Biomass-coal co-combustion: opportunity for affordable renewable energy

Abstract

This investigation explores the reasons for and technical challenges associated with co-combustion of biomass and coal in boilers designed for coal (mainly pulverized coal) combustion. Biomass-coal cocombustion represents a near-term, low-risk, low-cost, sustainable, renewable energy option that promises reduction in effective CO 2 emissions, reduction in SO x and often NO x emissions, and several societal benefits. Technical issues associated with cofiring include fuel supply, handling and storage challenges, potential increases in corrosion, decreases in overall efficiency, ash deposition issues, pollutant emissions, carbon burnout, impacts on ash marketing, impacts on SCR performance, and overall economics. Each of these issues has been investigated and this paper summarizes the state-ofthe-art in each area. The focus is on fireside issues. While each of the issues can be significant, the conclusion is that biomass residues represent possibly the best (cheapest and lowest risk) renewable energy option for many power producers.

Comparison of selected parameters of biomass and coal

“…They concluded that straw had higher fouling tendency than peat and bark, due to the large amounts of water-soluble compounds occurring in the straw. Moreover, Baxter (2005) found that the highly active alkali/alkaline metals (for instance K, Na, Ca) and Cl contents could simply form vapour phase chloride compounds/ ions. Due to the low melting points (<800°C), these chloride compounds (eg KCl) could deposit and form a sticky layer on the heat transfer surfaces or heat exchanger (Shao et al, 2012a).…”

Von Rittinger theory adapted to wood chip and pellet milling, in a laboratory scale hammermill

“…For most such co-combustion units the specification requires all of the particles to pass through a 6.34 mm mesh and the majority to pass through a 3 mm mesh [4]. However, some consumers sometimes apply stricter specifications [1,9].…”

“…Some power plants use biomass (generally wood-derived) in pulverized form, which requires milling before use to produce wood particles that have the requisite aerodynamic and combustion properties to be used in the process. In such systems the characteristics of the biomass particles are vitally important as they impact upon feeding, combustion kinetics, the combustion residue volume and the hearth temperature [5e8].For most such co-combustion units the specification requires all of the particles to pass through a 6.34 mm mesh and the majority to pass through a 3 mm mesh [4]. However, some consumers sometimes apply stricter specifications [1,9].…”

Assessment of ash melting temperature of birch and grass biomass pellet mixtures

“…In comparison with coal, the ash melting temperature of which exceeds 1000ºC [11] or 1150-1500 ºC [12], biomass has a comparatively low ash melting temperature (usually between 750 and 1000 ºC), as it has a very different ash chemical and mineralogical composition [9]. Straw ash melting temperature is 850-1100 ºC [13].…”