Formation of Fuel-Grade Ethanol
from Swine Waste via Gasification

 

B. Kaspers, J. Koger, R. Gould[1], A. Wossink[2], R. Edens[2], and T. van Kempen

 

Summary

The objective of this project is to investigate the application of gasification technologies to the treatment of swine waste for the ultimate production of fuel-grade ethanol.  This waste treatment system would reduce the negative environmental impact of current manure management systems.  The research objectives are: 1) to develop and test a system for harvesting swine manure in a form dry enough to be used as a gasification feedstock, 2) to establish the feasibility and the gasification conditions for the swine waste/amendments feedstock, 3) to characterize the end products of gasification (ethanol and mineral ash) and their potential markets, and 4) to conduct a rigorous economic analysis on the entire swine manure management model to determine its feasibility along with the factors that promote or impede its implementation.

 

Introduction

Ethanol production (primarily via fermentative methods) from crops and other “renewable” biomass sources has received much attention recently, but the current approach has problems.  Mainly, crop-based feedstocks are subject to seasonal fluctuations in supply, ultimately limiting ethanol generation.  Such feedstocks necessitate either lengthy storage of the perishable plant materials or stopping ethanol production altogether during the off-season.  Another dilemma faced is that some of the feedstocks currently used in ethanol production (e.g. corn stubble) have a greater value elsewhere (e.g. fertilizer).  More specifically, the energy cost in harvesting these feedstocks (e.g. corn stubble) as well as their lost value as soil amendments makes ethanol production costly to farmers (Pimental, 1992). Animal manures avoid many of these problems because they are a truly renewable feedstock.

 

The quantity of swine manure produced in the U.S, estimated at 5 billion kg dry matter per year, is sufficient to contribute substantially to ethanol supplies.  Assuming a conversion efficiency of 40%, there is a theoretical ethanol yield of 500 million gallons per year.  North Carolina is the second largest hog-producing state within the U.S. with a swine population large enough for gasification technology to be feasible.  Thus, ethanol production of 80 million gallons per year should theoretically be attainable. The most recent RFA (Renewable Fuels Association) Ethanol Report (May 11, 2000) concludes that replacing corn with less expensive feedstocks will result in substantial reductions in ethanol production costs.

 

Gasification of biomass has received much attention as a means to convert waste materials to a variety of energy forms (i.e. electricity, combustible gases, synfuels, various fuel alcohols, etc.).  Gasification is a two-step, endothermic process in which solid fuel is thermochemically converted into a low or medium Btu gas.  Pyrolysis (Step 1) of the biomass is followed by either direct or indirect oxygen-deprived combustion (Step 2) during the gasification process.  This process converts raw biomass into a combustible gas, retaining 60-70 % of the feedstock's original energy content.  Thermochem’s steam reformer is the system we are investigating to gasify our feedstock because this type of gasifier produces a hydrogen-rich, medium-Btu fuel gas.  This gasifier design percolates superheated steam through an indirectly heated inert fluidized bed of sand or a mineral material.  The organic feedstock injected into the bed undergoes a rapid sequence of pyrolysis and vaporization reactions.  Higher hydrocarbons released among the pyrolysis products are steam cracked and partially reformed to produce low molecular weight species.  This process produces a gas with nearly immeasurable environmental emissions of NOx, SOx, CO, and particulates. The main reason this particular gasifier design is favored is because of its hydrogen to carbon ratio (2:1) is ideal for ethanol synthesis. A recent cost and performance analysis of biomass (i.e. wood) gasification systems for combined power generation indicated that such a steam system (Battelle Columbus Laboratory) had the lowest capital cost and product electricity cost (Craig and Mann, 1997). 

 

There is an intensive effort, especially in North Carolina, to develop a better waste management strategy.  The ultimate goal of this project is to eliminate the land application of lagoon effluent. The elimination of this waste via gasification would abolish the need for land application of waste.

 

The primary obstacle to overcome in this project is converting the swine manure into a suitable feedstock for gasification.  Factors such as moisture content, density, and transportation requirements must be investigated.  The most common waste systems currently employed, the lagoon (1% Dry Matter (DM)) and slurry basin (10% DM), do not produce a waste stream which makes for a suitable feedstock for gasification and thus alternative waste management systems must be developed.  When the appropriate feedstocks are selected, the gasifier will be engineered to maximize product gas yields.

