ARTICLE pubs.acs.org/EF
Development of a Gasification System for Utilizing Fish Processing Waste and Coastal Small Diameter Wood in Rural Areas Shawn R. Freitas†,‡ and Juan A. Soria†,‡,* † ‡
University of Alaska Fairbanks, Agriculture and Forestry Experiment Station, 1509 S. Trunk Rd, Palmer Alaska 99645, United States University of Alaska Anchorage, Applied Environmental Science and Technology, Engineering Bld, 3211 Providence Drive, Anchorage, Alaska 99508-4614, United States ABSTRACT: Many small rural communities lack the infrastructure and demands of urban areas and find themselves constrained by fuel and energy limitations. Researching gasification processes that can be used by these communities can play a significant role in making them sustainable by reducing their environmental and economic impact through the use of locally available resources. This research focuses on developing a methodology and process to gasify a unique waste stream composed of salmon fishery wastes and small diameter alder, by taking a low-cost practical approach to create a system that could be locally sustainable and support existing infrastructure and heating needs. The waste stream was successfully processed and then gasified in a small-scale downdraft gasifier. Various combinations of feedstock mixtures and gasification conditions were determined using a response surface model and then trialed. The results showed a distinct set of optimal conditions and suggested that utilization of salmon processing waste in a downdraft fixed bed gasifier is feasible and could potentially generate a new function for this waste.
’ INTRODUCTION Thermochemical conversion based upon gasification is a proven and robust concept. Gasification is capable of employing a variety of feedstocks while consistently generating a functional and useful syngas product that can be converted into energy, fuels, and other value added chemicals.1 Gasification systems also have the ability to meet the bulk of their energy demands for conversion by using energy provided by the feedstock itself, making this thermochemical conversion process more sustainable than many others that require significant external sources of energy to operate.2,3 This flexibility regarding feedstocks and potential sustainability regarding operation has made gasification the focus of increased research, development and distributed energy planning.47 However, despite this potential, very little research has been done to support practical local and small-scale gasification implementations. Many small rural communities lack the infrastructure and demands of urban areas and find themselves constrained by fuel and energy availability.811 Researching practical gasification processes that can be used by these communities could play a significant role in making them sustainable by reducing their environmental and economic impact through the use of locally available resources. Additionally, gasification systems are often only scalable by means of interconnecting individual gasification units, due to size constraints on single reactor systems.2 This is seen as a direct benefit for implementations in small communities that can utilize smaller gasification operations and avoid the scalability issues sometimes faced by large scale systems.2 In coastal Alaska, many such communities generate unique waste streams in the harvesting and processing of fish. If this waste stream could be integrated into an appropriate gasification system, it could become an enabling resource, providing energy, revenue, and jobs. This research focused on developing a methodology and process to gasify a unique waste stream composed of salmon fishery wastes r 2011 American Chemical Society
and small diameter alder, by taking a low-cost practical approach to create a system that could be locally sustainable and support the existing infrastructure and heating needs. Many of Alaska's coastal communities are involved in harvesting and processing fish on a commercial scale.1214 These communities support Alaska's world class fisheries and generate a significant amount of fish processing waste. Importantly, fish processing is gaining notoriety as one of the few remaining industries that discharges its waste into the environment untreated.15 Typically fish processing waste is composed of heads, viscera, skeletons, and fins that are ground up and discharged through “slurry lines” into the ocean. This method is generally considered the only economical solution for waste disposal. Alaska's seafood industry generates vast amounts of seafood processing waste, with salmon processing waste alone accounting for approximately 100 000 t a year.16,17 This waste is high in oil and represents a significant energy resource if it can be leveraged as a feedstock for gasification. However, it is also high in moisture which creates an energy barrier that has to be overcome in order for this feedstock to function in a gasifier. Salmon processing waste was identified as a priority target to meet the goals of this gasification project, as salmon is the largest Alaskan fishery sector, responsible for producing the most waste, and offering the greatest advantages if developed as an energy resource. Salmon processing waste can have a moisture content as high as 80% and is largely composed of heads and viscera that have been shown to hold as much as 15% lipids.18 While the salmon lipid content has supported inquiry into its potential as a feedstock for biodiesel, the energy intensity of extracting the oil
Received: December 30, 2010 Revised: March 24, 2011 Published: March 29, 2011 2292
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Table 1. Tar Content in Gas from Updraft and Downdraft Gasifiers and Allowable Tar Content for End Usea gasifiertype (fixed bed) reported tar level in gas (mg/Nm3) updraft down draft
end use device reciprocating engine industrual gas turbine compressor
10 000150 000 501400
references 2226 2224,27,28
allowable tar loading in gas (mg/Nm3) references 10100 0.2525 50500
and wood composition and the resulting producer gas quality. Details on each step are provided in the materials and methods section, with the subsequent results and implications section discussing the relevance of the findings of this research.
