Nut Agricultural by-Products

Billions of metric tons of biomass are generated every year from agricultural processes, including liquid, solid and gaseous residues; that may be considered one of the most abundant, cheap and renewable resources on Earth [16]. Many of these residues are just incinerated or dumped without any kind of control causing several environmental problems, such as air pollution or soil erosion.

In this context, a more efficient utilization of agricultural wastes to yield a number of added-value resources is highly attractive to ensure sustainable and cleaner production processes that are economically viable, environmentally sound and socially beneficial.

As a consequence, the incorporation of agricultural wastes into different polymer matrices is a current trending topic in materials research with a raising number of results. The obtained biocomposites show relatively high strength, stiffness and low density by the incorporation of natural fibres [17]. In this context, Mande clearly defined two main categories of agricultural residues [18]:

Crop residues or primary biomass residues, generated in the farm, which are normally non-edible plant parts that are left in the field or orchard after the main crop has been harvested. These residues mainly include straw, stover, stubble, stalks, sticks, leaves, haulms, roots, branches, twigs, brushes, trimmings and pruning; and they are produced from different sources including seeds, fruits, nuts, vegetables and energy crops.

Agro-industrial residues or secondary biomass residues, generated during the postharvest processing, are those by-products or sub-products obtained from the post-harvesting processes or the transformation of the crop into valuable products. These products include residues from wood and food processing industries in the form of husks, hulls, peels, dust, straws, bagasse, sawdust, corncobs, pomace, etc. Regarding nut wastes, a huge quantity of residues with high environmental impact and greenhouse effects are generated and disposed every year (Table 3).

Table 3. Yield and estimated potential production of primary and secondary residues

from some nut crops [16]

Crop

Yield (MMT/year)

Main residue

Total residue production (MMT/year)a

Almonds

2.51

Brown hulls, shells,

0.88

seeds coating

0.10

Chestnuts

1.96

Shells (outer, inner)

0.39

Hazelnuts

0.89

Shells, kernel

0.44

Walnuts

2.54

Shells

1.70

a Residue production = (Yield x % of residue after crop processing) per 100.

MMT: Millions metric tons.

Flow diagram of final destination of agricultural residues [16]

Figure 2. Flow diagram of final destination of agricultural residues [16].

The main current applications of these residues are in the production of bio-energy due to the characteristics (non-edible, high energy potential, etc.) of lignocellulosic materials, generation of biomass-based energy fuels, industrial materials such as animal feed and compost and a great variety of industrial products, such as wood-based panels, paper, cardboard and bio-fertilizers; among others [16]. Figure 2 shows a scheme of the main postprocessing destination of agricultural residues.

The use of these residues for energy purposes contributes to avoiding disposal problems. As an example, a kinetic model based on the pyrolysis of Brazil nut shells between 350 and 850 °C was reported showing an efficient conversion of the shells into useful products19. Cardozo et al. compared the combustion behaviour of selected agricultural residues: sugar cane bagasse, sunflower husks and Brazil nut hells with commercially available wood pellets to evaluate the impact of the fuel properties on emission levels, oxygen levels, temperatures in the combustion chamber, and efficiencies based on the conversion of fuel compounds into gaseous products [20]. As a result, the type of agricultural residue had an impact on the power input, oxygen levels and combustion chamber temperature when compared with wood pellets. The power input of Brazil nut shells and sunflower husk pellets due to their high ash content were reduced and therefore high oxygen levels were measured. Also, higher NO, CO and SO2 emissions for Brazil nut shells when comparing with wood pellets were reported mainly due to their higher contents of nitrogen, sulphur and ash; and therefore, improvements in the combustion process are required. As a result, new applications of these residues as a valuable stock material to be used in a wide range of fields have been carried out in the industry including biomaterials, compost, fertilizers, food and feed or added value compounds.

Gasification technology has been reported as a suitable technique for biomass residues conversion and remains an economical alternative for the valorisation of cashew nut shells in small scale industries [21]. In order to evaluate the cashew net shells valorisation by gasification several experiments were satisfactorily conducted on a char obtained from the pyrolysis of this biomass and using a fixed bed reactor. As gasifying agents, carbon dioxide, steam and the mixture of carbon dioxide and steam were used at different temperatures for the gasification of fine particles.

In direct combustion applications these biomasses show some drawbacks: low combustion efficiency in spite of rather high calorific value; release of unhealthy smoke and soot deposits in closer environment. Pyrolysis might be an alternative valorisation pathway of such extractive rich feedstock, to convert it into either char (slow pyrolysis) or pyrolysis oils

(fast pyrolysis). In this context, Melzer et al. studied the usability of cashew nut shells in energetic terms [22]. In contrast to lignocellulosic biomass, these residues are rich in extractives. The feedstocks were characterised in a first step upon their physical and chemical properties before they were pyrolysed in a thermogravimetric system and a tubular reactor under rapid pyrolysis conditions. A detailed study of obtained pyrolysis oils showed that the extractives of cashew nut shells are not entirely cracked while vegetable oils decompose almost entirely. Furthermore, pyrolysis oils obtained from this study would contain valuable products which could be extracted from cashew nut shells, in particular chemicals appreciated in green chemistry such as phenols and amines.

 
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