|
Bio-Mass Anaerobic
Digester Fuel Cell System on Dairy Farms It is well known that dairy farms are quite capable of producing self-sufficient power through a remote stand-alone power system consisting of an anaerobic digester and engine-generator. However, combustion of fuel has led to environmentally harmful emissions, and low energy efficiency. This and other factors have led to a search for an alternative means to meet the energy needs of dairy farmers, and in so doing, the fuel cell came to mind. Our main concerns have been the emission of pollutants, maintenance, and system efficiency. The fact that energy (electrical and heat) can be obtained from hydrogen-rich gas e.g. hydrocarbons, promising higher energy efficiency and cleaner emissions, makes the fuel cell the more attractive energy producing alternative.
Let us consider a dairy farm that currently uses the anaerobic digester
engine-generator system to produce the needed heat and power, and only look at
an alternative (i.e. fuel cells) to the engine-generator component of an
anaerobic digester system on diary farms, then it is fair to say that the
equipment used e.g. plug flow digester, biogas storage equipment, milking
equipment, ventilation, lighting, etc. not directly related to the power
generator, will remain the same (i.e. the need for these equipment and the
electrical demand by these equipment will remain the same).
By implementing a fuel cell system in place of the engine-generator, the
aspects most likely to be affected are changes in energy efficiency
(electrical/heat), the need for maintenance, cleaner emissions and the reduction
of greenhouse gases into the atmosphere, and noise reduction which are the
expected benefits of a fuel cell system when replacing the engine-generator. To
help visualize the energy production process on dairy farms, Figure
1.1 is a flow diagram of an anaerobic digester CHP generating system.
Since the plug-flow digester is the most commonly
used digester on dairy farms; ultimately, the type of fuel cell will play an
important role in determining CHP capabilities, and the need for auxiliary
equipment e.g. reformers. This work
will provide general information on the different anaerobic digester and fuel
cell types, auxiliary equipment (e.g. reformer) if
some other digester fuel cell system is to be considered, as well as grid
interconnection requirements, etc. also included is a fuel cell circuit model.
However, the anaerobic digester fuel cell system this study is mainly concerned
with is the plug-flow digester SOFC (Solid Oxide Fuel Cell) system that
could potentially be used to meet the energy and environmental needs of dairy
farmers. Benefits of an Anaerobic Digester Fuel Cell System Stricter environmental standards have actually helped push the need for improving energy efficiency, conserving energy, and reducing energy costs for dairy farm owners. Implementing an anaerobic digester fuel cell system on dairy farms could meet these needs and even prove to be beneficial in the years to come. The expected benefits of such a system would include [1.1]:
However, the cost of a fuel cell system, ranging anywhere from $3,000-$6,000 per kW of power generating capacity, is the main disadvantage today when compared to an engine-generator. Depending on the type of fuel cell used, reformers may be needed which would further add to the complexity and cost of the fuel cell system.
[1.1] A. Ferguson, V. I. Ugursal. A Fuel Cell Model For Building Cogeneration Applications. Canadian Residential Energy End-use Data and Analysis Centre. [1.2] D. Scott. Advanced power generation from fuel cells -implications for coal. IEA Coal Research, 1993. [1.3] Y. Zhu, K.Tomsovic. Development of models for analyzing the load-following performance of microturbines and fuel cells. Elsevier, December 2001 [1.4]
C. Nelson, J. Lamb. Final
Report Summary: Haubenschild Farms
Anaerobic Digester. The Anaerobic Digesters and Energy Production on Dairy Farms The most common anaerobic digester found on a dairy farm is the plug-flow type digester system. Anaerobic (oxygen-free) digesters are designed to digest cow manure where decomposition releases biogas products. How this works is, a load of manure moves through the digester, typically it takes 20-25 days (retention time) for the methane producing bacteria to grow and reproduce in sufficient quantities. During this retention time, the methane producing bacteria in the organic waste is allowed to break down the solids while releasing biogas which is comprised of about 60% CH4, 40% CO2, and the highly corrosive H2S at approximately 0.2% - 0.4% according to [2.1]. At the end of the retention time, the fully decomposed manure is removed and replaced by a fresh manure load on a daily basis for continuous biogas production. Anaerobic
Digester Design Types Each of the following digester design type is capable of trapping biogas but differ in cost, climate suitability, and concentration of manure solids they can digest as described below. In general, anaerobic digesters should be kept at a normal operating temperature of about 100 degrees F in order to function properly. Although, higher temperature operation can speed up the process and reduce tank volume; more species of methane producing bacteria exists at the normal operating temperature. Further more, higher temperature operation of a digester is more prone to upset and can lead to further complications if close monitoring is not considered.
