What Is Biocrude?

In search of renewable fuels, as early as the mid-twentieth century, researchers started to convert biomass into petroleum-like liquids. For example, Berl (1944) treated biomass using alkaline water at 230°C to produce a viscous liquid that contained 60% carbon and 75% heating value of the starting material. The liquid, termed biocrude, contains 10–20 wt% oxygen and 30–36 MJ/kg heating value as opposed to <1 wt% and 42–46 MJ/kg for petroleum (Aitani, 2004). The high oxygen content imparts lower energy content, poor thermal stability, lower volatility, higher corrosivity, and tendency to polymerize over time (Peterson et al., 2008). Hence, biocrude needs to be deoxygenated to make it compatible with conventional petroleum. Because biocrude contains less oxygen and more heating value than biomass, the hydrothermal liquefaction process is also referred to as hydrothermal upgrading. Compared with bio-oil from fast pyrolysis, biocrude produced from hydrothermal liquefaction has higher energy value and lower moisture content but requires longer residence time and higher capital costs. Typical hydrothermal liquefaction conditions range from 280 to 380°C, 70 to 20 Mpa, with liquid water present and reaction occurring for 10–60 minutes.

Hydrothermal Medium

Generally, hydrothermal medium refers to water that has been heated and compressed simultaneously. A phase diagram of pure fluid is shown in Figure 10.1; in the case of water, the critical point is 374°C and 221 bar (Note: a mixture may have a different critical point depending on the components and concentrations).

Challenges with Corn-Based Ethanol

Today, the world's ethanol supply is mainly derived from U.S. corn or Brazilian sugarcane. Corn and other grain (as farmers planted corn instead of other grains) prices have soared internationally, and the corn-to-ethanol industry has been blamed for driving up food prices worldwide (Figure 6.1). The Food and Agriculture Organization of the United Nations and the World Bank have said that soaring world food prices are due in part to increased demand for biofuels. However, additional factors that contribute to increased food prices include higher energy prices and increased consumption of meat and dairy products in developing economies of China and India. Note that it requires about 13 kg of corn to raise 1 kg of meat; hence, a shift toward nonvegetarian diet will put even more pressure on the food supply. The U.S. energy bill of 2007 allows for the doubling of corn-based ethanol with a ceiling of 15 billion gallons per year. The bill also calls for an additional 21 billion gallons per year of advanced biofuels, including 16 billion gallons per year of ethanol from nonfood sources, by 2022. The energy policy means that corn is likely to rule the U.S. ethanol industry for many years. However, soaring food prices and questions about whether corn-for-fuel can reduce global warming have sparked a debate about whether the United States is going down the wrong road in the search for alternatives to fossil fuels.

Biofuel Economy

High petroleum prices, increasing trade deficits due to fuel import, depleting petroleum supplies, and increased concern over the greenhouse gas emissions from fossil fuels have driven interest in transportation biofuels (Hill et al., 2006). For many developing countries, petroleum import accounts for the major share of their outflow of foreign exchange, putting a high strain on the growth. For such countries, a locally produced fuel can help with the economy. The first-generation biofuels (ethanol and biodiesel) are already in the market, and the second-generation biofuels from biomass are emerging. The U.S. Energy Independence and Security Act of 2007 provides major support for the biofuel industry by setting a renewable fuel requirement for use of 36 billion gallons per year of biofuel by 2022, with corn ethanol limited to 15 billion gallons. Any other ethanol or biodiesel may be used to fulfill the balance of the mandate, but the balance must include 16 billion gallons per year of cellulosic ethanol by 2022 and 5 billion gallons per year of biodiesel by 2012.

With increasing global interest in biofuels, there is a considerable discussion on the biomass supply and conversion costs. Production and use of life cycle analysis are being carried out to determine whether biofuels provide any benefit over fossil fuels, accounting for various aspects such as farm yields, commodity and fuel prices, farm energy and agrichemical inputs, production plant efficiencies, coproducts, greenhouse gas emissions, and so on.

Introduction

Although fossil fuels (petroleum, coal, and natural gas) are still being created at a geological rate, they are being consumed at a much faster rate. Hence, fossil fuels are not renewed fast enough to be replenished. That is why fossil fuels are considered nonrenewable. Presently on Earth, renewable sources of energy are due to the sun's energy and its direct and indirect effects on the earth (solar radiation, wind, falling water, and various plants, i.e. biomass), gravitational forces (tides), and the heat of the earth's core (geothermal). Renewable energy sources (RES) will replenish themselves within our lifetime and may be used with suitable technology to produce predictable quantities of energy when required. Additionally, renewable resources are more evenly distributed than fossil and nuclear resources on the earth. The most important benefit of renewable energy systems is the decrease of environmental pollution.

