Agri biomass vs wood pellets — calorific value, ash, price. What suits a heat plant?

The question "agri or wood?" comes up in every conversation with a heat plant procurement department when planning the season. The answer is always the same — "it depends" — but that "depends" can be broken down into a dozen or so numbers that fit into a table and plug straight into a heat cost calculation.

This post is not here to sell a single product. BGT supplies both — A1/A2 wood pellets (bagged and bulk) and agri biomass: sunflower pellets, sunflower husk, PKS (Palm Kernel Shell) and olive stone. Our goal is for the heat plant to choose the fuel that delivers the lowest cost per GJ at an acceptable operational risk — not the one with the prettiest brochure.

Below we break both fuels down to first principles: what the standard says, what the lab analysis shows, what happens inside the boiler and how it translates into zlotys per gigajoule.

What qualifies as "agri biomass" and what as "wood pellets"

In both cases the framework is defined by the ISO 17225 series (formerly EN 14961). Same standard, different parts — and that changes everything.

Wood pellets fall under ISO 17225-2. Three classes: A1 (cleanest coniferous/deciduous wood, ash content <= 0.7%, nitrogen <= 0.3%), A2 (allows bark and higher ash, up to 1.2%) and B (industrial wood, including recycled, ash up to 2.0%, higher concentrations of N, S, Cl permitted). Diameter 6 or 8 mm, bulk density 600–750 kg/m3, moisture typically 6–10%.

Agri biomass falls under ISO 17225-6 (non-woody pellets) and 17225-7 (non-woody briquettes). Classes A and B for agricultural materials: straw, husks, residues from olive processing, vegetable oils, nuts. In practice the ones that matter most for heat plants are:

• Sunflower pellets (from husk) — a by-product of oil pressing.
• PKS (Palm Kernel Shell) — the shell of the oil palm kernel, a by-product of the oil industry in South-East Asia.
• Olive stone — a residue from olive oil production, mainly from Spain, Portugal, Tunisia and Greece.
• Bulk sunflower husk — delivered straight from the press.

Important: in commercial energy production the ISO class is the starting point. The real passport is KZR INIG or ISCC EU — without it the fuel will not go into the renewables balance and will not build an entry into a guarantee of origin.

Calorific value, ash, chlorine — comparison table

The table shows typical ranges from lab analyses of BGT deliveries over the last 18 months (LHV, as-received, averaged values). For a purchasing decision what matters is not just the averages but also the upper bounds — because those are what cause failures.

Parameter	Wood pellets A1	Wood pellets A2	Sunflower pellets	PKS	Olive stone
Calorific value LHV (MJ/kg)	17.0–19.0	16.5–18.0	17.0–19.0	18.0–21.0	18.0–21.0
Ash (%)	0.3–0.7	0.7–1.5	2.5–4.0	3.0–5.0	1.0–4.0
Moisture (%)	6–10	6–10	8–12	8–15	8–12
Sulphur S (%)	<= 0.04	<= 0.05	0.10–0.20	0.05–0.15	0.05–0.15
Nitrogen N (%)	<= 0.3	<= 0.5	0.8–1.5	0.3–0.8	0.3–0.8
Chlorine Cl (%)	<= 0.02	<= 0.03	0.08–0.20	0.10–0.30	0.10–0.30
Bulk density (kg/m3)	650–750	620–720	600–700	550–650	600–700
Ash deformation temp. DT (C)	> 1200	1100–1250	700–900	900–1150	1100–1300
Market certification	ENplus A1, KZR/ISCC	ENplus A2, KZR/ISCC	KZR/ISCC	KZR/ISCC, ISPO/RSPO	KZR/ISCC
Indicative price (PLN/t, DAP)	950–1150	850–1000	700–850	750–900	750–900
Cost (PLN/GJ)	50–65	47–58	37–47	36–48	36–47

Three conclusions you will not see in the advertising:

1. Agri wins on PLN/GJ — a gap of 10–20 PLN/GJ vs A1 pellets is not cosmetic. For a 20 MWth heat plant running 5000 h/year that comes out at several million PLN/year.
2. Agri loses on chlorine and ash — Cl at 0.20–0.30% and ash at 3–5% is an entirely different league from A1. That does not mean "you cannot burn it" — it means "you need a boiler designed for it".
3. PKS and olive have a higher calorific value than wood — this surprises people. 18–21 MJ/kg comes from the lignin and residual fat content.