Reasons
for Using Biogas There are several reasons for considering the use of biogas in fuel cells to supply energy. First, the methane (CH4) produced from biogas is of special interest because it is readily available on dairy farms. Secondly, usable energy (heat and electricity) can be obtained from biogas (about 60% CH4) because of its similar composition to natural gas (98% CH4). And finally, biogas can be purified and/or reformed before it is fed into a fuel cell which will essentially allow us to have a wider selection of fuel cell types to choose from. Energy
Production from Biogas Energy production will essentially depend on the amount of biogas produced from the manure fed into the digester. If we assume the cows are kept in an enclosed area most of the time and manure collection to be very efficient, then Table 2.1, which provides information on the amount of manure produced on a daily basis from various farm animals, can be used to estimate e.g. the amount of manure collected, and electricity produced from biogas, etc. Table 2.1 Daily manure production
As can be seen from the above table, a single dairy cow produces about 110 lbs of manure per day having a Total Solids (T.S.) content of about 13%. About 80% of the 13% of Total Solids is Volatile Solids (V.S.) that can potentially produce energy. It is estimated that for each 100 lbs of manure fed to a digester about 4 lbs of manure is converted into biogas and the rest leaves the digester as effluent [2.2]. We can estimate the amount of manure collected per day from dairy cows by:
where: hp = proportion of manure collected (%) MPF = manure production factor (ft3/day/cow) From this, we can find the daily amount of biogas produced and thus the heating value. From the heating value, the amount of electrical and heat energy that is available for use, depending on the energy system used (i.e. engine-generator or fuel cell system), can be calculated. With the help of Tables 2.2 and 2.3 these quantities can be obtained through the following:
where: GPF = gas production factor
DGP = daily gas produced
Table 2.2 Retention time and Gas Production Factor provided by [2.2]
The gas production factors listed in Table 2.2 depend on the retention time of the collected manure. A retention time of 25 days or up to one month is not unusual. To a certain extent, the longer the retention time, the more biogas that can be produced. From the daily gas production, the following can be calculated:
Daily Heat Value (Btu/day) = DGP x heat value of gas (2.4)
where: heat value of gas = the amount of heat energy available in Btu t = on-line time (%) hengine = electrical efficiency of engine-generator hcell = electrical efficiency of fuel cell type eengine = electrical energy produced from engine-generator (kWh/day) ecell = electrical energy produced from the fuel cell type (kWh/day) A list of heating values of gaseous fuels at various temperatures is given in Table 2.3.
The heating values listed in Table 2.3 were computed by using a Matlab script (energy_density.m) that found the available free energy at various temperatures for the reactions below. The available free energy was then converted into a heating value. Hydrogen: 2H2 + O2 " 2H2O (2.7) Methane: CH4 + 2O2 " 2H2O + CO2 (2.8)
We can use the information above to estimate energy
production, e.g. if a single dairy cow were to produce 110 lbs of manure this
would in turn produce about 65.6 ft3/day of biogas or 39.4 ft3/day
of methane with a retention time of 25 days.
At 25°C, using an engine generator with
an electrical efficiency of 25%, this would mean an electrical production of
about 2.64 kWh per day per cow from biogas having an energy density of 549.39
Btu/ft3. With a
constant flow of fuel and oxidant to a fuel cell, it would be ideal for all the
energy from the chemical reaction involving hydrogen, Equation (2.7), to be
available for use. Unfortunately,
this has yet to be obtained because of certain factors that affect the
performance of the fuel cell. However,
we can estimate the electrical production from a fuel cell by using the
electrical efficiency of the cell type used when fed with a constant flow of
hydrogen (from reformed biogas) and oxidant.
With that being said, the following Matlab script (electrical_production.m)
was created to do the following: § Computes the daily biogas production from dairy cows dependent on the farm size and generates a plot. Assumes a 30 day retention time and a 90% manure collection efficiency. Figure 2.1. §
Generates a plot of daily electrical
production from an engine-generator (
§
Generates a plot to estimate the daily
energy production from different fuel cell types (
§ Generates a plot to estimate the daily energy production using MCFC and SOFC from biogas. Figure 2.4. § Generates a plot to estimate the daily electrical energy production using MCFC and SOFC from methane (60% of biogas). Figure 2.5 The plots generated from electrical_production.m will allow us to estimate biogas production, and make comparisons between the engine-generator and fuel cells in terms of electrical production.
Efficiency
Comparison of Fuel Cells to Engine-Generators In fuel cells, the electrochemical reaction converts the chemical energy of the fuel into electrical and thermal energy to achieve high efficiencies. In other words, fuel cells generate electricity without burning the fuel. This process, in terms of energy efficiency, is not limited by temperature or what is called the Carnot efficiency as in the engine-generator [2.3], see Appendix A. Conventional power generation converts the chemical energy of the fuel (e.g. natural gas) into thermal energy by combustion. This thermal energy is transferred to a working fluid (e.g. water) that will convert the thermal energy to mechanical energy with the help of special engines (turbines). Finally, a generator converts the mechanical energy to electrical energy. This whole process is linked to and limited by the so-called Carnot efficiency. Table 2.5 compares typical fuel-to-power efficiencies of various types of prime movers. These efficiency figures do not account for increases due to the use of cogenerated heat.