The main RES and their usage forms are shown in Table 3.1. Some of these energy sources have been used by humans for over five thousand years. During the preindustrial era, RES were primarily used for heating and simple mechanical applications that did not reach high energy efficiency. In the industrial era, the energy use shifted from low energy value RES to much higher energy value coal and petroleum. In fact it is said that RES such as water and wind power probably would not have provided the same fast increase in industrial productivity as fossil fuels (Edinger and Kaul, 2000).

What Is Biodiesel?

Biodiesel refers to a renewable fuel for diesel engines that is derived from animal fats or vegetable oils (e.g., rapeseed oil, canola oil, soybean oil, sunflower oil, palm oil, used cooking oil, beef tallow, sheep tallow, and poultry oil). Biodiesel is a clear amber–yellow liquid with a viscosity similar to petroleum diesel (petrodiesel, diesel). With the flash point of 150°C, biodiesel is nonflammable and nonexplosive, in contrast to petrodiesel, which has a flash point of 64°C. This property makes biodiesel-fueled vehicles much safer in accidents than those powered by diesel or gasoline. Unlike petrodiesel, biodiesel is biodegradable and nontoxic and significantly reduces toxic and other emissions when burned as a fuel. Technically, biodiesel is a diesel engine fuel comprised of monoalkyl esters of long-chain fatty acids derived from animal fats or vegetable oils, designated B100, and meeting the requirements of the ASTM D-6751 standard. Some of its technical properties are listed in Table 7.1. Chemically, biodiesel is referred to as a monoalkyl ester, especially (m)ethylester, of long-chain fatty acids derived from natural lipids via the transesterification process. Biodiesel is typically produced by reacting a vegetable oil or animal fat with methanol or ethanol in the presence of a catalyst to yield methyl or ethyl esters (biodiesel) and glycerin (Demirbas, 2002a). Generally, methanol is preferred for transesterification, because it is less expensive than ethanol.

Biodiesel produces slightly lower power and torque, which results in a higher consumption than No. 2 diesel fuel for driving.

The spray equations have been studied and solved for many applications: single-component and multicomponent liquids, high-temperature and low-temperature gas environments, monodisperse and polydisperse droplet-size distributions, steady and unsteady flows, one-dimensional and multidimensional flows, laminar and turbulent regimes, subcritical and supercritical thermodynamic regimes, and recirculating (strongly elliptical) and nonrecirculating (hyperbolic, parabolic, or weakly elliptic) flows. The analyses discussed here will not be totally inclusive of all of the interesting analyses that have been performed; rather, only a selection is presented.

Spray flows can be classified in various ways. One important issue concerns whether the gas is turbulent or laminar. In this chapter, only laminar flows are considered; the turbulent situation is discussed in Chapter 10. Another issue concerns whether thermodynamic conditions are subcritical, on the one hand, or near critical to supercritical, on the other hand.

In the most general spray case, the gas and the droplets are not in thermal and kinematic equilibria, that is, the droplet temperature and the droplet velocity differ from those properties of the surrounding gas. Of course, heat transfer and drag forces result in the tendency to move toward equilibrium. The equilibrium case is sometimes described as a locally homogeneous flow. It is possible to have thermal equilibrium or kinematic equilibrium without the other. When thermal equilibrium exists, the analysis described in Chapter 7 is simplified because the droplet temperature Tl can be set equal to the gas temperature T and Eq. (7.82) or its alternative forms, Eq. (7.83) or Eq. (7.87), can be avoided.

The fluid dynamics and transport of sprays comprise an exciting field of broad importance. There are many interesting applications of spray theory related to energy and power, propulsion, heat exchange, and materials processing. Spray phenomena also have natural occurrences. Spray and droplet behaviors have a strong impact on vital economic and military issues. Examples include the diesel engine and gas-turbine engine for automotive, power-generation, and aerospace applications. Manufacturing technologies including droplet-based net form processing, coating, and painting are important applications. Applications involving medication, pesticides and insecticides, and other consumer uses add to the impressive list of important industries that use spray and droplet technologies. These industries involve annual production certainly measured in tens of billions of dollars and possibly higher. Many applications are still under development. The potentials for improved performance, improved market shares, reduced costs, and new products and applications are immense. Continuing effort is needed to optimize the designs of spray and droplet applications and to develop strategies and technologies for active control of sprays in order to achieve the huge potential.

