In recent years microalgae are gaining importance mainly due to their potential for fuel production with zero carbon emissions. In the actual context, algal fuel is economically unfeasible compared to petroleum-derived fuel (which costs around US$0.55/L to U.S. con- sumers). To successfully make the transition from fossil fuels to biofuels, it is necessary to achieve the same or better quality (chemical and physical characteristics) for at least the same price. At this point, for most of the world, economics have greater influence than the eco- friendly characteristics (renewable sources and less polluting gas emissions) offered by biofuels.
The main reason for this economical limitation of biofuels manufactured from algae is the high costs of culture media and downstream processes (extraction, purification, and transformation) on an industrial scale. To make algal oil technologies economically feasible, these steps might be improved. In terms of culture media, it is in vogue to use wastewater as a partial or complete source of nutrients (carbon dioxide, nitrogen, phosphorous, potassium, magnesium, and some micronutrients) for algal growth as an alternative to reduce cultivation costs, whereas in terms of oil recuperation and transformation fast pyrolysis is a cheap
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Biofuels from Algae #2014 Elsevier B.V. All rights reserved.
alternative. This chapter describes a patented technology for biofuel production through fast pyrolysis from lipid-rich microalgae.
7.1.1 The Energetic Issue
Over the last hundred years, world energy consumption has increased greatly. In just the last 38 years, energy demand increased 99%; between 1973 and 2011, consumption went from 6.111 to 12.150 million of tons of petroleum (International Agency of Energy, 2011). According to the same study, 81% of that energy came from fossil-dependent sources such as petroleum, coal, and natural gas.
The scientific community continues to discuss whether global warming is caused by the excessive increase of carbon dioxide in the atmosphere, but this idea is generally accepted. This situation has caused a rush to development of economically feasible and sustainable technologies, those independent of fossil sources. Among these new technolo- gies, microalgal technologies have gained importance and are being widely explored due to their capacity to absorb carbon dioxide from atmosphere via photosynthesis and their high capacity to accumulate lipids, which can in turn be transformed into different forms of energy.
The independence of organic carbon sources for growth opens the possibility to develop technologies using wastewater that are unfeasible for heterotrophic microorganisms. At the same time, microalgae have many advantages compared to vascular plants (Benemann and Oswald, 1996): All physiological functions are carried out in a single cell, they don’t differ- entiate into specialized cells and they multiply much faster, they carry low costs for harvest and transportation (Miyamoto, 1997), they consume less water (Sheehan et al., 1998), and they have the possibility to be cultured under conditions (such as infertile land) not suitable for the production of conventional crops (Miyamoto, 1997).
7.1.2 Culture Medium
One of the big challenges of microalgae culture is the search for alternative (and cheap) culture media. Some microalgal species can accumulate up to 70% of lipids but only when cultured in a specific balanced medium, as mentioned byChisti (2007).
Medium costs are difficult to estimate because much depends on the species of microalga to be cultured. In the literature, medium cost is described as between US$0.27 and $0.588 per kg algal biomass (Molina-Grima et al., 2003; Tapie and Bernard, 1988). Such high cost, the major drawback in biofuel production processes involving microalgae, makes these processes unfeasible. (Just the biomass production step represents almost 40% of the price of the final product.) The necessity to exploit inexpensive and abundantly produced nutritional sources to substitute artificial media is clear.
In this context, the patented technology developed by the company Ourofino Agronego´cio, in partnership with the Laboratory of Biotechnological Processes (Federal University of Parana´, Brazil), is a very interesting and economical alternative for the production of biofuels from high-lipid-content microalgal biomass cultured in wastewater from ethanol distilleries.
(The present technology was patented: PI0705520-0.)
144 7. PRODUCTION OF BIOFUELS FROM ALGAL BIOMASS BY FAST PYROLYSIS
7.1.3 Vinasse
Vinasseis a liquid residue from the sugarcane-based ethanol industry. After sugarcane juice fermentation by yeast, ethanol concentration in the fermented broth is no more than 10% v/v (due to its toxicity). During distillation, the ethanol is recuperated and everything left is called vinasse. It is produced in high volumes (12–15 liters for each liter of ethanol) and is rich in minerals (Rego and Herna´ndez, 2006). Ethanol production in Brazil in 2012 is esti- mated at 27.9 billion liters (Empresa de Pesquisa Energe´tica, 2012), which means production of vinasse is around 365 billion liters.
