Elsevier

Bioresource Technology

Volume 100, Issue 22, November 2009, Pages 5466-5471
Bioresource Technology

Thermochemical conversion of livestock wastes: Carbonization of swine solids

https://doi.org/10.1016/j.biortech.2009.03.005Get rights and content

Abstract

Slow pyrolysis or carbonization promotes the conversion of animal manures such as swine manure into charcoal. In this paper, the carbonizing kinetics of swine solids taken from different treatment stages were investigated with a thermogravimetric analyzer. Compared to their biologically stabilized counterpart (lagoon sludge) with an activation energy of 160 kJ mol−1, the activation energies for fresh swine solid samples such as homogenized flushed manure and dewatered solids were much lower between 92 and 95 kJ mol−1. Compared to the kinetics of first order decomposition of cellulose, the pyrolytic decomposition of the swine manures were more complex with the reaction orders varying at 3.7 and 5.0. The two different mathematical methods employed in this paper yielded the similar values of activation energy (E) and pre-exponential factor (A), confirming the validity of these methods. The results of this study provide useful information for development of farm-scale swine solid carbonization process.

Introduction

According to the recent study jointly sponsored by the USDA and US DOE, US agricultural lands currently have 35 million dry tons of available, sustainable animal manure (Perlack et al., 2005). The energy content of this sustainable animal manure can be estimated from its higher heat values (HHV), ranging from 7.9 MJ kg TS−1 for soil surfaced feedlot manure to 18.2 MJ kg TS−1 for flushed dairy manure (Cantrell et al., 2007). Using a simple arithmetic average HHV of different animal manures (13.4 MJ kg TS−1), the annual energy content of the 35 million dry tons of manure is estimated to be approximately 0.43 EJ (i.e., 0.43 × 1018 J). This is about 15% of the total biomass energy consumed in the US annually (Perlack et al., 2005). Assuming an efficiency of 20% for extracting useful energy from the manure (Denmark, 2002) and an energy value of about $50 per barrel of oil equivalent (BOE), this sustainable animal manure can provide energy with an approximate worth of 0.7 billion US dollars per year. This simple calculation clearly demonstrates that effective utilization of this abundant renewable energy source can have significant impact on the US agricultural energy budget and economy.

There are two pathways of extracting renewable energy from animal manure – biochemical and thermochemical pathways. Biochemical pathways utilize microorganisms or enzymes to convert animal manure into useful energy. Anaerobic digestion of animal manure for methane production is the most common biochemical means of extracting useful energy. Although anaerobic digestion technology is well established and robust, the process is very slow, requiring processing times of days and weeks. In addition, anaerobic digestion still leaves substantial amounts of sludge and supernatant that require further treatment and disposal. In contrast, emerging TCC technologies only require treatment times in the span of minutes to hours. Furthermore, they convert most organic matter into energy-rich and valuable end products such as combustible gases, liquids, and charcoals. These TCC end products can be used as energy intermediates for combined heat and power generation (CHP) or feedstocks for downstream catalytic conversion processes to produce higher value products such as liquid transportation fuels.

There are many TCC technologies that can be integrated with existing animal manure management practices to extract useful energy. Recently Cantrell et al. (in press) reviewed gasification, fast pyrolysis, hydrothermal gasification, and carbonization (slow pyrolysis) technologies for livestock waste-to-bioenergy generation applications. Among these TCC technologies, carbonization of animal manure for producing charcoal (or bio-char) may offer many advantages and usages for farmers. Compared with other sophisticated TCC technologies, such as fluidized bed gasification and fast pyrolysis, carbonization (or slow pyrolysis) requires relatively low technical resources, making the technology suitable for farm-scale process. The bio-char produced from livestock wastes can easily be transported and stored without nuisance odor and deterioration. It can be readily used as a cooking fuel and feedstock for existing coal power plants. Bio-char is superior in quality to charcoal made from coal due to its low sulfur content and high reactivity. In addition, bio-char can be activated with steam or chemicals to produce activated carbon. When compared to commercially activated charcoals, activated carbon produced from broiler litter demonstrated higher performance rates in adsorbing heavy metals (Lima and Marshall, 2005). The production cost of activated carbon from broiler litter was $1.44 kg−1, which was comparable to that of activated carbon from other renewable biomass sources (Lima et al., 2007).