22,2931 22,32,33 7,22
a
All research shown was completed using only biomass feedstock. Ranges in gasifier tar production are due to differences in design. Ranges in allowable tar loading for end use devices are a function of the specific make and model of the devices and the gas cleanup processes used.22
from heads and viscera makes the process less attractive, requiring nearly twice the energy input to extract as soybean oil.19 Gasification of commercially available wood pellets soaked in salmon waste and utilized in an updraft fixed bed gasifier has been reported.20,21 Wood pellets were chosen for their low moisture content, which reduced the overall impact of the slurry's high moisture. The approach successfully highlighted the importance of using a low moisture matrix with fish wastes to bring overall moisture to an appropriate level. However, this approach could not have supported significant utilization of the salmon waste. In commercial pellet manufacturing, extrusion systems effectively seal the sides of the pellets, resulting in limited pore spaces where fish waste could be adsorbed. A pellet soaked in salmon waste would swell with water and likely show very little inclusion of the salmon waste into its mass. As the goal of this research is to use as much of the waste stream as possible, investigation into the direct contribution of salmon waste to the mixture across a range of concentrations was required. While Rowland and Dewitt were successful in gasifying this mixture in an updraft gasifier, a downdraft gasification system typically produces a higher quality gas with less tar, more capable of supplying fuel for reciprocating engines, and direct heat applications needed in rural areas.2,3,22 A comparison of tar content in the produced gases from updraft and downdraft gasifiers can be seen in Table 1. The required tar levels for various end user applications are also shown. To support the use of as much fish waste as possible toward the production of a functional fuel gas, this research involved actively mixing salmon waste with low moisture, small diameter alder wood sawdust (Alnus rubra) and compounding this mixture into pellets for gasification in a downdraft fixed bed gasifier. This supported the investigation of the fish contribution to the producer gas generated and allowed gasification of the waste stream in a way that could be locally sustainable and support existing infrastructure. This process consisted of four steps; initial characterization of the feedstock resources which needed to be locally available and underutilized, addressing the unique processing requirements for incorporating high moisture fish waste slurry into a drier sawdust biomass and compacting the resulting mixture into a functional pellet, modifications to a fixed bed downdraft gasifier in order to accommodate the manufactured pellets, oxidant requirements, gas conditioning needed for this novel feedstock, and optimization of the system using response surface modeling (RSM) to provide baseline information on process relationships important for modeling the optimal fish
’ EXPERIMENTAL SECTION Red alder (Alnus Rubra) was collected during March 2009 from the Palmer Agricultural and Forestry Experiment Station (AFES) located in Palmer, Alaska. Collecting and processing the alder to prepare it for pelletizing required several steps. Twelve to eighteen centimeter diameter alders were harvested and processed to produce chips using a Vermeer BC935 and Yard Machines model 24A chippers. The chips were dried to average moisture of 4 wt % and processed in a Wiley mill (Thomas Scientific) to pass through a 2 mm mesh. The materials were sealed in containers prior to pelletizing. Seventy six liters of mixed Red salmon (Oncorhynchus nerka) and Pink salmon (Oncorhynchus gorbuscha) processing waste were received from the Alaskan U.S. Department of Agriculture Agricultural Research Service in a frozen, slurry form. Salmon slurry was composed of nineteen liters 2006 Pink salmon viscera (origin Alaska Pacific Seafood Cannery, Kodiak, Alaska), 19 L of 2006 red salmon heads (origin Alaska Pacific Seafood Cannery, Kodiak, Alaska), 