Which boilers: wood's versatility vs agri's specialisation

Wood pellets are a universal fuel. A 15 kW domestic boiler, a 2 MW district boiler house, a 200 MW power block — A1/A2 will work everywhere. Retort burners, forced-draught burners, fixed grates, moving grates, fluidised-bed boilers — no significant contraindications. Why: low ash, high ash deformation temperature (> 1200 C), low chlorine.

Agri biomass requires a deliberately designed boiler. In practice:

• Low-power retort burners (domestic boilers) — agri is out. The ash bakes onto the retort and chlorine destroys the heat exchanger within 2–3 seasons.
• Low-power fixed grate — occasionally yes, but it needs frequent cleaning and firing as a blend (max 20–30% agri).
• Stepped moving grate in 5–50 MW boilers — the native technology for agri. Ash removed continuously, furnace temperatures controllable.
• Fluidised-bed boilers (BFB, CFB) 20+ MW — the best option. The fluidised sand buffers ash melting and chlorine binds with calcium additives. In a CFB boiler burning a 70% agri / 30% wood blend is standard.
• Multi-fuel boilers in commercial energy production — designed from the outset for agri + wood + sludge + RDF in varying proportions.

A simple rule: if the boiler is older than 10 years and was designed for coal or pure wood pellets — do not load pure agri into it without consulting the service engineer. Risk: slagging, high-temperature corrosion, blocked flue gas ducts.

Corrosion, slagging, clinker — what you pay for when the fuel is cheap

Three destructive mechanisms which occur more strongly in agri than in wood:

1. High-temperature corrosion (Cl + K). Chlorine reacting with potassium forms potassium chlorides (KCl), which deposit on superheaters and heat exchangers at 400–600 C. KCl is one of the most corrosive compounds in the power sector — in extreme cases it destroys superheater tubes within a dozen or so months. A1 wood pellets have Cl <= 0.02%, sunflower pellets 0.08–0.20%. A tenfold difference.

2. Slagging and clinker formation (low ash deformation temperature). Agri ash melts at 700–900 C (sunflower) up to 1100 C (olive), while furnace temperatures often sit at 1200–1400 C. The ash melts, runs across the grates, sets into hard clinker. The result: downtime to hammer out clinker, ruined grates, drop in efficiency.

3. Duct fouling. Ash at 3–5% vs 0.5% is not a linear ratio in operating costs — it is an exponential increase in the frequency of cleaning, dedusting and disposal. Add to the balance: the cost of ash disposal (for A1 wood pellets the ash goes onto the field as a K/Ca fertiliser — for high-Cl agri it goes to landfill).

All three problems can be controlled — with calcium additives, sulphur sorbents, boiler selection and temperature control. But these are additional costs and additional procedures which do not exist when burning A1 pellets.

Economics: why PLN/GJ is the only honest comparison

The classic mistake: comparing price per tonne.

A1 pellets at 1050 PLN/t vs sunflower pellets at 780 PLN/t — a gap of 270 PLN/t (26%). Looks like a clear win for agri.

But the energy locked in a tonne is different. Convert it:

• A1: 1050 PLN / (18 MJ/kg x 1000 kg / 1000 = 18 GJ/t) = 58 PLN/GJ
• Sunflower: 780 PLN / (18 MJ/kg x 1000 / 1000 = 18 GJ/t) = 43 PLN/GJ

Real difference: 15 PLN/GJ, i.e. around 26%. In this case the same in percentage terms, but for PKS or olive, where calorific value is 20 MJ/kg, the percentage rises to 30%+.

Hidden costs have to be added to PLN/GJ — otherwise the comparison lies:

• Cost of ash disposal: 50–150 PLN/t of ash x ~3x more ash from agri.
• Cost of additives (kaolin, lime) when burning high-chlorine agri: 3–8 PLN/GJ.
• Cost of increased service frequency and parts replacement: 2–5 PLN/GJ (heavily boiler-dependent).
• Cost of storage (agri has lower density and absorbs moisture faster): 1–3 PLN/GJ.

After these corrections the real "full cost" GJ from agri is often 42–52 PLN/GJ vs 55–62 PLN/GJ from A1. Still in favour of agri, but the gap is 8–15 PLN/GJ, not 15–20.

Certification: ENplus, KZR, ISCC — who requires what

ENplus — commercial certification for wood pellets, key for the retail market and small boiler houses. Classes A1/A2/B. For a heat plant, on its own it is not enough.

