Softwood versus Hardwood

Table of contents:

Definitions of Softwood and Hardwood

Softwood (e.g. pine, spruce, larch)
Softwoods come from gymnosperms, where the seeds don't have any protective covering - like in pine cones; more precisely, softwoods come from the coniferous subcategory of gymnosperms.

At a cellular level, 90-95% of the wood is made of long, thin tracheid cells that run the length of the timber, with most of the rest being ray cells that run from the centre to the outside. Many species also have a small number of resin cells for transporting sap and resin, and some have a small number of parenchyma cells, shorter and fatter than the tracheids.

Softwood generally have a lot more sap and resin than hardwoods, giving more energy when they burn.
Hardwood (e.g. oak, ash, birch)
Hardwoods come from angiosperms, where the seed in encased in a fruit or a nut. More specifically, they come from dicots - the other branch, monocots, has plants like palms and bamboo. In temperate climates like in Europe, hardwoods are deciduous, losing their leaves in winter, but this isn't necessarily the case in warmer parts of the world.

At a cellular level, most of the wood is made of fibres, with an aspect ratio of about 100:1. Between these there are vessel, shorter and fatter but joined together to form long pipelines. Some species also have small numbers of tracheid and parenchyma cells, as in softwoods, with much thinner cell walls than the other cell types. And then ray cells run from the centre to the outside.

How they differ

In terms of energy, there is little difference on a weight basis between softwood and hardwood. But softwood is significantly less dense than hardwood, so in terms of volume you need to put a lot more wood into the kiln. So you either need to stoke a lot more frequently, or you need a larger firebox, if firing with softwood.
Now turning to emissions, with most parameters differences in firing technique and/or in the dryness of the wood make more difference than the species. One exception is in NO_x emissions, as hardwoods generally have more nitrogen in the wood, and so will create more NO_x emissions, regardless of firing style. The other exception is particulate matter, but this is a bit more complex. At lower temperatures in the firebox the resins in softwood give off a lot more PAHs during the devolatilisation stage, but at higher temperatures these will decompose; overall hardwoods have higher ash content, which means more particulates over the entire temperature range. Also, it's worth noting that bark produces about 10x more particulates than normal wood. So for clean burning it makes sense to start off with hardwoods, and then switch to softwoods if your aim is low particulates (and, of course, you want to keep out of reduction), but if you're more concerned about NO_x then you want to stick with softwoods throughout.

Property	Softwood	Hardwood	Notes
Energy per kg (dry), MJ/kg	~20.5 - 21.5	~19-20	Softwood ~5–8% more energy per kg. Difference due to the resins in softwoods outputting more energy than the higher cellulose of hardwoods.
Density (dry), kg/m³	~350–550	~600–900	Major practical difference. You need ~1.6–2× the volume of softwood to match hardwood by weight. Affects stoking frequency.
Lignin content, %	26–34	18–25	Lignin burns hotter and slower; contributes to ember phase. Higher lignin → more CO₂ per kg.
Resin / extractives	Higher (pine, spruce especially)	Lower	Resins boost calorific value; also drive PAH spike during volatile phase if combustion is incomplete.
Nitrogen content, %	~ 0.05–0.1	~0.1–0.5%; birch highest	Direct NOx effect. Hardwood produces more fuel-NOx. Cannot be reduced by better combustion.
Ash content, %	0.2–0.5	0.3–1.5	Higher ash → more fly ash (PM) and potential for atmospheric deposition on ware. Bark greatly increases this.
PM₂.₅ emissions	Generally lower (esp. pellets)	Generally higher (correlated with ash)	Effect dominated by combustion quality; species difference smaller than wet/dry effect.
PAH emissions	Higher during devolatilisation if incomplete	Mixed — some studies higher, some lower	Genuinely contradictory in literature. Combustion temperature is more important than species.
Burn rate / flame	Fast; bright, high flame; less ember	Slow; longer embers; more consistent	Softwood better for rapid heat rise; hardwood better for steady hold temperature. Classic kiln-firing strategy: softwood to climb, hardwood to hold.