In the first edition of this book and in this second edition, I have attempted to provide some scientific foundation for movement toward the goals of optimal design and effective application of active controls. The book, however, does not focus on design and controls. Rather, I discuss the fluid mechanics and transport phenomena that govern the behavior of sprays and droplets in the many important applications.

There are various complications that occur when a multicomponent liquid is considered (Landis and Mills, 1974, and Sirignano and Law, 1978). Different components vaporize at different rates, creating concentration gradients in the liquid phase and causing liquid-phase mass diffusion. The theory requires the coupled solutions of liquid-phase species-continuity equations, multicomponent phase-equilibrium relations (typically Raoult's Law), and gas-phase multicomponent energy and species-continuity equations. Liquid-phase mass diffusion is commonly much slower than liquid-phase heat diffusion so that thin diffusion layers can occur near the surface, especially at high ambient temperatures at which the surface-regression rate is large. The more volatile substances tend to vaporize faster at first until their surface-concentration values are diminished and further vaporization of those quantities becomes liquid-phase mass diffusion controlled.

Mass diffusion in the liquid phase is very slow compared with heat diffusion in the liquid and extremely slow compared with momentum, heat, or mass diffusion in the gas film or compared with momentum diffusion in the liquid. In fact, the characteristic time for the liquid-phase mass diffusion based on droplet radius is typically longer than the droplet lifetime. Nevertheless, this mass diffusion is of primary importance in the vaporization process for a multicomponent fuel. At first, early in the droplet lifetime, the more volatile substances in the fuel at the droplet surface will vaporize, leaving only the less volatile material that vaporizes more slowly. More volatile material still exists in the droplet interior and will tend to diffuse toward the surface because of concentration gradients created by the prior vaporization.

Consider the gas phase that surrounds or adjoins liquid droplets, films, pools, and/or streams. Figure B.1 presents a diagram that portrays a wide variety of two-phase reacting flow situations included here. Laminar multicomponent flow with viscosity, Fourier heat conduction, and Fickian mass diffusion will be analyzed. Diffusivities for all species will be assumed identical; the Lewis number value will be unity; radiation will be neglected; and kinetic energy will be neglected in comparison with thermal energy so that, with regard to the energetics, pressure will be considered uniform over the space although the pressure gradient can be significant in the momentum balance (small Mach number). We will analyze, as a general case, the situation in which a one-step exothermic reaction occurs between the ambient gas and the vapor from the bulk liquid. We will refer to the liquid as the fuel and the ambient gas as the oxidizer, although the opposite situation, e.g., oxygen droplet in hydrogen gas, can be analyzed in exactly the same fashion. The nonreacting case can be the special subcase in which the reaction rate becomes zero.

Both steady and unsteady scenarios will be discussed. The roles of the gas-phase energy equation and the species equations will be emphasized. Of course, they must be coupled with the continuity and momentum equations and with a gas equation of state. In addition, the solution of conservation equations for the liquid phase might be required.

There is interest in the droplet-vaporization problem from two different aspects. First, we wish to understand the fluid-dynamic and -transport phenomena associated with the transient heating and vaporization of a droplet. Second, but just as important, we must develop models for droplet heating, vaporization, and acceleration that are sufficiently accurate and simple to use in a spray analysis involving so many droplets that each droplet's behavior cannot be distinguished; rather, an average behavior of droplets in a vicinity is described. We can meet the first goal by examining both approximate analyses and finite-difference analyses of the governing Navier–Stokes equations. The second goal can be addressed at this time with only approximate analyses because the Navier–Stokes resolution for the detailed flow field around each droplet is too costly in a practical spray problem. However, correlations from Navier–Stokes solutions provide useful inputs into approximate analyses. The models discussed herein apply to droplet vaporization, heating, and acceleration and to droplet condensation, cooling, and deceleration for a droplet isolated from other droplets. The governing partial differential equations reflecting the conservation laws are presented in Appendix A. Several coordinate systems are considered. Formulations with primitive velocity variables and formulations with stream functions are discussed. Appendix B discusses some conserved scalar variables whose analytical use can be very convenient and powerful under certain ideal conditions.

