African Boxthorn Biochar Project

African Boxthorn (Lycium ferocissimum) is a densely branched, perennial shrub growing typically 2 – 3m in height and with branches that end in sturdy thorns. African Boxthorn is a Weed of National Significance due to its invasiveness impacts, potential for spread and significant environmental and socioeconomic impacts. (Commonwealth of Australia and the Australian Weeds Committee, 2012)

Once introduced, it displaces native vegetation in both coastal and inland environments, reducing biodiversity values of the local environment. It readily invades agricultural land, reducing access and usability. Boxthorn provides prime habitat for invasive animals such as rabbits. It provides impenetrable barriers to livestock, reducing access to pasture and water.

The fruit provides a breeding place for undesirable insects such as fruit fly, and a food source for birds and foxes, which in turn disperse the fruit’s seed. Being spread by animals including birds, African Boxthorn is very challenging to contain and can readily spread to new areas including relatively remote coastal islands.

Current best management practice is to remove the whole plant with an excavator or by hand, pile in heaps and burn it when conditions are favorable.

The Northern Agricultural Catchments Council (NACC) approached Energy Farmers Australia to trial the pyrolysis of African Boxthorn into biochar as a means to finding an alternative use for this plant once removed from the local environment.

The objectives of the trial included

• Materials handling and pre-processing requirements
• The pyrolysis process
• Determine biochar/energy output and biochar quality.

Materials Handling

In order for the African Boxthorn to be pyrolised through the Energy Farmers biochar kiln, the material had to be broken down from whole trees and shrubs into smaller particles. As part of the project, Energy Farmers engaged Aussie Tree Services to travel to Dongara where the African Boxthorn piles were located and chip approximately two cubic meters. Aussie Tree Services then transported the chipped material back to Geraldton by truck where Energy Farmers bagged it into bulka bags.

After the piles had been chipped, it became clear that the African Boxthorn needed further processing, a one pass operation with a wood chipper used by most tree lopping companies leave whole sticks and stems that cannot be moved by conventional augers.

Energy Farmers proceeded to process the material through a tub grinder which significantly reduced the particle size of the African Boxthorn into a more manageable form.

Pyrolysis

Pyrolysis of the African Boxthorn was conducted on the 28/5/2014.

Process temperatures were high, signifying a highly combustible material and ideal process conditions. Maximum temperature achieved in the gasification chamber was 982°C with the average being 715°C. Flare Temperature averaged approx. 380°C, ideally, flare temps should be in excess of 400°C to burn off any volatiles however, considering the short run time of the process and the fact that flare temps were on the increase before shut down, Energy Farmers is confident that the gas released is of high quality and well within acceptable emission standards.

Biochar yield was in the region of 20 – 30% which meets the target yield for biochar production through the EFA process Biochar temperature averaged approx. 400°C which is optimal for good quality biochar. Energy produced was approx. 480 MJ thermal or 133kW of heat. Being a mobile system it would be hard and expensive to convert the heat to electricity however, process heat could be used to dry the incoming African Boxthorn which would increase the throughput capacity of the machine.

In conclusion this project demonstrated that while there are challenges with materials handling, pyrolysis of African Boxthorn is possible and mobile pyrolysis could potentially provide an “on-site” solution for the treatment of boxthorn waste, as opposed to the current practice of burning Boxthorn piles.

The biochar market in Australia is a small and expanding market. Currently, the retail price of woody based biochar’s is in the region of $2-5/liter. Therefore, there is potential to process African Boxthorn onsite to produce biochar’s to sell into this market.

Considering the fact that African Boxthorn is a weed of national significance and very widespread throughout Australia. EFA recommend a more in depth investigation to determine benefits and the potential to deliverer a positive business case for adopting the process.