Executive Summary: Biochar from wood residues and boiler ash

Summary of Project and Key Findings:
Sawmills and other wood product manufacturers produce large quantities of woody biomass in the form of wood chips, sawdust, shavings, off-cuts and bark. These mill residues are commonly sold as raw material for the manufacture of pulp and engineered wood panels, for landscaping applications (e.g. bark mulch) and as fuel for combustion in boilers (e.g. hog fuel). Many wood residue streams are potentially available, including sawdust from sawmills with geothermal heat or over supply of residues in relation to their heat demand. Other potential resources are municipal wood wastes such as untreated demolition waste. This resource has been estimated to be in the order of 200,000 oven dry tonnes per annum nationally. Despite its high use rate as a fuel, there are still quantities of sawdust which end up in the landfill. There is potential for increasing the value of this waste feedstock by converting it to higher value products such as bio-energy and biochar, using well engineered slow-pyrolysis technology.

This project is focussed on increasing forest resource efficiency by converting low value wood residues to a higher value product (biochar), helping to increase its current reuse and recovery and decrease disposal to landfills. Due to its high carbon content and surface properties, when biochar is added to soil it can have multiple benefits such as increase of water holding capacity, increase of organic matter content, decrease of nutrient leaching, increase of pH and plant productivity thereby reducing the inputs of chemical fertilisers. High quality biochar (>70% C by weight) is estimated to worth NZ$350 – 600/tonne in the current market, whilst the raw material – sawdust – has an energy value of $42 per green tonne.

The project was completed in collaboration with Soilpro (http://soilpro.co.nz), a company that manufactures soil conditioners using high carbon boiler ash sourced from wood processing mills and composts made from woody residues. Onion and potato growers in Pukekohe and kiwifruit growers in Te Puke, have been able to achieve improved product quality yields using high carbon boiler ash products.

High quality biochar was produced by pyrolysing various wood residues (sawdust, wood chips, shavings and bark peelings) including sawdust on its own and as a mixture. Temperatures of 400 and 550 C were used, along with and 30 min and 1 hr residence times, in a oxygen free environment. The resultant biochar had a carbon content of >70% which complied with International Biochar Standards, compared to the feedstocks which have a carbon content of approximately 50%. Higher temperature produced biochar with higher carbon contents, but at a cost of lower char yields. In some instances high carbon boiler ashes can comply with International biochar standards. High carbon boiler ashes are produced due to burning of wet wood residues (~55 to 60% moisture) with insufficient air and grate residence time in wood processing mill boilers. As a result this product is dependent on the feedstock composition (hence it is inconsistent) and has considerable amount of klinker and unburnt wood residues (14 to18%). Therefore this feedstock needs screening and testing of the product to assure conformity with the biochar standard.

Techno-economic analysis of two different scale biochar making operations showed that, commercial biochar making operations to be financially viable, biochar should be sold at > $400/tonne and the metrics showed sensitivity to recovery of a gate fee (i.e. using wood waste streams) and utilization of syngas in the process. Financial metrics improved with the size of the operation. Currently imported biochar sells around NZ$2000/tonne (based on retail price) and surveys done in the USA and Australia have reported an average prices of NZ$3826 and NZ$1946, respectively. Therefore, the required price of $400/tonne for biochar making operations to be viable, appears realistic. Co-locating pyrolysis plants near forest industry or wood processing operations, will help to reduce costs of transportation and handling of feedstocks and will help to further improve the financial metrics.

In summary, this work indicates that ash from some biomass boilers, where it contains a high proportion of unburnt carbon, can be a viable biochar. The financial performance of biochar operations is dependent on scale and product price, but these operations should be viable given the current markets.