CO-FIRING COAL WITH BLACK PELLETS

China added 78GW of coal-fired power to its energy mix in 2025, and coal storage yards like this one in Ningbo City are becoming more commonplace across the country. But coal can be co-fired with steam explosion black pellets to help nations meet their climate targets. 

Conventional biomass pellets must be transported and stored in specialized, climate-controlled containers and silos because exposure to moisture causes them to swell. 

Infiltration of water reduces the structural integrity of white pellets, so they ultimately revert back to being sawdust once dry again.

This disintegration of white wood pellets can result in significant fuel loss. It also causes conveyor systems to clog and, in large volumes, it can also trigger a biological fermentation process which leads to self-combustion. 

The steam explosion process softens and redistributes hydrophobic lignin, making black pellets highly water-resistant. This property enables power utilities to store steam-treated black pellets in open-air stockyards previously reserved for coal, and expose them to precipitation alongside coal. [2]

Black pellets' physical durability minimizes degredation and dust generation during handling, and it greatly reduces explosion risks in generating station facilities. 

Conventional white pellet biomass is fibrous and elastic, and difficult to process in original and traditional milling equipment at coal-fired power generating stations. 

The replacement of milling equipment for processing white wood pellets often constitutes a significatn proportion of the capital expense incurrred to retrofit facilities. 

Due to high moisture contents found in white pellets, processing them causes spikes in mill power consumption, severe mechanical wear and an ultimately uneven particle size distribution. 

Expensive plant modifications are inevitable to process white wood pellets.

Steam-treated black pellets are reinforced, by the redistribution of lignin. They possess much higher mechanical strength than white pellets as a result, with a Hardgrove Grindability Index (HGI) position which approaches that of coal.

Power generating stations can feed steam-treated black pellets directly into existing coal pulverizers, reducing the need for significant capital expenditure. Power requirements for pulverizing steam exposion black pellets with existing coal power station infrastructure are significantly lower than the power requirements for pulverizing white pellets. [2]

When processed with existing coal infrastructure's ball mills and vertical spindles, steam-treated black pellets fracture cleanly into a homogenous, fine, granular powder, rather than tearing into strings or causing motor overloads or losing biofuel volume during throughput.

Steam-treated black pellets possess all of the positive qualities of coal – high energy density, compatible grindability, total water-resistance – but burning them creates just six percent of coal’s carbon footprint, making them an ideal fuel for an energy transition pathway. 

Dropping steam-treated black pellets into a coal furnace for co-firing with the fossil fuel quickens, improves and supports stable combustion thermodynamics. [3]

Standard white wood pellets contains large volumes of volatile matter, in the form of fast-burning chemicals and moisture [4]. They cause hot pockets of fire that destabilize furnace burns which can then become difficult to control. This heat-shock is also capable rupturing furnace boiler pipes.

The steam explosion process drives off a high proportion of those fast-burning chemicals before the black pellet reaches the power plant, allowing both fuels to be co-fired and consumed in synchronization. 

Steam-treated black pellets release their flammable gases at the same speed as coal, and reach and complete their char burn-out phase at the same time as coal. 

The optimal co-firing properties of steam-treated black pellets allow for precise control of furnace temperatures and eliminate any need for significant burner re-configuration or modification.

Traditional white wood biomass pellets contain elevated concentrations of alkali metals combined with chlorine.

During combustion in facility furnaces their potassium and sodium mineral deposits quickly become volatile at low temperature, and form molten eutectic salts that splash onto furnace walls. [4] 

Over time, these salts form rock-hard slag which begins to corrode furnace walls, impedes heat transfer, reduce boiler efficiency and shorten the lifespan of furnaces.

The condensate draining phase of Valmet’s high-pressure steam explosion leaches away the majority of these water-soluble alkali minerals and chlorides. [5]

Ash produced by steam-treated black pellets doesn’t melt and splash onto furnace boiler tubes, to leave slagging and fouling. 

Instead, it resists intense heat to form a dry powdery dust that drops into ash collectors without damage to furnace walls and reducing critical convective heat transfer.

Black pellets manufactured using steam explosion offer improved co-firing characteristics on preceding white wood pellets. Their net calorific value, low volatility, high bulk density, hydrophobicity and Hardgrove Grindability Index make them an ideal match for coal.