Could leftover nutshells be the next renewable energy source? Challenge your students to find out using calorimetry.
The world is waking up to the harm caused by burning fossil fuels. As a result, many countries are turning towards more environmentally friendly alternatives, including energy derived from biomassw1. Although these sources are generally considered better than non-renewables, one argument at the heart of most bioenergy debates is that farmland is often diverted for the production of the biomass, which comes at a cost to the food supply. One obvious way to avoid this ‘food versus fuel’ dilemma (and reduce carbon emissions in the process) is to derive bioenergy from waste.
In recent years, one material that we normally think of as a food source – nuts – has started to be used as an energy source. At present, whole nuts are used, but would burning leftover nutshells be a more sustainable way to use this material? And could waste nutshells help to meet future energy demands? In the following activities, we challenge students to find out.
Using a homemade calorimeter, students aged 14–19 determine the amount of heat energy released by various nutshells. By comparing their results to data from scientific literature, the students evaluate whether nutshells could replace other renewable energy sources that are currently in use.
Constructing the calorimeter takes 1–2 hours. We suggest that students work in groups of 2–3 and build one calorimeter per group. If your school already has calorimeters that can be put in contact with a flame, you can omit this part of the activity and start immediately with the experiment.
For one combustion calorimeter (figure 1):
In the same groups, students burn various nutshells and kernels using their calorimeter and calculate the amount of heat energy that is transferred from the kernel or shell to the water in the beaker. Although the aim of the experiment is to determine the amount of energy released by nutshells (not kernels), students also need to burn the kernels to compare their results to published values, as these values are more widely available than those for nutshells.
For the calculations, students use the following equation:
Q = m x c x ΔΤ
Where:
Q = heat energy transferred (J)
m = mass of water (kg)
c = specific heat capacity of water (4200 J kg-1 K-1)
ΔΤ = change in temperature (K)
We suggest that students burn kernels and nutshells for four different types of nut, in which case the whole experiment will take approximately 2 hours including calculations and repetitions.
This activity should not be carried out if any students or staff involved have an allergy to nuts. Remember that remnants may stay in the air following the grinding or burning of nuts, which could affect other individuals with severe allergies that use the same room afterwards.
Safety goggles should be worn and special care should be taken with the open flames.
Mean heat energy per kg (x 104 kJ/kg) | ||
---|---|---|
Nut type | Kernel | Shell |
Walnut | 1.4595 | 0.5320 |
Hazelnut | 1.4175 | 0.5285 |
Almond | 1.1690 | 1.0745 |
Pistachio | 0.6685 | 0.5180 |
Students should compare their own results for nut kernels to values that are widely available in scientific literature. This allows them to assess the reliability of their results for nutshells. Students can search for the literature themselves or use the values provided in table 2. Our students found that their own experimental values for nut kernels differed by roughly 50% from the published values they obtained (Brufau et al., 2006). This large difference provides an opportunity to discuss why the experimental setup and procedure may cause such a discrepancy. Later, students will calculate a correction factor by which to amend their data. There are a number of sources of error that you can discuss, for example:
Overall, however, the biggest source of error in calorimetry occurs as a result of heat being lost to the surroundings. To deal with this, students need to calibrate their calorimeter. In the next activity, they do this by measuring the combustion energy of a paraffin wax candle with a specific mass.
Mean heat energy per kg (x 104 kJ/kg) | |||
---|---|---|---|
Nut kernel | Experimental values without calibration | Experimental values with calibration | Published values |
Walnut | 1.460 | 2.840 | 2.735 |
Hazelnut | 1.418 | 2.757 | 2.679 |
Almond | 1.169 | 2.274 | 2.302 |
Pistachio | 0.669 | 1.300 | 1.147 |
Students calculate the amount of heat energy that the water in the beaker absorbs from the heat of the candle, and compare this to the amount of heat energy that the candle releases. By comparing the two values, students can calculate the heat loss and the error in their measurements. This procedure takes approximately 1 hour including repetitions.
D = 4.2 x 104 / 2.1589 x 104
= 1.945
For the final part of the activity, students evaluate whether nutshells could replace other renewable energy sources. To do this, students can obtain data from scientific literature regarding the energy released by burning existing sources of biomass, such as hardwood, wood pellets or olive stones, and compare these to their own experimental results for nutshells. Alternatively, students could use the values in table 3, which are sourced from published literature. They can then plot this information on a graph (figure 6).
Mean heat energy per kg (x 104 kJ/kg) | |||
---|---|---|---|
Biomass (nutshells and other sources) | Experimental values without calibration | Experimental values with calibration | Published values |
Walnut | 0.5321 | 1.0349 | - |
Hazelnut | 0.5285 | 1.0279 | - |
Almond | 1.0745 | 2.0899 | - |
Pistachio | 0.5180 | 1.0075 | - |
Hardwood | - | - | 1.5823 |
Wood pellet | - | - | 1.9088 |
Olive stone | - | - | 1.8944 |
Overall, our results showed that the energy released from the combustion of nutshells was approximately 46% lower than the energy released by these three other renewable sources. However, note that our energy value for almond shells was actually comparable to that for wood pellet and even better than those for hardwood or olive stones.
If time allowed, students could actually perform the calorimetry experiments for these other sources of biomass themselves, and compare their own values for hardwood, for example, to their values for nutshells.
To fully assess whether nutshells could be a suitable alternative, students should also consider the advantages and disadvantages of using nutshells for bioenergy. For example, the major advantage is that shells are a waste material, so the production cost is very low. Nutshells also contain very little moisture, so they don’t require any further drying – unlike (for example) wood pellets – which reduces processing costs. However, the amount of energy produced by certain nuts is much lower than other potential sources, as the results show. In addition, using nutshells as a bioenergy source is feasible only in countries where the production of nuts – and in particular, almond nuts – is high, such as the USA, Spain, Iran, Italy and Syriaw2.
Overall, our students appreciated the importance of using nutshells as a source of energy, and came to the conclusion that using shells – which would otherwise go to waste – for bioenergy production is a valuable and innovative practice. What conclusions will your own students draw?
The authors would like to warmly thank their students, Giannis Charalambidis, Alexis Nikas and Giannis Mantzaridis, for their assistance with the experiments described in this article, which was written in memory of Giannis Mantzaridis, who died in 2017.
Their thanks also go to Dr Georgios Memetzidis for fruitful discussions regarding the construction and calibration of the calorimeter. They would also like to express their gratitude to their current headmaster, Dr Konstantinos Keramidas, for his interest and continued support.
Comments
Other uses for the shells of nuts
1. I have experienced car parks on some wine farms in South Africa that use nut shells instead of gravel.
2. Shells, such as almond shells, are being considered as a raw material to make activated charcoal by first treating the shells with concentrated sulphuric acid and then heating to 600 oC... See: Adsorption of lead by chemically activated carbons from three lignocellulosic precursors, ,T.Brudeya,et al, Journal of Analytical and Applied Pyrolysis
Volume 120, July 2016, Pages 450-463.