.

Additives are (usually) minerals injected in the plant or premixed with the fuel, in order to capture critical inorganic elements (e.g. Cl, K, P, heavy metals) and promote specific interactions (e.g. to increase ash melting temperatures). Examples are: aluminosilicates (kaolin), Ca-based (gypsum), S-based (ammonium sulphate) minerals.

Which are ash-related problems in thermal processes for bio-waste fuels?

Ash agglomeration (fluidized beds): rapid formation of agglomerates (ash and bed material particles) in fluidized beds, which causes improper fluidization till interruption of operations. Two agglomeration mechanisms are identified: coating and melt induced agglomeration. It happens already at low temperatures (< 800°C).

Ash slagging: deposition of (partially) molten ash particles on the furnace water walls, bottom grid/roof, more in general on radiant heat exposed surfaces (high temperatures, e.g. > 1000°C).

Ash fouling. High-temperature fouling (around 1000°C) is the deposition condensed and impacting compounds on the banks in the upper and medium temperature section of the boiler (convective heat exposed surfaces). Low-temperature fouling (300-600°C) is the deposition (often solid particles) on the economizers banks of a boiler. Fouling deposits are often (partially) removed with sootblowers, cleaning devices which use steam or water.

Corrosion. High temperature corrosion (already at 500°C metal temperature) is a metal wastage mechanisms occurring in superheater sections, often related to Cl and heavy metals. Low temperature corrosion, in convective sections (e.g. around 200°C), is often related to condensation of acidic compounds (e.g. S, Cl) and hygroscopic salts.

Erosion/wear: metal wastage, e.g. due to the presence of hard compounds in the fuel ash and increasing with increased gas flow velocities in the installation.

Flue gas conditioning disturbances: ash can influence the operation of the flue gas cleaning systems, e.g. by deactivation of catalysts or disturbance of the performance flue gases cleaning systems.

Those problems affect the thermal efficiency of the plant and increase maintenance costs therefore reducing the system’s profitability. Once the problems occur, boilers inspections and cleaning are then required.

Can you suggest references to understand bio-waste ash challenges?

We recommend the keynote lecture from Prof. Hupa, by The Combustion Institute (August 2016).

Can you suggest standards to be considered for the fuel analyses?

Helpful standards concerning analytical analyses are: moisture (EN 14774), ash content (EN 14775), C, H, N; S, Cl, F (EN 15104; EN 15289), ash oxides (CaO, K2O, SiO2 etc.) (EN 15290, by XRF or ICP), volatile matter (EN 15148), calorific value (EN 14918), ash fusibility (e.g. deformation temperature) (CEN/TS 15370, ISO 540), water soluble K, Na (EN 16995). ISOEN

Why to use BIOFACT?

BIOFACT provides at low cost a confidence on results comparable to pilot testing. It is easy to use and rapid to apply for a quick fuel screening. The best use of BIOFACT is in a funnel strategy to screen and compare fuels. The tool generates comparable results to aid fuel selection, design and boiler operational decisions. It helps preventing the sourcing of unsuitable fuels and premature plant outages.

BIOFACT Fuel Dashboards: how risks are computed and how models are validated?

Indicators are computed with theoretically based predictive models calibrated with sets of experimental data. Models are validated with experimental data from the technical literature (lab-, pilot- and full-scale tests) and from operational experience from industrial partners. The fuel – technology matching matrix is built linking fuel properties with requirements from boiler technologies and is validated with REX from industrial partners.

What is a calibration/reference fuel? When fuel data are not available, which information is used?

A calibration fuel or reference material is a fuel for which the behaviour is well known, e.g. for which plant results are available. The BIOFACT results for the calibration fuel help to compare outcomes for unknown fuels (relative comparisons).

When fuel properties are not available, data are retrieved from own databases.

BIOFACT Report: how it works?

Detailed and comprehensive simulation, delivered as a technical report, are based on softwares and allow to explore specific ash related risks and design or operational aspects of bio-waste boilers.