Theoretically, fuel cells promise much higher energy efficiency (~ 60% fuel-to-power efficiency, approximately 40% of energy produced is wasted as heat) than the engine generator (~ 25% fuel-to-power efficiency, approximately 75% wasted as heat) as seen in Table 2.5. If half of the wasted heat is recovered and used, we can say that using the engine-generator considering cogeneration has an efficiency of about 62.5% and the fuel cell would have a cogeneration efficiency of about 80%. One of the primary reasons for considering the use of the fuel cell is the capability of high energy conversion efficiency. There are four aspects of efficiency that must be considered when determining the efficiency of a fuel cell:
Such cells are running at about 50-65% efficiency. The operating voltage is simply measured during the operation of a cell. The lower operating voltage is the result of activation, concentration, and impedance losses. 2. Current efficiency is based on an assumed stoichiometric equation and is defined in several ways:
To illustrate what is meant by current efficiency, consider the following balanced reaction: Anode: 2H2 D 4H+ + 4e- Cathode: O2 + 4H+ + 4e- D 2H2O Overall: 2H2(g) + O2(g) g 2H2O If the sole product is to be H2O, then all the current produced from the fuel cell consuming hydrogen should be used in forming H2O and the current efficiency is 100%. If e.g. any side products or intermediates are formed, then the end product is formed at a lower efficiency. The following are several reasons that result in a lower current efficiency:
Most practical fuel cells however are likely to use air as the oxidant and so having a high current efficiency becomes a secondary need.
4. The comparative thermal efficiency (eT) makes it possible to make the comparison of fuel cells to heat engines. This is defined as:
Observe that even when eG = 100%, the comparative thermal efficiency of the fuel cell may still be less than 100%. The following example is used to make a comparison of a fuel cell to heat engine. Consider the reaction: H2(g) + ½ O2(g) g H2O(l) For
which DG = -56.69 kcal and DH
= -68.32 kcal at 298°K,
it follows that a fuel cell with a free energy efficiency of 60% is as efficient
as a heat engine burning hydrogen at a thermal efficiency of
However, when electrical energy is required from a particular fuel, then the efficiency of the fuel cell should be compared with the overall efficiency of the heat engine burning the fuel and the generator converting mechanical energy to electricity. Heat
Recovery Utilization Usable heat should be recovered from high temperature operating fuel cells. About 40% of fuel energy from high temperature operating fuel cells is rejected as waste heat. Recovery of this heat for heating the digester, and space heating and hot water needs on the dairy farm are essential. Properly sized heat exchangers can recover about 7,000 Btus of heat per hour for each kilowatt of generator load [2.2]. For example, 140,000 Btu/hr of heat could be recovered from a cogenerator operating at 20 kilowatts (7000 Btu/hr-kW x 20kW = 140,000 Btu/hr) using a heat exchanger. Figure 2.6 is a diagram of a fuel cell cogeneration system.
Figure 2.6 Components of a fuel cell cogeneration system The equipment used is an important aspect to be considered for proper sizing of a fuel cell system and the energy usage on a dairy farm will certainly depend on the farm size. The energy usage (electricity and heat) by a typical dairy farm can be categorized by the equipment used and is summarized here. This will provide baseline information on the types of equipment used that contribute to the dairy farm electrical load. The NYSERDA audit shows that the majority of the energy used by dairy farms is due to milk production equipment that consists of milk cooling, water heating equipment, and vacuum pumps, other sources of energy usage are due to lighting and ventilation. All together, these equipment, account for 92% of the total energy used on a dairy farm. The remaining 8% will consist of feeding and manure handling equipment, and miscellaneous uses. Generally, the electrical energy consumption on a typical dairy farm ranges from 800 to 1200 kWh of electricity per cow annually, and averaging about 100 dairy cows in size according to an energy audit for dairy farms across central and northern New York. Figure 2.7 provided by [2.1] shows the energy needs of a 100 cow dairy farm and the potential energy that can be delivered by this farm size using a generator. Also shown is the energy profile of a 600 cow dairy and the energy a farm this size could potentially deliver.
Referring
back to the example we had earlier, the electrical production from the manure of
a cow is approximately 2.64 kWh per day, then 100 cows would have the potential
to deliver 11 kW of energy per day (
Fuel
Cell Sizing To size a fuel cell one could use a load profile of a dairy farm to determine sizing or if potential gas production has been estimated, using the same idea for sizing an engine-generator the cogenerator fuel cell should have one kilowatt of capacity per 600-650 cubic foot of daily biogas production. For a 250-cow dairy farm with a digester producing 15,000 ft3 of biogas per day, a 25 kilowatt fuel cell generator is suggested. For continuously operated cogenerators, 1 kW of cogenerator capacity per 10 cow equivalents is also reasonable starting point. Fuel
Cell System Components A power generation fuel cell system may incorporate several auxiliary devices required for operation and are listed below:
REFERENCES [2.1] P. Wright.
Overview of Anaerobic Digestion Systems for Dairy Farms.
Agricultural and Biological Engineering Department, |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