19 L of 2008 pink salmon heads (origin Ocean Beauty Cannery, Kodiak, Alaska), and 19 L of 2008 pink salmon viscera (origin Ocean Beauty Cannery, Kodiak, Alaska). The utilization of two different salmon species and samples from a two year time period reduced the probability that the effects of the salmon on the experiments would be specific to one only one year or only one type of salmon, thus broadening the applicability of any results. This salmon was collected from processing facilities and frozen at the Fishery Industrial Technology Center in Kodiak, Alaska. It was thawed on October 28, 2008, mixed, and ground down to a 0.3 cm mesh slurry, retaining some bone. The slurry was then refrozen in buckets and sent to Palmer AFES on November 3, 2008. A response surface model (RSM) was used to design the experiments and analyze the collected data. The model was generated using Stat-Ease, Design Expert 7 software, and used salmon % in the pellets and airflow as the independent variables to build the optimization model and determine the experimentally derived optimum operating conditions for the downdraft gasifier based on the % salmon contained in the feedstock pellet (025% w/w) and the airflow into the gasifier (0.0021 to 0.155 m3/s), which enables the partial oxidative conditions needed to properly thermochemically convert the feedstocks into a producer gas. Optimum gasifier conditions were a function of the flame temperature for these experiments, which was used as the response parameter (°C) in the model. Salmon slurry and alder sawdust were mixed on a percent-by-weight basis in 5 kg batches, with salmon occupying between 0 and 25% of the mixture composition based on the RSM. Alder sawdust was weighed and placed into a Kushlan Model 350 WSB concrete mixer; then salmon slurry was weighed and slowly poured into the active mixer to prevent clumping. Each batch was rotated and stirred in the concrete mixer for approximately 15 min, ensuring homogeneity. Each individual mixture was placed into a labeled fivegallon bucket and sealed for storage until pelletizing. Feedstock fuel pellets were created using a 20 ton hydraulic press and a section of steel tubing approximately 3” in diameter and 6” long. To create a pellet, two cups of the salmon/alder 2293
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Energy & Fuels mixture were loaded into the steel tubing and the hydraulic press was used to compress the mixture into a feedstock fuel pellet that was then extruded out of the steel tubing. The average pressure applied to the mixture inside the tubing was 5000 psi, held for 15 s. All mixtures were pelletized with this method. Completed feedstock fuel pellets were loaded into paper bags for storage until use. Moisture content was determined for all raw materials, all mixtures, and all feedstock fuel pellets using ASTM D-444-92.34 All analysis was done in triplicate. Compositional analysis for alder was done using ASTM 1108-96 for extractives,35 ASTM D1106-96 for Klason lignin,36 and carbohydrates by difference. Compositional analysis for salmon was taken from the literature.1618 Elemental C, H, N percent composition was also determined for mixtures. 0.070.09 g samples were analyzed using a LECO Elemental Analyzer and ASTM D5373-93.37 Energy content was determined for all raw materials and mixtures. 0.30.6 g samples were pelletized using a California benchtop pelletizer and then analyzed using IKA C200 calorimeter and ASTM E711-87.38 The gasifier used was a commercially available Gasifier Experimenters Kit (GEK) by All Power Laboratories (Berkeley, CA). The GEK is a modified Imbert style downdraft gasifier. It is designed to gasify commercial wood pellets and is tubular in shape. Several modifications to the original GEK design were required for gasification of the experimental feedstock fuel. All modifications were fabricated and installed at the UAF Palmer facility. Gasifier air flow was measured with a Testo 405 stick anemometer. Air flows were tested at the top of the gasifier and were determined to be between 0.0021 m3/s with a 2.5 cm diameter opening and 0.0135 m3/s with a 