Introductory descriptions of vaporizing droplet behavior can be found in the works of Chigier (1981), Clift et al. (1978), Glassman (1987), Kanury (1975), Kuo (1986), Lefebvre (1989), and Williams (1985).

Introduction

It is intended herein to present the current status of the fundamental understanding about the liquid-atomization processes for various injection configurations. The limitations of the theory and the need for future work will be made apparent. This chapter is not intended to be a guide for the practicing engineer; the current state of the art is based on empirical approaches that are discussed in Chapter 1. Rather, this chapter reviews theoretical research that should eventually lead to improved design methodology and design tools for liquid-atomization systems. Other overviews of the theory can be found in Lefebvre (1989), Bayvel and Orzechowski (1993), Sirignano and Mehring (2000, 2005), and Lin (2003).

The atomization problem could be divided according to three subdomains of the fluid mechanical field. The upstream subdomain lies within the liquid-supply piping, plenum chamber, and orifice (nozzle) of the injector hardware. More than the mass flow and average velocity from the orifice into the combustion chamber are important here; velocity and pressure fluctuations in the liquid that are due to turbulence, collapse of bubbles formed through cavitation, supply-pressure unsteadiness, and/or active-control devices are critical in affecting the temporal and spatial variation of the liquid flow over the orifice exit cross section. A small amount of research has been performed on this subject.

The second subdomain involves the liquid stream from the orifice exit to the downstream point where disintegration of the stream begins. The neighboring gas flow (or gas and droplet flow) is part of this subdomain.

The interactions of a spray with a turbulent gas flow is important in many applications (e.g., most power and propulsion applications). Two general types of studies exist. In one type, the global and statistical properties associated with a cloud or spray within a turbulent field are considered. In the other type, detailed attention is given to how individual particles behave in a turbulent or vortical field. Some studies consider both perspectives. Most of the research work in the field has been performed on the former type of study. Faeth (1987), Crowe et al. (1988), and Crowe et al. (1996) give helpful reviews of this type of research.

The interactive turbulent fields can be separated into homogeneous turbulent fields and free-shear flows (e.g., jets and mixing layers). In some theoretical studies, two-dimensional vortical structures interacting with a spray have been examined. Most of the studies deal with situations in which the contribution of the spray to the generation of the turbulence field is secondary, that is, there is a forced gas flow whose mass flux and kinetic-energy flux substantially exceed the flux values for the liquid component of the dilute flow. The turbulent kinetic-energy flux of the gas flow is much less than the mean kinetic-energy flux of the gas flow and is comparable to the mean kinetic energy of the liquid flow. Therefore the turbulent field is much more likely in this situation to receive kinetic energy transferred from the mean gas flow than kinetic energy transferred from the mean liquid flow.

High pressures and supercritical conditions in liquid-fueled diesel engines, jet engines, and liquid rocket engines present a challenge to the modelling and the fundamental understanding of the mechanisms controlling the mixing and combustion behavior of these devices. Accordingly, there has been a reemergence of investigations to provide a detailed description of the fundamental phenomena inherent in these conditions. Unresolved and controversial topics of interest include prediction of phase equilibria at high and supercritical pressures (Curtis and Farrell, 1988; Litchford and Jeng, 1990; Hsieh et al., 1991; Delplanque and Sirignano, 1993; Poplow, 1994; Yang and Lin, 1994; Delplanque and Potier, 1995; Haldenwang et al., 1996), including the choice of a proper equation of state, definition of the critical interface, importance of liquid diffusion, significance of transport-property singularities in the neighborhood of the critical mixing conditions, and influence of convection (including secondary atomization); d2-law behavior at supercritical conditions (Daou et al., 1995); droplet-lifetime predictions (Yang et al., 1992; Delplanque and Sirignano, 1993, 1994; Yang and Lin, 1994; Delplanque and Potier, 1995; Haldenwang et al., 1996); dense spray behavior (Delplanque and Sirignano, 1995; Jiang and Chiang, 1994a, 1994b, 1996); combustion-product condensation (Litchford and Jeng, 1990; Litchford et al., 1992; Delplanque and Sirignano, 1994; Daou et al., 1995); and flame structures at high and supercritical pressures (Daou et al., 1995). The actual combustion process is characterized by the supercritical combustion of relatively dense sprays in a highly convective environment.