The major problem related to vinasse is its high chemical and biological oxygen demand:
29,000 and 17,000 mgO2/L (Elia Neto and Nakahodo, 1995), respectively, 100 times more pollutant than average domestic wastewater. Vinasse pollutant strength is mainly due to high organic matter content and the presence of three important nutrients: nitrogen, phos- phorous, and potassium (Bittencourt et al., 1978). Due to its composition, vinasse is largely used as fertilizer in sugarcane cultivation. Theoretically, the amount of vinasse allowed per area is regulated by the Brazilian government, but inspection is difficult to be carried out, leading to indiscriminate use.
According toManha˜es et al (2003), soils irrigated with vinasse have high concentrations of nutrients at depths that can contaminate groundwater. Around 60% of the Brazilian ethanol is produced in Sa˜o Paulo state (UNICA, 2010), which is located on the Guarani Aquifer, the second largest underground freshwater reserve in the world.
Given the clear environmental risk caused by poor allocation of vinasse, it is of great importance to apply technical and scientific knowledge for its better distribution, allowing further relocation in water bodies. When used in microalgae cultivation, biological and chem- ical oxygen demand (BOD and COD, respectively) can reach more than 90% reduction in BOD and more than 80% reduction in COD (DalmasNeto, 2012) in the first cycle of cultivation.
Considering three cycles, reduction in BOD and COD can reach more than 95%.
7.1.4 Market Value
To successfully make the transition from fossil fuels to biofuels, it is necessary to achieve a similar or better quality product (chemical and physical characteristics) for at least the same price. This shift toward biofuels will take place if petroleum prices increase so much that the prices of petroleum-derived fuels become greater than those of biofuels.
Unfortunately, the eco-friendly characteristics of biofuels (renewable sources and less pol- luting gas emissions) are not sufficient to lead the transition if no economic benefit is generated.
If we examine gasoline prices since 1997, the strong price increase becomes clear (Figure 7.1). It is accepted that prices of petroleum-based fuels will keep increasing, a situa- tion that forces humankind to search for new sources of energy.
7.1.5 Pyrolysis
Pyrolysisis a physical-chemical process in which biomass is heated to between 400C and 800C, resulting in the production of a solid phase rich in carbon and a volatile phase 145
7.1 INTRODUCTION
composed of gases and condensable organic vapors (Mesa-Pe´rez et al., 2005). These organic vapors condensate in two different phases: bio-oil and acid extract (Beenackers and Bridgwater, 1989).
Through pyrolysis, carbon-carbon bonds are broken, forming carbon-oxygen bonds. It is a redox process in which part of the biomass is reduced to carbon (coal) while the other part is oxidized and hydrolyzed yielding phenols, carbohydrate, aldehydes, ketones, and carboxylic acids, which combine to form more complex molecules such as esters and polymers (Rocha et al., 2004).
Due to the extreme conditions to which biomass is submitted, many simultaneous reac- tions occur, resulting in gaseous, liquid, and solid products:
1. Gas phase.Consists primarily of low-weight products that have moderate vapor pressure at room temperature and do not vaporize at pyrolysis temperature.
2. Liquid phase.Further subdivided into two other phases determined by density differences:
• Bio-oil, which is a mixture of many compounds with high molecular weight that became vapors at pyrolysis temperature but condense at room temperature.
• Acid extract (or aqueous extract), which consists of an aqueous phase with numerous soluble and/or suspended substances.
3. Solid phase.Also known asbiochar, the solid phase is composed of an extremely porous matrix, very similar to charcoal (DalmasNeto, 2012).
Pyrolysis conditions can be manipulated to produce preferably one phase or the other. Residence time is one of the factors that most influence the final result. To produce incondensable gases, high residence time at high temperature is generally used;
higher yields of solids are generally achieved by very high residence time at low temper- atures (allowing polymerization reactions) (Sa´nchez, 2003). For preferential production of the liquid phase, fast pyrolysis is often chosen.Table 7.1 summarizes the conditions and main effects of residence time and temperature in gaseous, liquid, and solid product generation. Other pyrolysis technologies and their characteristics are presented in Table 7.2.
0.00 0.50 1.00 1.50 2.00 2.50 3.00
June 1997
June 1999
June 2001
June 2003
June 2005
June 2007
June 2009
June 2011
FIGURE 7.1 Price fluctuations of gasoline, New York, NY, USA, 1997–2011.(Adapted from U.S. Energy Information Administration.)
146 7. PRODUCTION OF BIOFUELS FROM ALGAL BIOMASS BY FAST PYROLYSIS
Due to its tendency to preferentially form bio-oil, coupled with high-speed reaction and greater productivity, fast pyrolysis is the best model for the production of biofuels from algae.