15.2 cm diameter opening. The airflows specified by the RSM were achieved by running the gasifier fan and testing the varying airflows generated by different diameter openings. All airflows were tested with the Imbert nozzle hoses closed to help isolate and consolidate the airflow. High temperature K-type thermocouples were used to measure temperatures inside the gasifier. Temperature measurements were taken at the top of the reduction bell, at the base of the char bed, inside the gas cowling, inside the first stage scrubber, near the gas outlet, and inside the burner. These locations were chosen to analyze the gasification flame front and the gas temperature progression during operation. Thermocouples were attached to an Omega instruments TC-08 data logger and an Acer One portable computer running Omega Engineering Logging software version 5.2.6 to allow for continuous and simultaneous temperature measurement during operation. To reduce moisture in the produced gas, a water jacketed second stage scrubber was fabricated and installed to cool the gas and settle out moisture and heavy pyrolysis products. This scrubber used two 1.2 m long cooling jackets at 22 and 45 degree angles in the vertical direction and 90 degrees from each other to condense and collect the moisture of the producer gas. Water from the producer gas was collected and measured in a P trap and removed via a ball valve. The gasifier was designed for commercially available wood pellets and had to be modified to accept the salmon/alder feedstock fuel pellets. A tubular frame of wire mesh approximately 7.6 cm wide to match the diameter of the pellets and was secured to the reduction bell opening. The tubular frame was approximately 60.9 cm long which allowed it to span the distance from the reduction bell opening to the top of the gasifier. The tubular design allowed consistent loading of the feedstock fuel pellets during operation, and the wire mesh allowed air flow
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through the feedstock fuel pellets without affecting the air flow characteristics. Additional modifications to the gasifier unit included the construction of lighter aluminum radial fans and an electronic fan control unit to direct the producer gas to the burner. Operation of the gasifier and the collection of data was standardized and repeated for each trial. The gasifier was initiated using 400 g of hardwood charcoal placed inside the reduction bell. This charcoal layer was allowed to combust and evolve into a char bed with temperatures exceeding 900 °C. As the combustion process subsides, a decrease in the char bed temperature ensues, and it begins to stabilize. At this stage, the reactor is lined out, and the conditions affecting heat and oxidation do not fluctuate much, creating an environment suitable for introduction of new variables. Every trial run started during this stage, and when the temperature of the char bed decreased to 800 °C, approx 1.5 kg of feedstock fuel pellets were loaded into the reactor. As soon as the feedstock fuel pellets were added, a timer was started, a produced gas burner was lit, and a fresh jar for collecting moisture from the second stage scrubber was loaded. The gasification trial lasted until all of the feedstock fuel pellets had been consumed and the producer gas flame was selfextinguished. During this period, the flame temperature was measured constantly. Once the flame was extinguished, the timer was stopped, and the moisture collecting jar was replaced. Occasionally the flame would weaken or sputter due to a blockage or bridging of the feedstock fuel pellets, affecting the flow of feedstock fuel into the gasifier. This was addressed by briefly mechanically agitating the feedstock fuel pellets with a metal rod to break the blockage, with contact limited to the feedstock rather than the hot char bed. After the feedstock fuel bed had been reduced to a level below the reduction bell, where more feedstock could be introduced, the process was repeated. This process was done 13 times to capture the data required to satisfy the experimental design and response surface model.
