Biochar: A possibility for solid waste disposal

Biochar is the product of the pyrolysis of biomass created in the absence of oxygen. It is characterised by having a high carbon content and by its high stability in the environment.

Over time, trends in research on greenhouse gasses, and legislation to mitigate their effects have increased. "Many of the world's governments participate in agreements to reduce carbon emissions. It's highly unlikely that going green will be a passing fad" (Lop 2012). This trend in legislation and research has led to scientific breakthroughs and adaptations. From the use of food and waste crops to produce biofuels, lower emissions from industries, to the "green" wave of conscience growing among consumers, global warming-mitigation technologies are entering our everyday life. Biochar is one of the more recent developments to emerge.

Biochar is significantly different to pyrolysis, which has been extensively investigated for waste produced by the leather industry, in that it is carried out in the absence of oxygen. This anaerobic process causes carbonisation of the waste products and encapsulates non-volatile elements within the carbonised structure of the product rather than producing an ash with leachable materials. These differences make biochar an ideal option to investigate for waste disposal in the leather industry.

The product of pyrolysis is not only the solid biochar; there is also a gas fraction that is emitted during processing (figure 1). This fraction can be condensed down to a liquid biofuel or vented off and combusted as a flammable waste. Both products are energy rich and this property can be made use of. Some solid product can be used interchangeably with charcoal (for example, to provide cooking heat). The liquid fraction can be further refined to produce a biofuel; however, it is commonly vented back into the pyrolysis system and ignited to help heat the reactor, as in this way the energy costs for producing biochar are reduced.

Figure 1: LASRA Biochar reactor illustrating flaring of the gases produced.

Following pyrolysis, the waste material is lighter, more homogenous, more chemically stable, and resilient to toxin leaching. As a result, this process could make disposal of fellmongery and tannery wastes cheaper and easier. A survey conducted by LASRA (Foster, 2008) showed that in New Zealand almost 40% of waste produced by the leather industry is land filled. This equates to approximately 21,000 tonnes of waste that could be processed cheaply to leave a safer and more useful product. Figure 2 shows the make-up of this land filled waste.
Figure 2: Breakdown of waste dumped in New Zealand landfills by the leather industry. (Adapted from Foster, 2008).
Biochar research carried out at LASRA

The aim of this work was to investigate whether biochar could be used in the leather industry to safely and cost-effectively dispose of wastes. The study was broken down into two parts; firstly, could leather wastes be used to form biochars; and if so were the biochars safe, cheap to produce, and stable.
To test the first part of the aim, a pyrolyser was designed. The reactor was a vessel that fitted in the muffle furnace, was airtight, had a one-way valve for gas release, and was made of heat resistant stainless steel.
Samples of industry wastes were collected from a range of fellmongery and tanning stages; from green fleshings through to offcuts of finished crust.

Pyrolysis
100g of the material to be pyrolysed was weighed out and put in the reactor. A clay seal was added and the lid bolted down. The reactor was then purged with nitrogen gas to remove the excess free oxygen from the environment, and made airtight to prevent oxygen re-entering. The reactor was placed inside a muffle furnace for heating.

Biochar
The biochars produced had a range of physical and chemical properties that varied according to the temperature and waste material used when generating them. For leather wastes it appears that a temperature of 300°C is too low for proper carbonisation to occur. Biochars prepared at this temperature retained many of the properties of the leather waste and were not suitable for use in waste disposal. At the other end of the scale, the 600°C biochars appeared to have been completely carbonised. They had the same structural appearance as the initial waste product, but were shrunken and brittle.

Figure 3: A selection of solid waste materials (top) and their respective biochars (bottom) produced at LASRA from lime fleshings, screened effluent, wet-blue trimmings, buffing dust, and crust leather respectively

To test the stability and safety of the biochars that were produced, samples were leached in acid in an attempt to extract chrome. Soil conditions are predominantly acidic; therefore if the biochar is stable in acid it should be stable in soil as well. A solution of 1M hydrochloric acid was used for the test. As this should be significantly stronger than the weak humic acids in the soil, the absence of chrome in the acid solution after leaching would be a very good indication that there would not be chrome leaching in the soil.

Wet-blue shavings and biochars prepared from wet-blue at 300°C and 600°C were extracted with the acid solution for one hour. The solution was then filtered to remove the particulate matter, and the total chrome content of the liquid was measured using a Varian SpectrAA 220 atomic absorption spectrometer. The results are shown in figure 4.

Figure 4: The amount of chrome leached from the material into the acid solutions

As can be seen in figure 4, less chrome was extractable from the 300oC biochar than from the untreated chrome shavings, and none could be extracted from the biochar prepared at 600oC. This showed that once heated to this temperature, the biochar was stable and not liable to leach chrome.

In summary, it has been shown that wastes from the leather industry can be pyrolysed to form stable and safe biochars. Those produced by low temperature pyrolysis (=300°C) did not undergo carbonisation; however medium temperature pyrolysis (400°C -600°C) of leather wastes produced carbonised products that could be considered biochars. Biochars prepared at low temperatures were unstable and leached chrome into acid solution. The 600°C biochar was stable and no chrome was present in the acid leachate. Due to the oxygen-free environment in which the biochar is produced, it is expected that the chrome present will be in the form of chrome III, and so will be considered environmentally safe.

Cost analysis
Designs for pyrolysis units range from cheap, site built batch processing units to complex large scale continuous units. Since the reactor need only be stainless steel and has no pressure certification requirement because gas produced is vented or flared, a small simple batch unit could be easily prepared onsite by a competent engineer at relatively low cost.
Running costs will depend on the waste material being processed. Energy is required to heat the reactor for carbonisation to occur. During processing, the gas fraction is vented off. This could be flared off, but a more productive use for it would be to use it to help heat the reactor, so lessening the amount of external energy required. The major cost of heating the material in the production of biochar is the removal of moisture. We conservatively estimate an energy breakeven point of 53% moisture content.

Design factors
There are some aspects that need to be appreciated before biochar production could be implemented in the fellmongering and tanning industries:
- The system must be anaerobic. The addition of oxygen will initiate combustion and the material will burn instead of carbonise.

• The temperature must be maintained. If it varies then the product produced will change.
• Heating rates must be controlled. Faster heating will produce more gas fraction and less biochar; slow heating will produce more biochar and less gas. The correct balance must be found.
• The quantities that need processing will dictate the production method that can be used.
• A flammable gas is produced during the process. This means that the reactor cannot be completely sealed, and the gas must be dealt with safely.
• There is an unpleasant smell related to production, the nature of which depends on the material being processed. The smell may impact on the ability to get consents etc.

Conclusions
For the leather industry, biochar production could be used as a cost-effective means of waste disposal. It is cheaper than dumping the waste in landfill and decreases the mass and volume of the waste that industry generates, and so is a more economical option for waste disposal than landfill.
Biochar makes leather waste safer for the environment, as it sequesters the chrome that otherwise leaches out of the waste. This in turn has the potential to lower the fees that must be paid for disposing of wastes.
If leather wastes are pyrolysed to biochar, the leather industry will become more environmentally friendly in other ways. There will be less carbon released to contribute to global warming, and in addition biochar can be used to improve soil quality.

References
Foster, N. (2008). Solid wastes survey and looking at options. In, Report of the 59th Annual LASRA Conference (pp. 59-68). Palmerston North, NZ: Leather and Shoe Research Association.
Lop, P. (2012) Is Going Green Just Another Trend?. Retrieved 5th November 2012 from
http://www.life123.com/home-garden/green-living/green-homes/is-going-green-just-another-trend.shtml