Making carbon powder using fireplace and weak acid
The result of your first pyrolysis must be charcoal plus the mineral constituants that are present in wood ashes. Now the wood ashes are usually a mixture of : $40 - 70$ % $\ce{CaCO3}$, $5 - 10$ % $\ce{Na2CO3}$,  $5 - 10$ % $\ce{K2CO3}$, $2 - 3$ % $\ce{K2SO4}$, $2 - 5$ % $\ce{Ca3(PO4)2}$, $5 - 10$ % $\ce{MgCO3}$, $2 - 10$ % $\ce{SiO2}$, &lt; $1$ % $\ce{FePO4}$. Washing your charcoal with water and hydrochloric acid must remove all these substances, except $\ce{SiO2}$. This impurity cannot be removed.