’ RESULTS AND DISCUSSION Project requirements limited manipulation of the fish slurry, requiring it to be used as it was received. The high moisture was addressed by following research20,21 showing the benefits of using a drier matrix, but also ensuring complete incorporation of fish into the feedstock pellet production. Fresh alder (green) used in this study had a moisture content of approximately 25%, which was brought down to 4% for use. This stood in stark comparison to the salmon processing waste, which exhibited a moisture content of 70%. The various mixtures produced by the compounding of these two biomass raw materials ranged from 20.7% to 4% moisture. All of the mixtures were below 25% moisture, and as the percentage of salmon increased, so did the moisture content . Feed Selection. Analysis of the feedstocks in the first step of the development process revealed significant differences in energy content. The fresh salmon processing waste had an energy content of 9.2 kJ/g versus the energy content of the fresh alder at 14.6 kJ/g. When dried, the salmon processing waste had an energy content of 25.4 kJ/g and the alder an energy content of 19.1 kJ/g. The energy content of the mixture pellets ranged from 16 kJ/g to 18.7 kJ/g. The fluctuating energy content is attributed to the varying moisture and fish oil present in the mixtures. This relationship can be seen in Table 2. The effects of the moisture and fish oil were also seen during gasification in the form of condensate removed from the produced gas and the salmon 2294
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Table 2. RSM Trials with Associated Conditions and Mixture Compositions mixture
mixture
mixture
mixture
mixture
mixture
energy content(kl/g)
moisture content (%)
oxygen content (%)
carbon content (%)
hydrogen content (%)
nitrogen content (%)
trial
salmon (%)
alder (%)
air flow (m3/s)
1
12.5
87.5
0.0021
2
12.5
87.5
0.0088
3
12.5
87.5
0.0088
17.5
12.0
50.1
42.6
6.5
0.8
4
3.6
96.4
0.0041
18.5
4.4
47.5
45.8
6.4
0.6
5
0
0.0088
18.7
3.6
47.6
45.9
6.1
0.6
6 7
3.6 21.3
96.4 78.7
0.0135 0.0135
18.5 16.4
4.4 16.3
47.5 52.7
45.8 39.8
6.4 6.7
0.6 1.0
8
12.5
87.5
0.0088
17.5
12.0
50.1
42.6
6.5
0.8
9
12.5
87.5
0.0088
17.5
12.0
50.1
42.6
6.5
0.8
10
25
75
0.0088
16.0
20.8
52.9
39.3
5.8
1.1
11
12.5
87.5
0.0155
17.5
12.0
50.1
42.6
6.5
0.8
12
12.5
87.5
0.0088
17.5
12.0
50.1
42.6
6.5
0.8
13
21.3
78.7
0.0041
16.4
16.3
52.7
39.8
6.7
1.0
100
17.5 175
12.0
50.1
42.6
6.5
0.8
12.0
50.1
42.6
6.5
0.8
Table 3. Chemical composition of alder and other woods (normalized to 100%). All analysis utilize similar procedures and all compositions are shown on a dry basis wood
carbohvdrates (%)
Klason lignin (%)
extractives (%)
references
Alnus rubra - red alder
59.0
32.2
8.8
Pinus Ponderosa - ponderosa pine
62.8
30.4
6.7
40
Eucaiyptus giobuius - eucalyptus
63.8
26.6
9.6
41
Table 4. Characterization of Salmon Waste and Various Feedstocks Used in Meat Waste Gasification Research. a moisture
heating
ash (%, dry
protein (%,
fat (%,
carbohydrate
source
(%)
value (kJ/kg)
basis)
dry basis)
dry basis)
(%,dry basis)
salmon processing waste
76.9
25496
12.8
78.4
8.8
NA
pork processi ng waste specific risk material
72.6 27.0
26000 20089
10.5 26.1
20.5 NA
50.5 NA
18.5 NA
references
45 46
high risk material
33.2
18918
25.5
NA
NA.
NA
46
blood stuff
25.8
23265
1.4
NA
NA
NA
46
diary biomass
25.3
17185
20.1
NA
NA
NA
47
a
This table shows three different types of meat and bone meal (MBM). Specific risk material (SRM) is composed of bones and nervouse tissue. High risk material (HRM) is composed of any animal parts that are not SRM. Blood stuff (BS) is composed exclusively of dried animal blood.46 Dairy biomass (DB) is dairy cattle excrement and milking leftovers often described as a meat processing waste.47
content effect on the flame temperatures, suggesting that higher moisture could lead to a depression of the flame temperature. The compositional analysis for alder showed that the levels of extractives, lignin, and carbohydrates fit typical wood characteristics and indicated that the gasification performance of alder should be similar to that witnessed with other types of woods.2,7,39 A comparative analysis of wood compositions is shown in Table 3. The composition of the salmon waste suggested that it would be similar to that witnessed in other meat gasification research4244 and that the lipid content might be high enough to offset heat losses due to moisture. A comparative analysis of various feedstocks used in other meat gasification research is shown in Table 4. Elementally the nitrogen and oxygen increased with increases in the salmon content and the carbon content increased with increases in the wood content. This can also be seen in Table 2. Analysis of coastal resources suggested that fast growing, small-diameter
