Table 2 Essential feedstock and solid fuel characteristic

Physical

Bulk density^a

Density of feedstock. It influences the economics of storage collection and transportation

Ratio of measured mass (using any analytical balance) and calculated volume of feedstock

Lestari et al. (2017), Goulart and Maia (2013), Zhang et al. (2012)

Green density^b

The density of the solid fuel immediately after ejection from the mold

Ratio of mass to volume; measuring the mass and calculating the volume of the briquettes/pellet

Oladeji et al. (2016), Lestari et al. (2017), Goulart and Maia (2013)

Relaxed density^b

The density of the solid fuel after drying. It is the density of the fuel when it had achieved a stable weight

Ratio of mass to volume of the briquettes/pellet

Oladeji et al. (2016), Lestari et al. (2017), Goulart and Maia (2013)

Water-resistance/porosity index (PI)^b

The quantity of water the fuel will be able to absorb when exposed to a humid environment. Porosity affects the heat and mass transfer, airflow velocity, which in turn influences the heat conductivity, conversion efficiency, emissions and burning rate

It is calculated using the following expression:

\({\mathrm{PI}}= \frac{\mathrm{MW}}{\mathrm{MF}}\times 100\) (1)

where \({\mathrm{MW}}\) is the mass of water absorbed while \({\mathrm{MF}}\) is the mass of fuel sample

Tuates et al. (2016b), Oyelaran and Tudunwada (2015), Zhang et al. (2012)

Particle distribution^a

The particle size distribution influence the heat, diffusion, flowability, bonding and reaction rate

It is determined by performing sieve analysis

Tuates et al. (2016b), Zhang et al. (2012)

Mechanical

Compressive strength^b

Measure the resistance of the solid fuel to squeezing and pressing forces

It can be determined using universal testing machine (UTM) in accordance with established standards

Paper and Luttrell (2012)

Durability/shatter index^b

Measure the degree of fuel breakage and shattering tendency under sudden forces

It can be determined by performing a drop test. Fuel with known weight and dimensions would be dropped on the concrete floor from a height of 1 m

Calculate the shatter index (SI) after 4 drops

\(\% {\text {weight loss}}= \frac{{w}_{1}-{w}_{2}}{{w}_{1}}\) (2)

\({\mathrm{SI}}=100-\% {\text{weight loss}}\)

\({w}_{1}\) and \({w}_{2}\) are the weight of the fuel before and after shattering, respectively

Paper and Luttrell (2012)

Impact/attrition resistance^b

Measure the resistance of the solid fuel to impact and grinding forces

Tumbler could be employed to determine attrition resistance. A fuel of known weight is placed in a tumbler rotating at about 12 revolutions per minute for about 4 min. After the tumbling process, fuels are taken out and weighed. The expression used for the shatter resistance will be adopted to calculate the abrasive resistance

Paper and Luttrell (2012)

Combustion/thermal

Proximate analysis^a

This analysis will reveal the feedstock moisture (MC), volatile matter (VM), ash (AC), and fixed carbon (FC) contents

The MC, VM, AC, FC can be determined following the procedures of ASTM E1871-82 (2006), E872-82 (2006) E1755-01 (2007) and E1756-08 (2008), respectively

Ikelle et al. (2014), Ajimotokan et al. (2019b), Chou et al. (2009), Young and Khennas (2003), Ghaffar et al. (2015), Shuma and Madyira (2019)

Thermogravimetric analyses (TGA)^a

Provide information on the thermal breakdown profile of feedstock. It measures the fuel percentage weight loss as a function of temperature and presents a peculiar shape as the resulting thermogram for fuel materials

Determine using thermogravimetric analyzer

Raj, et al. (2015), Anukam et al. (2017)

Calorific value^a

Reveals the feedstock energy potential

It is determined using bomb calorimeter or calculated from the results of proximate and ultimate analyses

Ikelle et al. (2014), Ghaffar et al. (2015), Shuma and Madyira (2019), Djatkov et al. (2018)

Energy density/thermal efficiency^c

Describe the amount of energy stored per unit volume. Thermal efficiency is the percentage of fuel energy available for power generation

It is usually measured by performing a water boiling test

Odusote and Muraina (2017)

Ignition time^c

Ignition time is the average time taken to achieve steady glowing fire while burning the fuel

Fuel ignition time is determined by burning a known quantity of the fuel in a charcoal stove

Oyelaran and Tudunwada (2015), Odusote and Muraina (2017)

Combustion rate (CR)^c

It is the time taken to burn a known mass of fuel completely

\({\mathrm{CR}}= \frac{\text{Total mass of burnt sample}}{\text{burning time}}\)(3)

Oyelaran and Tudunwada 2015, Odusote and Muraina (2017)

Chemical

Ultimate analysis^a

Reveal the contents of hydrogen, nitrogen, sulfur, chlorine, oxygen, and carbon in the feedstock

Hydrogen, nitrogen, and carbon may be determined using an elemental analyzer, while sulfur may be determined using an atomic emission spectrometer

Thulu et al. (2016), Lestari et al. (2017), Gado et al. (2013)

Chemical bonds and constituents and crystalline nature of feedstock^a

Estimate the quality and quantity of the chemical constituents and crystalline nature of feedstock used for fuel production. Identification of the chemical bonds in the molecule and generate an infrared retention range of the compounds

These can be determine using Fourier transform spectroscopy (FTIR)

Onukak et al. (2017), Raj, et al. (2015), Anukam et al. (2017)

Analysis of surface morphology^b

It is used in portraying and distinguishing minerals and material formed together with surface components. SEM is used for viewing the surface morphology solid fuel to establish the suitability of fuel for a given application

Scanning electron microscope (SEM)

Promdee et al. (2017), Onukak et al. (2017)

Elemental composition^a

Used for quantitative and qualitative determination of elemental composition feedstock

X-ray fluorescence

Promdee et al. (2017)