Biofuel potentials (BFP) of cassava feedstocks.
Many cassava producing nations lack suitable energy availability and sufficiency. Just 10% of the population in Haiti receive power from the national electric grid. The proportion is 7% for Mozambique. In both countries, deforestation is extensive due to dependence on wood and charcoal for 70 and 85% of energy requirement respectively. In the case of Ghana, although biomass accounts for 64% of national energy supply, the dependence on primary biomass energy sources like wood and charcoal has led to increased loss of biodiversity, soil erosion and health problems. Prospects for the use of cassava peeling residues as a source of biomethane to mitigate these constraints have received little attention. In this chapter, the advantages and benefits of biomass energy, along with the potential for cassava as a feedstock and rationale for anaerobic digestion are highlighted. Depending on the quantity of cassava root processed by individual countries, the energy recovered from cassava peeling residues could satisfy up to 100% of national energy requirements.
- cassava residues
- anaerobic digestion
- renewable energy
In July 2015, world population was estimated at over 7.3 × 109 persons and will exceed 9.7 × 109 persons in July 2050 . At the same time, planet earth’s capacity to sustain life is diminishing. Issues such as land use conflicts, rural poverty, food insecurity, energy insecurity and environmental pollution are posing serious threats to humanity. Global energy supply is dependent on fossil fuels, which account for over 78% of final energy consumption . Fossil fuels are depleting non-renewables, and their use exacerbates anthropogenic forcing of environmental perturbations including carbon dioxide and other greenhouse gas emissions, acid rain, biodiversity and ozone layer depletions. Advancing global energy supply system toward renewable bioenergy could constrain these adverse impacts.
The poor economic development and progress in developing countries have been attributed in part to inadequate suitable energy supply. Mainly developing countries produce cassava. However, most of the production occurs in rural areas where fuel/electricity availability is limited. In Mozambique for instance, only 7% of households (1% in rural areas) has access to electricity , and 85% of total energy consumed comes from firewood and charcoal . In Haiti, 10% of the population receive power from the national electric grid while wood and charcoal account for 70% of the nation’s energy use [5, 6]. These circumstances have led to extensive deforestation and soil erosion in both countries; with just 1.5% of land forested in Haiti , and 219,000 hectares of land deforested per year in Mozambique . In the case of Ghana, although biomass accounts for 64% of national energy supply, the dependence on primary biomass energy sources like wood and charcoal has led to increased deforestation, land degradation, loss of biodiversity, soil erosion, and health problems . Creative use of cassava as an energy crop would help to mitigate environmental degradation and energy paucity issues, as well as minimize the health problems associated with the combustion of firewood and charcoal in cassava producing nations.
Biomass energy should be of interest to developing and developed countries. This is because biomass alleviates reliance on limited fossil fuel sources, creates employment, and contributes to economic development and revitalization of rural communities. Biomass is a clean energy source that dramatically improves the environment by generating far less air emissions than fossil fuels, reduces the amount of waste sent to landfills, and decreases reliance on chemical fertilizer. Moreover, biomass energy is renewable and therefore sustainable. Renewable energy supplied 19% of global energy consumption in 2012 and in 2013, accounted for more than 56% of net additions to global power capacity with about 6.5 million people employed . In 2015, renewable energy sales in Europe was 150 billion euros (≈ US$ 178 billion) .
These trends demonstrate the growing utility, benefits and advantages of renewable energy of which biomass energy is a major component. However, the use of edible biomass (food crops such as sugarcane, corn (maize), soybean, palm oil, etc.) for biofuel (bioethanol, biodiesel, etc.) production has raised ethical concerns about competition and diversion of land and food to fuel production. Perhaps a reasonable alternative is biofuel production from biomass originating from nonfood sources such as, agricultural residues, food processing residues, lignocelluloses, and microalgae. Nonfood cassava peeling residues (CPR) could come to the rescue.