alder would be a practical, local, and renewable resource to provide the appropriate matrix to leverage salmon waste attributes for gasification. The relevance of the first step was in setting the baseline conditions and stand-alone raw material characterization that guided the analysis of the contribution of each raw material to the total value of the gasification system. Feed Pretreatment. The second step of the process was the feedstock compounding to meet the requirements of the gasifier and also the economic and sustainability goals of the project. This was achieved by using a common hydraulic press to manufacture large 7.6 cm diameter pellets, approximately 710 cm in length. The gasifier required 1.5 kg of pellets for each experiment, or approximately 10 of these individual pellets. The results of the characterization of these pellet mixtures shows that the pellets are compatible for use in a downdraft gasifier, and that the incorporation of salmon slurry with alder sawdust could be performed without the need for specialized equipment, which is 2295
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Figure 1. Trial downdraft gasifier design and gas cleanup system. The numbers represent the following parts of the gasifier; 1 is the gasifier opening and cap, 2 is the insulated reaction chamber (it has tubing coiled around it that brings air from the top of the gasifier to the bottom of the reaction chamber), 3 is the reduction bell (with a mesh pellet feeder), 4 is the char bed ash grate, 5 is the ash collection area, 6 is the entrance to the outer gas cowling, 7 is the exit to the outer gas cowling, 8 is the cyclone settler, 9 is the tar collection jar, 10 is the first stage scrubber (carbon and cinder filled), 11 is the fan/blower, 12 is the entrance to the second stage scrubber (water jacketed), 13 is the moisture collection jar, 14 is the gas outlet hose, and 15 is the burner.
Figure 2. Trial number and average reduction bell temperature vs average flame temperature.
advantageous for remote locations in Alaska with limited infrastructure and resources. Gasification. The third step in the process was proving the operation of the modified downdraft gasifier with the novel feedstock. The gasifier and gas cleanup design can be seen in Figure 1. The gasifier was operated as a batch reactor, where each pellet mixture was introduced separately and data collection in each of the 13 individual experiments was carried out, noting the behavior of the reduction bell and temperature of the producer gas flame, as well as for the other sites where thermocouples were installed. Data from the reduction bell enabled the determination
of the state of thermochemical conversion occurring at any given time. As gasification in a downdraft fixed bed reactor involves the initial thermochemical breakdown of the feedstock pellets into pyrolysis products above the reduction bell region, monitoring the reduction bell temperature was a direct indicator of the degree of thermochemical conversion during partial oxidation or gasification. The char bed temperature averages, as shown in Figure 2, had periods in which active gasification and nonactive gasification processes happened. Active gasification was defined as a period of producer gas flame generation between a lineout and a self-extinguishing flame. The nonactive gasification periods 2296
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Energy & Fuels were a function of the gasifier being operated as a batch reactor. More seamless active gasification periods are achievable when the reactor is operated using a continuous feedstock input rather than as a batch unit. This experimental design required a batch input for each feedstock trial to support analysis, but if scaling or commercialization of this approach were to occur, an appropriate continuous feeding mechanism would need to be developed. There were 13 gasification trials based on the RSM. It is important to note that experimental runs were staged in such a way that the salmon % of the pellets increased from the initial runs, which were done with pellets containing no salmon. The reason for this was to test the behavior of the gasifier as more salmon wastes were introduced, and to remove any variability that may exist from alternating between high salmon content and low salmon content pellet feedstocks. Proper gasification of biomass in these reactor systems operated as a batch, occurs as a cycle between partial oxidation (gasification) and full oxidation of biomass (combustion). Close inspection of the relationships between the reduction bell curves and the producer flame temperature curves in Figure 2 shows that every time feedstock fuel pellets were added, the reduction bell temperature dropped, and the producer gas flame temperature increased. The inverse is true, as well. This occurred because when feedstock fuel pellets were added, the exposure of the reduction bell to air was reduced, leading to pyrolysis and partial oxidation. Correspondingly, the flame temperature increased dramatically as a result of this gasification, and the generation of a combustible gas proceeded. As the feedstock pellets were consumed, increasing oxidative conditions entailed, resulting in an increase in the reduction bell temperatures. Once combustion became the primary reaction, no producer gas formed, and the flame self-extinguished. When the temperature in the reduction bell rose, it signified the oxidative consumption of the combustible products by the char bed, and the flame temperature would drop due to the reduced availability of these combustible gases. These trends were remarkably consistent unless bridging occurred. When the reaction was complete, the char bed in the reduction bell would migrate