Bio-char can also be applied to soil as an amendment. Soil application of bio-char not only improves soil fertility and increases crop production, it also offers significant environmental and potential economical benefits (Laird, 2008). Because of its adsorptive capacity, bio-char prevents leaching of pesticides and nutrients from soil. Soil application of bio-char represents a carbon-negative process whereby the environment realizes a reduction in both atmospheric CO2 and global warming (Lehmann, 2007a, Lehmann, 2007b). Once soil application of bio-char is eligible for carbon credit, farmers can generate significant income from producing and applying bio-char to the soil promoting the “Win-Win-Win” for a “Charcoal Vision” (Laird, 2008, Lehmann, 2007b).

Despite these advantages, relatively little information is available in the technical literature about the kinetics of carbonizing biomass. This information is reported by only handful of researchers (Antal and Gronli, 2003, Caballero and Conesa, 2005, Conesa et al., 1995, Jauhianinen et al., 2004, Mok and Antal, 1983; Narayan and Antal, 1996, Varhegyi et al., 1989, Varhegyi et al., 1993, Varhegyi et al., 1994, Vlaev et al., 2003). However, research on livestock wastes carbonization has not been studied in the same rigor (or enthusiasm) as lignocellulosic biomass. Only a handful of research papers on poultry litter carbonization have been published (Kim and Agblevor, 2007, Whitely et al., 2006). As for carbonizing swine solids, this paper is the first in reporting kinetics of thermal decomposition of swine solids. The swine solids in this study were collected at different stages of a 5600-head feeder-to-finish waste treatment system. The carbonization was studied by thermogravimetric analysis using a one-step global decomposition model. The kinetic information can be used for later design and operation of the carbonization process.

Thermogravimetric analysis (TGA) is a widely used technique to study reaction mechanisms and kinetics of solids undergoing thermal decomposition (Caballero et al., 1995, Gauer and Reed, 1998). Pyrolytic decomposition of biomass takes place through a complex network of parallel and competitive reactions. Different kinetic models have been proposed based on different reaction mechanisms (Bradbury et al., 1979, Caballero and Conesa, 2005, Conesa et al., 1995, Jauhianinen et al., 2004, Shafizadeh, 1982, Varhegyi et al., 1993, Varhegyi et al., 1994, Vlaev et al., 2003). Even after 30 years of research, there is still no consensus concerning pyrolysis kinetics of relatively simple cellulose biomass (Gronli and Melaaen, 2000, Varhegyi et al., 1994). Investigating detailed pathways for thermal decomposition of complex biomass such as swine manure is beyond the scope of this study. Instead, we utilized the following one-step global decomposition kinetic model for our swine manure samples.Swine maureChar+Volatiles(condensable and non-condensable gases)

The reaction rate is dependent upon both the temperature, and the volatile matter content. The temperature dependence is usually expressed as a reaction coefficient for using an Arrhenius equation. The volatile matter dependence can be expressed as an nth order reaction equation.dα(1-α)n=Aβexp(-E/RT)dTwhere, A is the pre-exponential factor (min−1), E is the activation energy (kJ mol−1), R is the gas constant (8.314 J mol−1 K−1), T is the temperature (K), t is the time (min), n is the order of reaction, α is the fractional conversion, β is the constant heating rate or dT/dt (K min−1).

The fractional conversion or the extent of reaction is defined as:α=mo-mTmo-mfwhere, mo is the initial mass (g), mT is the mass at temperature T (g), mf is the final residual mass (g).