2. Potential of cassava as feedstock for bioenergy production
Cassava (Manihot esculenta Crantz) is a mostly vegetatively propagated perennial root crop that grows well in tropical climates. Nevertheless, the roots (main reason for growing cassava) are very perishable once taken from the soil and go to waste unless processed in some way soon after harvest. Most processing requires removal of peels (cortex and periderm), head, and tail ends. These components usually discarded as waste, engender environmental pollution. In this chapter, the components are referred to collectively as cassava peeling residues (CPR), and instead of being discarded as waste, would be put to bioenergy production function. The CPR is generated during production of numerous cassava root based food products like akpakpuru, attieke, casabe, chickwangue, farina (farinha de mandioca), fufu, fuku, gaplek, gari, ijapu, konkonte, lafun, landang, peujeum, and thundam [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. Because more than 65% of global annual cassava output is processed for human consumption, enormous quantity of CPR is generated. This nonfood organic matter is potential good feedstock for anaerobic digestion (AD) processes that generate bioenergy.
There are numerous other reasons for the attraction of cassava crop as source of food and bioenergy.
Cassava provides economic and subsistence value for 800–1000 million people in more than 90 countries including Angola, Barbados, Brazil, Cambodia, China, Cook Islands, Democratic Republic of Congo, Dominica, Ghana, Haiti, India, Indonesia, Lao Peoples Democratic Republic, Mozambique, Nigeria, Suriname, Thailand, Uganda, United Republic of Tanzania, and Vietnam [23, 24, 25].
It is the fourth most important food crop in developing nations. Cassava is also world’s third largest source of food carbohydrates and the top food energy supplier for tropical and subtropical regions. About 30% of all calories consumed in Mozambique come from cassava . In Zaire, cassava roots provide 60% of the daily caloric intake, while 20% of protein come from cassava leaves . In addition, Cassava can be biofortified with vitamin A, iron and zinc to eliminate hidden hunger and improve the nutritional status of vulnerable groups.
Cassava presents numerous agro-climatic advantages and benefits as well. First, it has high biological efficiency as the edible root portion lies underground and does not require support from stems and branches. It is easily cultivated by stem cuttings for multiplication and planting purposes, and requires minimum agricultural inputs (fertilizers, pesticides, etc.). With the possible exception of sugarcane, cassava’s productivity in terms of calories per unit land area per unit of time is significantly higher than that of other staple food crops; and its production requires energy input that constitutes just 5–6% of the energy output of the entire cassava biomass .
Cassava can be planted most time of the year and is available all year long with more than 2 years harvest window. Cassava is adaptable to various farming systems. It can be intercropped with beans, yams, and other annual crops. It is tolerant of various climatic conditions (e.g., high drought; temperature: 8–33°C; rainfall: 500–6000 mm per annum; relative humidity: 15–90%; and elevation: sea level–2500 m). Cassava is also productive on soils with pH of 3–9.5. It can thus be cultivated on marginal lands where other crops such as corn, wheat, rice and sugarcane cannot be grown well [29, 30]. Cassava has high efficiency of photosynthetic CO2 assimilation. The photosynthetic rate of cassava is 40–50 μmol CO2 m−2 s−1 under high solar radiation. That of rice is around 20 μmol CO2 m−2 s−1 .
Cassava root is endowed with high starch content of excellent functional and structural qualities. The cassava starch can be transformed into products with huge industrial applications and is of major economic importance in Brazil, India, Indonesia, Philippines, China, Thailand, South East Asia, and in the tropical regions of the world.
Cassava is a major ingredient for livestock feeds.
Cassava is important in the provision of bioenergy such as bioethanol and biogas. For instance, the yield of bioethanol from cassava (6000 kg/ha) is higher than that of sugarcane (4900 kg/ha), carrot (4500 kg/ha), sweet sorghum (2800 kg/ha), Rice (2250 kg/ha), Maize (2050 kg/ha), and wheat (1560 kg/ha) . A feasibility case study in Kenya using biogas engine for backup power generation showed ample savings over the use of diesel engine. Biogas engine saved 17 tons of carbon dioxide emissions, 18% reduction in net present cost, 20% reduction in levelized cost of electricity, and 30% reduction in capital cost .