upward toward the air supply at the surface, stopping the production of combustible gas and increasing the temperature of the reduction bell while extinguishing the flame. Carrying out the experiments on a scale equal to that which could be practiced in the regions of study was an invaluable aspect of the data generated. Additionally, the use of a downdraft gasifier and effective but simple gas cleanup processes were important for generating a gas product that could be used to support operation of reciprocating engines and boilers. Optimization. The fourth step of the process was the optimization of the entire gasification system to determine the effects of the independent variables on the performance of the system, namely, the salmon percentage of the pellet feedstock and the volume of air flow. Optimization was done using the RSM to establish a statistically significant framework based on experimental data, leading to the determination of the process relationships needed to understand the behavior of the gasification system as the independent variables changed from low salmon concentrations to high and from low airflows up to the unrestricted maximum flow available through this downdraft gasifier design. In doing so, and by measuring the resulting producer gas flame temperature, the optimal feedstock pellet composition and air flow to work with a given feedstock were determined. The experimental results of the 13 gasifier trials were used to construct a first order design, in which linear regression was
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used to identify areas of nonlinear response. This indicated whether the experimental variables were properly designed, and identified the region of the independent variables where curvature or a deviation from linear response was present. It is in this nonlinear region where the determination of the optimal range of experimental conditions was discovered by constructing a second order analysis using higher models other than linear (i.e., squared, quadratic and cubic). The second order analysis produced a 3D surface map that detailed the changes in the measured temperature response as a result of varying the independent variables in a continuum scale, within the limits of the experimental setup. The first order design showed a lack of fit, or the presence of areas of nonlinearity. The subsequent higher order analysis yielded the best model fit with the observed experimental data to be the quadratic model. The resulting R squared value for the quadratic model was 0.80 supporting the validity of the model and experimental data generated. The analysis of variance for this model showed an F value of 5.67 for the response, meaning it had significantly explained the observed variance and had low statistical independence. For the model variables, air flow had an F value of 16.56 compared to the salmon percentage that had an F value of 0.64, meaning that the air flow generally elicited the strongest and most direct response from the model compared to the salmon content. A normal plot of residuals also showed that the temperature results for the flame showed very little variance from the ideal line. A second order analysis was performed using the quadratic terms in the model and applying canonical variables, where the zero point, or point of origin, is shifted and rotated in a coordinate axis to match the second order surface. The stationary point is the zero point for our canonical variables v, as expressed in the following equation: f ðv1 , v2 Þ ¼ f vð0Þ þ λv1 þ λv2 þ λv1 2 v2 2 þ λv1 2 þ λv2 2 ð1Þ Where vx are the design variables expressed as canonical coordinates, f (vx) is the response as a function of the canonical variables and λ are the coefficient of the quadratic terms that indicate the vector of the function. In actual terms, the equation for the optimization of salmon and small diameter alder gasification is: flame temp ¼ 183:34 þ 4:35ðfishÞ þ 58853:32ðairflowÞ þ 210:03ðfish airflowÞ 0:311ðfish2 Þ 2:64e6ðairflow 2 Þ ð2Þ The second order model was used to build a contour map with 125 interval points for the response surface surrounding the center points, and limits set by the R-value (0.05) as shown in Figure 3. The figure shows the 3D model depicting the overlaid response data for both independent variables. This 3D graph shows that the optimal conditions to generate a high temperature flame are a salmon content in the range of 12.521.3% and airflow of around 0.0135 m3/s. The lowest flame temperatures would be generated by the extremes of airflow, salmon and their combination. The region of optimal performance encompassed a greater proportion of salmon, in accordance with the statistically derived F value. The practical interpretation of the salmon contribution to the gasification system is that the gasifier setup can handle additional salmon better than operating with too little or too much oxidant. Analysis showed the statistical relevance of airflow in the model and, correspondingly, airflow must be carefully adjusted 2297
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Figure 3. 3D contour map generated from the response surface model and depicting the overlaid temperature response data for the independent variables; air flow (m3/s) and fish (%).