KZR INIG — the Polish certification scheme for biomass for energy purposes, recognised by URE. Required for renewable energy guarantees of origin. Covers both wood and agri.

ISCC EU — the European counterpart to KZR, required for sales to the EU outside Poland. Key for PKS and olive (imports).

RSPO / ISPO — for PKS from Asia, "no deforestation" verification.

For a commercial heat plant the minimum is KZR + an ISO 17225 class certificate. Without that there is no way into the support scheme.

BGT supplies all documents as standard — including a batch-by-batch analysis report. This is not a bonus, it is a prerequisite for the heat plant to be able to account for the fuel at all.

Seasonal and logistical availability

• Wood pellets — available year-round, domestic and EU production. Price peaks: September–January. Heating-season contracts are best signed in March–May.
• Sunflower pellets — sunflower harvest season VIII–X (Ukraine, Romania, Poland). Lowest prices XI–I. Available year-round, but stored at the producer.
• PKS — sea import from Indonesia, Malaysia, Thailand. Delivery cycle 8–12 weeks. Prices are stable, sensitive to USD rate and freight rates.
• Olive stone — pressing season XI–III (Mediterranean basin). Best availability I–IV. Truck or rail import from Spain/Portugal.

For a heat plant with large annual demand, an annual contract with deliveries split by quarter is the only way to avoid spot-market risk. BGT prefers framework annual contracts with a fixed delivery schedule and a price formula.

Emissions and CO2 footprint

Formally both fuels are neutral — CO2 released in combustion equals CO2 taken up by the plant/tree during growth. That is the assumption of the standard and of the ETS.

The real difference lies in the logistics footprint:

• Domestic wood pellets / from the Baltic states — 30–80 kg CO2eq/t (road/rail transport).
• Sunflower pellets from Ukraine/Romania — 40–90 kg CO2eq/t.
• Olive stone from Spain — 90–130 kg CO2eq/t.
• PKS from Indonesia — 150–220 kg CO2eq/t (sea freight + inland transport).

In the GJ balance the differences are small (a few percent), but for ESG reporting and Scope 3 disclosures they can matter. It is a good idea to have this visible on the delivery SDK.

FAQ

Can I burn agri biomass in a boiler designed for wood pellets? Usually yes, but with limits. A blend of up to 20–30% agri in wood often requires no changes. Above that — you need to consult the boiler manufacturer, monitor temperatures and inspect more frequently. Pure agri in a small domestic boiler: not recommended.

How much can I save by switching from A1 to PKS in a 20 MW fluidised-bed boiler? Order of magnitude: 8–15 PLN/GJ net (after operating costs). At 5000 operating hours and 88% efficiency that is around 2.5–4.5 million PLN/year. The real payback on investment in modernising the fuel handling system for agri is measured in months, not years.

Are A2 pellets a compromise between A1 and agri? No. A2 is still wood pellets with slightly higher ash (bark) — a different risk from agri. The commercial-technical compromise is wood + agri blends in controlled proportions.

What about RDF fuel and sludge? They are outside this comparison — different standards, different collection schemes, different emissions regime. BGT does not trade in RDF or sludge.

How often should fuel analyses be done? For a heat plant on contract: every delivery (report from the press or producer) + a quarterly control analysis at an independent laboratory. BGT adds its own analysis to every weighing.

Can agri be burnt without KZR? Physically yes, but it cannot be entered into the renewables balance to URE nor accounted for in the ETS as biomass. Economically it makes no sense.

Summary: who's wood for, who's agri for, who's the blend for

Short and decisive:

A1 / A2 wood pellets: district boiler houses, public utility buildings, boilers older than 10 years without modernisation, sites in dense urban settings with NOx/PM emission constraints. Heat plants up to 10 MW with a simple fixed grate.

Agri biomass (PKS, olive, sunflower): 10+ MW industrial boilers with moving grate or fluidised bed, CHP plants, sugar mills, wood-processing plants with in-house energy. Anywhere the process engineers can manage ash and chlorine.

Blends (70% wood + 30% agri or vice versa): a practical starting point for a heat plant that wants to bring down its fuel cost but does not yet have experience with pure agri. Test under controlled conditions, measure efficiency, measure flue gas composition.

If you do not know which category you are in — write to us. BGT holds several dozen such conversations with heat plants every week and we have real benchmarks from many sites. We will send analyses, a GJ cost comparison for your boiler and a proposal for a supply contract for the 2026/27 season.