Energy recycling from biomass residues and wastes is increasingly attractive because the sustainability of analyzed feedstock favors biomass waste flows over dedicatedly cultivated energy crops . Therefore, utilization of nonfood cassava processing residues such as CPR in biomethane production via the anaerobic digestion technology is prudent and beneficial. Nevertheless, in order to properly assess and quantify the value and contribution of CPR to the energy mix of cassava producing nations, establishment of Biofuel Potential (BFP) of CPR is necessary. Relatively very few studies have been published on biomethane production from cassava feedstocks. Most of the studies utilized cassava starch extraction wastewater. Other cassava feedstocks used were stillage (wastewater) from cassava ethanol production; cassava stem residue; whole cassava root; effluent from cassava flour and meal industry; and cassava peeling residue (CPR). However, CPR constitutes about 19% fresh weight of the root and is perhaps the most abundant residue from cassava root processing. It is easy to generate and does not require water usage. Therefore, analyses of energy impact of cassava crop in this chapter will use CPR as the feedstock of choice in renewable biomethane production. Table 1 summarizes the biofuel potentials of CPR and other cassava feedstocks.
|S/N||Feed stock||BFP yield||BFP units||References|
|1.||Cassava peeling residue||377||L CH4/(kg VS)|||
|2.||Cassava peeling residue||180–310||L CH4/(kg VS)|||
|3.||Cassava peeling residue||280||L CH4/(kg VS)|||
|4.||Cassava peeling residue||87.1||L biogas/(Total mass of slurry)|||
|5.||Cassava peeling residue||68.7||L biogas/(Total mass of slurry)|||
|6.||Cassava starch extraction wastewater||360||L biogas/(kg COD removed)|||
|7.||Cassava starch extraction wastewater||130–325||L biogas/(kg dry matter)|||
|8.||Cassava starch extraction wastewater||134–316||L CH4/(kg VSS Day)|||
|9.||Cassava starch extraction wastewater||140||Nm3 per Mg dry mass of COD|||
|10.||Cassava starch extraction wastewater||11.3||L CH4/(kg VSS Day)|||
|11.||Cassava starch extraction wastewater||0.52–3.70||L biogas/(L wastewater Day)|||
|12.||Cassava starch extraction wastewater||0.40–0.55||L CH4/(L effluent Day)|||
|13.||Cassava stillage||215–380||L CH4/(kg VS)|||
|14.||Cassava stillage||132–259||L CH4/(kg VS)|||
|15.||Cassava stillage||249||L CH4/(kg VS)|||
|16.||Cassava stillage||158–248||L CH4/(kg VS)|||
|17.||Cassava stillage||220–230||L CH4/(kg COD added)|||
|18.||Cassava tubers||660||L biogas/(kg VS)|||
|19.||Cassava tubers||475–510||L biogas/(kg VS)|||
|20.||Cassava stem residues after starch extraction||153||Nm3 per Mg dry mass of stem residues|||
|21.||Cassava flour and meal industry effluent||14.5||L biogas/Day|||
3. Rationale for anaerobic digestion technology
Anaerobic digestion (AD) is a biochemical process that converts organic matter to biogas (a mixture of methane and carbon dioxide). This is achieved through the action of a mixed culture of naturally occurring microorganisms under near oxygen free ambient environmental conditions. The following attributes are among the numerous advantages and benefits of AD technology:
Flexible technology; energy efficient; prevents emission of volatile hazardous compounds (air pollution control); biotransformation and biodegradation of xenobiotics; treatment of seasonal effluents (e.g., wastewaters from sugar and fish processing industries); system stability and minimal operational difficulties such as bulking and biomass washout; higher loading rates and concentrations operations, from 20 to 40 kg BOD removed/m2 per day; reduced mass and volume of waste sludge; high waste stabilization; and Low construction, treatment and maintenance costs are typical examples.
AD can accommodate tighter restrictions on sludge disposal site location, air pollution, hazardous waste disposal, odor control, and other environmental regulations.
AD is attractive as a means of generating alternative energy such as biogas used for electricity and heat production, and to feed gas networks.
Among biofuel systems, AD is a highly energy positive process. AD generates energy as methane; with about 3.53 kWh/(kg COD) produced as biogas while aerobic treatment operations consume 0.5–2.0 kWh/(kg O2) .