to keep the gasification system performing under ideal conditions. By contrast, changes to feedstock had more subtle effects that may have been diluted as the produced gas passed through the gas scrubbing system, homogenizing its composition through the removal of condensables. This allows for more flexibility regarding the amount of salmon waste in the feedstock and is of paramount importance, as the experimental results and the optimization analysis suggest that effective gasification of salmon wastes can occur with feedstocks containing up to 18% salmon by weight before a measured reduction in performance is observed. These results agree with gasification fundamentals regarding the delicate balance between the steps of depolymerization, volatilization, partial oxidation, and complete oxidation. Thermal decomposition in a gasifier is endothermic until combustion is achieved through complete oxidation and a certain level of exothermic release is generated. Consequently, choosing how much oxidant and feedstock to introduce and in what combination can drastically affect the level of exothermic release that supports the endothermic conversions. The optimization and analysis support the importance of this balance in showing that there is indeed an optimum combination of oxidant and feedstock for producing a hotly burning syngas.
’ CONCLUSIONS Utilization of salmon processing waste in a downdraft fixed bed gasifier was demonstrated as feasible, generating a new function for a waste resource that, due to existing legislation, may soon require different methods for disposal.4851 Current disposal methods in the United States are regulated by the Magnuson-Stevens Fishery Conservation and Management Act and the Marine Protection, Research, and Sanctuaries Act. As
these laws are updated to reflect new methods and technology for salmon processing waste utilization, discharge of these wastes into the ocean will become more difficult. By showing that salmon processing waste is a feasible feedstock for gasification and that this conversion can be achieved using practical and available resource in coastal areas, we provide another alternative to “slurry lines”. The conversion of fish wastes into a functional fuel gas is viable and creates an energy option that can be considered alongside fish meal and fish oil options. Through this development, salmon processing waste may someday play a role in supporting the Alaskan fishing industry that generates it. Feed selection showed that salmon waste slurry could be utilized if it was blended with a drier, locally available biomass like alder. Feed pretreatment demonstrated that adequate pellets could be made using a hydraulic press rather than a mechanically complex pelletizer. These pellets were proven to be compatible for use in a downdraft gasifier and verified that incorporation of salmon slurry with alder sawdust could be performed without the need for specialized equipment. These characteristics of the process are advantageous for remote areas with limited infrastructure and resources. Gasification established that all trials had similar trends, supporting performance comparisons. The gasification experiments also provided performance data at a scale similar to that which could be practiced in the regions of study, showing the feasibility of using of a downdraft gasifier and an effective but simple gas cleanup processes for generating a gas product that could be used to support operation of reciprocating engines and boilers. Optimization of the gasification system demonstrated that salmon wastes can be introduced as a feedstock for gasification up to 18% on a mass basis of the pellet, before negatively impacting the system performance. The gasification system was more sensitive to airflow variations, showing 2298
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Energy & Fuels optimal conditions at 0.0135 m3/s, for feedstocks containing between 12.5 and 18% salmon. The data suggests that the simple gas cleanup mechanism designed and constructed to remove moisture from the producer gas stream prior to producer gas combustion could provide design flexibility regarding feedstock resource requirements. This research improves on the utilization of this waste versus an updraft gasifier, because it produces a higher quality gas. With Alaska's seafood industry generating approximately 100 000 t of salmon processing waste a year,16,17 the success of this project creates a valuable opportunity for improving the economics of disposal. In support of the remote and fragmented fisheries activities in Alaska, the research was able to leverage locally available, under-utilized resources to produce an alternative chemical energy source that is both compatible with existing infrastructure and potentially sustainable. The successful gasification of salmon processing waste by this project also provides an example of the value of experimental modifications to commercially available equipment, coupled with modeling tools designed to investigate optimal reactor conditions.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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