 

Results and Discussion

Initially, fresh fecal samples were collected from our swine research facility (Jan. 2000) to corroborate literature findings that claim swine feces is typically 20-30% dry matter.  The mean DM for the fresh fecal samples obtained from grower/finisher pigs fell within the reported range at 28.6%.  These DM values were not significantly (p= .26) different between the various sized (50-200 lbs.) grower-finisher pigs.  The mean energy value of the samples was found to be 4361 cal/g.  However, the energy values displayed a decreasing trend as body size increased (p= .16).  This trend can be explained by the increase in digestion time that occurs with an increase in the animal’s body weight.  In comparison to other potential feedstocks for gasification (Table 1), swine waste has a high enough energy value to make gasification feasible.


Table 1. Energy content ranking (highest to lowest) among possible feedstocks

Material

Energy content (cal/g)

Corn cob

4928

Birch wood

4613

Swine waste

4361

Corn straw

4253

Wheat straw

4247

Rice straw

3903

 

A thorough investigation of existing swine waste management systems within the U.S. suggests a low probability for obtaining feces with a desired DM content for steam reformation (60-80%) from currently employed systems (most commonly the lagoon and the slurry basin).  Some alternative housing systems like hoop structures (found primarily in the Midwest) and dry waste systems (Hog high-rise in Ohio) have been examined.  Samples were obtained from three hoop facilities in Indiana because they utilize a deep bedding system which could yield a dry waste.  Analysis of these samples determined that this waste stream was unsuitable as a possible feedstock for steam reformation with a mean dry matter content of only 41%. Analysis of samples for DM and energy content from the high-rise in Ohio will be conducted in the future.

 

European swine research facilities have shown that a conveyor belt collection system seems favorable for obtaining a drier waste stream.  Thus, we designed a small-scale (single pen) belt unit with plans to construct a large-scale (100 pigs) model in the summer of 2000.  These units should provide us with a suitable feedstock for steam reformation without having to employ additional drying mechanisms.

 

Initially, we set up a housing structure to simulate a belt system, in order to measure ammonia emissions.  The system consisted of grower/finisher pigs housed within a pen on tenderfoot flooring with PVC sheets slanted six inches below it, allowing the urine to drain away from the manure. This structure was housed within one of our enclosed chambers where ammonia levels were monitored using an FTIR (Fourier Transform-Infrared) spectrophotometer.  There was no increase in ammonia emission over the three days the animals were housed there, in contrast to the usually observed increase in ammonia emissions.  This finding suggests that an innovative manure collection system like the conveyor belt will dry the manure as well as reduce odors within the swine housing facility, making it a more environmentally friendly system.

 

Next, we built a small-scale model, consisting of one pen with tenderfoot flooring and a plastic belt running below it.  Our first pilot trial was with grower/finisher pigs averaging 31 kg.  Although this system required improvements, a DM of 60% was achieved, indicating the system could produce a feedstock for gasification.  This trial also examined DM as a reflection of times between collections off the belt.  Dry matter seemed to be the highest when the belt was moved one foot each day over a three-day period.  Further investigation into the collection periods will be examined in subsequent pilot trials after which a larger scale unit will be built.

 

Implications

Our research thus far has shown that swine manure can be a suitable feedstock for gasification.  The belt system (an alternative waste management system) has the potential to dry the swine waste to more than 60% DM.  Investigation into possible amendments of North Carolina's cash crop wastes (i.e. peanut shells, wood shavings, wheat straw, etc.) remains a possibility for producing an even drier feedstock. Alterations to the steam reformer will be performed to optimize product gas composition for ethanol production and to allow for flexibility in feedstocks (with or without amendments, varying dry matter contents, etc.).  Also, the ash product produced in the steam reformer will be examined for use as a mineral source in animal feeds or as a fertilizer. The final conclusion regarding the feasability of gasifying swine waste will be dependent upon the economic analysis of the entire housing and gasification system.  A decision support system (DSS) will be developed that stimulates and optimizes the whole chain from animal production to manure spreading or processing.  The system will assess the logistics, economics, and environmental effects for each of the elements of the chain.  An economic/environmental sensitivity analysis of gasification as a manure processing technology will be performed by changing the options (such as subsidies on ethanol), constraints (particularly the regulatory context), and model assumptions step by step.  The results will be compared to an environmentally sustainable system based on current technologies, waste disposal by land application at agronomic rates that avoid eutrophic consequences.

 

Literature Cited

Craig, K.R. and Mann, M.K. “Cost and Performance Analysis of Three Integrated Biomass Gasification Combined Cycle Power Systems”. DOE BioPower Program Technical Reports, Aug. 1997.

Pimental, D. 1992. Energy inputs in production agriculture. Energy in World Agriculture. ed. R.C. Fluck. Amsterdam; Elsevier. Pgs. 13-29.



[1] Mechanical & Aerospace Engineering

[2] Agricultural and Resource Economics