AD is used to produce hygienic digestate; a good source of soil organic amendment, compost and biofertilizer that can be sold for income generation.
In addition, AD is versatile, with commercial equipment in varied types, shapes, sizes and operating modes. These include BIOCEL, Bioferm, GICON and SEBAC (sequential batch anaerobic composting); as well as the ABR (anaerobic baffled reactor), AF (anaerobic filter), CSTR (completely stirred tank reactor), EGSB (expanded granular sludge bed), UASB (upflow anaerobic sludge bed), fixed dome, floating cover, and balloon/tube digesters. Figure 1 highlights the operating principles of a simple floating cover anaerobic digester.
4. Technical feasibility of anaerobic digestion of CPR
Information obtained from available literature were analyzed to determine critical values relevant to CPR biogasification characteristics. Tables 2 and 3 summarize estimates and assumptions concluded from the analyses. These data and assumptions were also used to perform the mass balance computations presented in Figure 2.
|S/N||Variable of interest||Unit||Value assumed||Explanation/justification||Source/references|
|1.||CPR mass fraction of root||(%)||19||Derived from literature data||Table 2|
|2.||CPR moisture content, wet basis||(%)||67||Mean of four replications|||
|3.||CPR methane capacity||(L CH4/(kg VS))||303||Derived from Table 1||[10, 47, 48]|
|4.||Methane obtained from CPR generated by processing 1 tonne (1000 kg) of roots||(kg)||12.55||Derived from 252 L CH4/(kg VS CPR) = 10.44 kg CH4|||
|5.||Proportion of cassava root output processed||(%)||66 & 100||More than two–thirds of total production processed for human food|||
|6.||Quantity of cassava root output processed||(kg)||Varies per country||Based on 66 and 100% of individual country’s 2014 cassava output (see Table 4)||[24, 67]|
To compute the energy obtained from methane generated by AD of CPR, the following equations were used.
where Tme = Thermal energy content of methane (MJ), M = Mass of methane (kg), HHV = Heat of combustion of methane (MJ/kg), Eme = Electrical energy equivalent of Tme (MWh), ϵ = Conversion efficiency; thermal energy to electrical energy (%), Note: This work used HHV = 55.53 MJ/kg, ϵ = 25% and 3600 MJ = 1 MWh.
Table 4 presents the 2014 cassava output and energy consumption patterns of cassava producing countries. Many of these countries are net energy importers; lacking in local energy capacity and sufficiency. However, in 2014, the global cassava output was over 268 million tonnes. Based on the equations, mass balances and analyses already posited in this chapter, 1 tonne of cassava root yielded 190 kg CPR. This CPR is transformed to 12.55 kg of methane; producing 697 MJ thermal energy or about 174 MJ electrical energy (≈ 0.0484 MWh). Therefore, the 268 million tonnes global cassava root output in 2014 would produce 51 million tonnes of CPR. This quantity of CPR would generate 3363.4 million kg of bio-methane which translates to 186.8 × 109 MJ of thermal energy; equivalent to 46.7 × 109 MJ of electrical energy (≈ 13 × 106 MWh). This is an enormous quantity of energy that could satisfy all the yearly energy needed by Slovenia or Turkmenistan. This energy should be recovered for the benefit and rescue of cassava producing nations. The foregoing analyses were applied to individual cassava producing nations to estimate the energy recoverable from their CPR. The results obtained are also presented in Table 4. It could be seen that the ability of recoverable energy from CPR to provide national energy requirement depends on the quantity or proportion of national cassava root output processed.