The three kinetic parameters or the kinetic triplet (E, A, and n) can be estimated from fitting the thermogravimetric data into differential or integrated forms of Eq. (1) (Gauer and Reed, 1998, Flynn, 1997a, Flynn, 1997b, Garcia-Nunez et al., 2008, Kim and Agblevor, 2007, Li et al., 2008). The integral method is generally believed to be more reliable then differential methods; however, the temperature integral (the middle exponential integral of Eq. (4)) cannot be solved analytically.G(α)=Aβ0Te-E/RTdT=0αdα(1-α)n

Many forms of approximation of the temperature integral have been developed (Gauer and Reed, 1998, Flynn, 1997a, Flynn, 1997b). When e-E/RT is expressed as an asymptotic series, the temperature integral can be integrated and simplified by ignoring higher-order terms, yielding Eq. (5). Hereafter the approximation will be called the Coats and Refern method (Coats and Redfern, 1964, Gauer and Reed, 1998, Guo and Lua, 2001).G(α)=ART2βE1-2RTEe-E/RT

Eq. (5) can be expressed in logarithmic forms for n = 1 and n  1.ln-ln(1-α)T2=lnARβE1-2RTE-ERT(forn=1)ln1-(1-α)1-nT2(1-n)=lnARβE1-2RTE-ERT(forn1)

Recently, Chen and Liu (2006) developed a new approximation for the temperature integral and the corresponding kinetic equation can be expressed as:ln-ln(1-α)T2=lnARβE3(E/RT)2+16(E/RT)+43(E/RT)2+22(E/RT)+30-ERTforn=1ln1-(1-α)1-nT2(1-n)=lnARβE3(E/RT)2+16(E/RT)+43(E/RT)2+22(E/RT)+30-ERTforn1

Chen and Liu (2006) claimed that these new equations, when compared to previously developed kinetic equations, provided a better approximation for the evaluation of non-isothermal kinetic parameters with greater accuracy; hereafter, it will be called the Chen and Liu method. Using any of these methods (Eqs. (6), (7), (8), (9)), the activation energy (E) can be estimated from the slope of a line established from fitting the TG data: lnG(α)T2 vs. 1/T. Since the order of reaction is usually not known beforehand, it is first necessary to fit the TG data with an assumed value of n. If the assumed reaction order adequately represents the reaction, the line becomes straight. If not, another reaction rate is assumed and the fitted line is examined for straightness. The pre-exponential factor A can be calculated from the intersection of the best fitted line, which is the first term of Eqs. (6), (7), (8), (9). The mean values of A can be calculated over the temperature range of interest.A¯=T1T2A·TdTT1T2TdTwhere A¯ is the mean pre-exponential factor in the temperature range of T1 and T2 (min−1).

Section snippets

Swine solid samples

A 5600-head finishing swine operation used a waste management system that combined solid–liquid separation with nitrogen and phosphorous removal from the liquid phase (Vanotti et al., 2007). The system was constructed and operated by Super Soils System USA of Clinton, NC. Homogenized flushed manure house effluent was passed through a mobile liquid–solid separation unit with polyacrylamide (PAM) flocculation and rotary press dewatering system. The solid phase was transported off-farm for compost

Results and discussion

Fig. 1 shows the thermogravimetric diagrams (TGs) of three different swine solids taken from various points in typical swine operation; dry solids extracted from homogenized flushed manure house effluent (HT), dry solids obtained after polymer-enhanced solid–liquid separation (SS) and dry solids from anaerobic lagoon sludge (LS). Thermal decomposition of these dry swine manure solids was achieved in three different temperature regimes; drying, pyrolytic decomposition, and gasification regimes.

Conclusions

Carbonizing kinetics of swine solids taken from different stages of waste treatment were investigated with a thermogravimetric analyzer. Activation energy for fresh swine solid samples such as homogenized flushed manure (HT) and the dewatered solids (SS) were much lower than that of biologically stabilized lagoon sludge (LS). The onset pyrolysis temperature of LS with higher activation energy was slightly higher than that of other solids with lower E. The reaction orders of 3.7 and 5.0 were

Acknowledgements

This research was part of USDA-ARS National Program 206: Manure and By-product Utilization; ARS Project No. 6657-13630-003-00D “Innovative Animal Manure Treatment Technologies for Enhanced Environmental Quality”. The authors are grateful to both Cody Alexander and Aprel Ellison for help with sample collection and preparation and to Jerry Martin II for assistance in equipment handling and analyses.

Mention of trade names or commercial products in this publication is solely for the purpose of

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