|S/N||Country||2014 Cassava output (×109 kg) b||2014 National energy consumption (MW.h/Y) c||Energy from CPR if 100% of national cassava output was processed (MW.h/Y)||Potential of CPR to provide national energy requirement if 100% national cassava output was processed (%)||Energy from CPR if 66% of national cassava output was processed (MW.h/Y)||Potential of CPR to provide national energy requirement if 66% national cassava output was processed (%)|
|6.||Dem. Rep. of Congo||14.683266||9,300,000||710523.2417||7.640034857||468945.3395||5.042423006|
|13.||United Rep. of Tanzania||4.992759||5,000,000||241599.608||4.83199216||159455.7413||3.189114826|
|26.||Lao People’s Dem. Rep.||1.629805||3,900,000||78866.26395||2.022211896||52051.73421||1.334659851|
|33.||Centr. Afric. Rep.||0.699764||200,000||33861.57996||16.93078998||22348.64277||11.17432139|
|38.||Venezuela (Boli. Rep.)||0.357876||78,000,000||17317.61964||0.022202076||11429.62896||0.01465337|
|42.||Bolivia (Plu. State of)||0.245808||7,500,000||11894.64912||0.158595322||7850.468419||0.104672912|
|49.||Papua New Guinea||0.148213||3,000,000||7172.02707||0.239067569||4733.537866||0.157784596|
|68.||Taiwan, China Rep||0.013017||249,500,000||629.89263||0.000252462||415.7291358||0.000166625|
|70.||Micronesia (Fed. States)||0.008891||178,000||430.23549||0.241705331||283.9554234||0.159525519|
|77.||Trinidad and Tobago||0.003194||9,100,000||154.55766||0.001698436||102.0080556||0.001120968|
|80.||Wallis and Futuna Islands||0.001874||—||90.68286||—||59.8506876||—|
|82.||Sao Tome and Principe||0.001349||65,100||65.27811||0.100273594||43.0835526||0.066180572|
5. Applications, utilizations and dividends of biomethane from CPR
The anaerobic digestion of CPR would generate biogas, which could be used as is or upgraded to obtain more efficient biomethane. The energy content of either fuel could be put to various applications and utilities. These include:
Fuel for stoves in cooking; boiling, frying, roasting, etc.
Fuel for lamps in lighting; illumination, reading, playing, etc.
Fuel for transportation; cars, trucks, sea vessels, etc.
Electrical power in processing operations; drying, grinding, heating, pumping, refrigeration, washing, etc.
The digester effluent (digestate) could be utilized for soil amendment and/or serve as biofertilizer for enhanced crop production. This was demonstrated to increase potato yield .
Perhaps the critical humane benefits are the reduction of drudgery and burden on the one hand and the improvement of health conditions on the other hand. This is due to reduced time spent on fetching firewood and charcoal for domestic fuel, and the reduced exposure to their combustion products.
Domestic combustion of the firewood releases health-impairing pollutants like carbon monoxide, hydrocarbons, smoke and other particulate matter. These combustion products may cause nausea, sneezing, eye and respiratory irritations ; pneumonia, lung cancer, and respiratory infections ; and reduced birth weight .
Biomethane utilization reduced firewood consumption by 74% in China  and 84% in Sri Lanka , thereby minimizing the drudgery, burden, and health hazards associated with use of firewood for domestic energy.
Figure 3 presents pathways of the production and utilization of biomethane from CPR and other renewable feedstocks.
Global energy security, sustainability and renewability could be enhanced by harnessing non-food biomass. The work presented in this chapter demonstrated that anaerobic digestion of cassava peeling residue (CPR) generated biomethane that could come to the energy rescue of cassava producing nations. Depending on the specific country and proportion of national cassava output processed, recovered biomethane from CPR could provide up to 7% national energy requirement in Haiti; 8% in Comoros; 10% in Cambodia; 11% in Nigeria; 31% in Rwanda; and 100% in Sierra Leone. The biomethane could be put to various applications and utilities that minimize the drudgery and burden of gathering wood and charcoal for domestic fuel. As additional dividends, use of the biomethane would prevent implications of the combustion products of these solid fuels that degrade air quality and impair human health. The time saved from fetching firewood may be put to economic, educational and social activities. Furthermore, the digester effluent (digestate) could be sold for soil amendment and as organic fertilizer, or applied to agricultural land for increased crop yield. Either way more revenue is generated for economic empowerment. Therefore, anaerobic digestion of CPR would help cassava producing nations to not only mitigate their energy insufficiency, but also address issues such as climate change, environmental degradation, poverty alleviation, rural development, and the sanitation and health hazards associated with the use of wood and charcoal as fuel.