Tom L. Richard

Challenges in Scaling Up Biofuels Infrastructure

Rapid growth in demand for lignocellulosic bioenergy will require major changes in supply chain infrastructure. Even with densification and preprocessing, transport volumes by mid-century are likely to exceed the combined capacity of current agricultural and energy supply chains, including grain, petroleum, and coal. Efficient supply chains can be achieved through decentralized conversion processes that facilitate local sourcing, satellite preprocessing and densification for long-distance transport, and business models that reward biomass growers both nearby and afar. Integrated systems that are cost-effective and energy-efficient will require new ways of thinking about agriculture, energy infrastructure, and rural economic development. Implementing these integrated systems will require innovation and investment in novel technologies, efficient value chains, and socioeconomic and policy frameworks; all are needed to support an expanded biofuels infrastructure that can meet the challenges of scale.

Moisture Relationships in Composting Processes

Moisture is a key environmental factor that affects many aspects of the composting process. Biodegradation kinetics are affected by moisture through changes in oxygen diffusion, water potential and water activity, and microbial growth rates. These relationships are made more complex by the dynamic nature of the composting process, with changes in particle size and structure occurring over time. A deductive model of the effects of moisture on composting kinetics has defined these relationships based on fundamental physical properties and biological mechanisms. This study applies this model to experimental data from a manure and papermill sludge composting system. The results demonstrate that the optimum moisture content for biodegradation can vary widely for different compost mixtures and times in the composting process, ranging from near 50 to over 70% on a wet basis. While there is a significant reduction in biodegradation rate when operating outside the optimum range, the results also suggest opportunities to mitigate this effect through manipulation of substrate density and particle size. This framework for engineering analysis demonstrates the importance and challenges of maintaining optimum moisture content in dynamic composting systems, where biological drying, metabolic water production, and changes in compaction and porosity are all occurring over time.

Simultaneous Cellulose Degradation and Electricity Production by Enterobacter cloacae in a Microbial Fuel Cell

ABSTRACT Electricity can be directly generated by bacteria in microbial fuel cells (MFCs) from many different biodegradable substrates. When cellulose is used as the substrate, electricity generation requires a microbial community with both cellulolytic and exoelectrogenic activities. Cellulose degradation with electricity production by a pure culture has not been previously demonstrated without addition of an exogenous mediator. Using a specially designed U-tube MFC, we enriched a consortium of exoelectrogenic bacteria capable of using cellulose as the sole electron donor. After 19 dilution-to-extinction serial transfers of the consortium, 16S rRNA gene-based community analysis using denaturing gradient gel electrophoresis and band sequencing revealed that the dominant bacterium was Enterobacter cloacae. An isolate designated E. cloacae FR from the enrichment was found to be 100% identical to E. cloacae ATCC 13047T based on a partial 16S rRNA sequence. In polarization tests using the U-tube MFC and cellulose as a substrate, strain FR produced 4.9 ± 0.01 mW/m2, compared to 5.4 ± 0.3 mW/m2 for strain ATCC 13047T. These results demonstrate for the first time that it is possible to generate electricity from cellulose using a single bacterial strain without exogenous mediators.

Carbon, nutrient, and mass loss during composting

Hoop manure (a mixture of partially decomposed pig manure and cornstalks from swine fed in hoop structures) was the subject of a nitrogen mass balance during the feeding period. The manure was then composted in windrows to investigate C, nutrient, and mass loss during the composting process. Feeding cycle mass balance results indicated that N losses from the bedded pack ranged from 24 to 36%. Composting treatments included construction with and without a manure spreader and subsequent management with and without turning. Significantly greater losses of mass, C, K, and Na were found in the turned windrow treatment. However, composting in turned windrows proceeded at a much faster rate, with temperatures dropping out of the thermophilic range within 21 days. Composting without turning was less rapid, with temperatures remaining in the thermophilic range to the end of the 42-day trial. Mass reduction and C loss was significantly higher in the turned windrows than in the unturned windrows. Nitrogen loss was between 37 and 60% of the initial N, with no significant effect from turning. It appears that the low initial C:N ratio (between 9:1 and 12:1) was the most critical factor affecting the N loss in this composting process. Phosphorus, K, and Na losses were also high during composting, which could be due to runoff and leaching from the hoop manure. These elements may be significant contributors to surface and groundwater pollution through runoff and leaching. Additional research is planned to understand the extent of losses through volatilization, runoff, and leaching during composting.

Air-filled porosity and permeability relationships during solid-state fermentation.

An experimental apparatus was constructed to measure the structural parameters of organic porous media, i.,e. mechanical strength, air‐filled porosity, air permeability, and the Ergun particle size. These parameters are critical to the engineering of aerobic bioconversion systems and were measured for a straw‐manure mixture before and after 13 days of in‐vessel composting. Porosity was measured using air pycnometry at four (day 0) and five (day 13) moisture levels, with each moisture level tested at a range of different densities. Tested wet bulk densities varied with moisture level, but dry bulk densities generally ranged from 100 to 200 kg m−3. At each moisture/density combination, pressure drop was measured at airflow rates ranging from 0.001 to 0.05 m sec−1, representing the range of airflow rates found in both intensive and extensive composting. Measured air‐filled porosities were accurately predicted from measurements of bulk density, moisture, and organic matter content. Reductions in air‐filled porosity at increasing moisture content were accompanied by an increase in permeability, apparently due to aggregations of fines. This aggregation was quantified by calculating an effective particle size from the Ergun permeability relationship, which increased from 0.0002 m at 50% moisture to 0.0021 m at 79% moisture. The range of airflow velocities reported in composting systems requires consideration of the second‐order drag force term, particularly at velocities approaching 0.05 m s−1 for the higher moisture treatments tested. Calculated permeabilities for the matrix ranged from 10−10 to 10−7 m2, varying with both air‐filled porosity and moisture. Mechanical strength characterization provided a means to predict the effects of compaction on air‐filled porosity and permeability of porous media beds. The results of this investigation extend porous media theory to the organic matrices common in solid‐state fermentations and help build a framework for quantitative and mechanistic engineering design.

Take a Closer Look: Biofuels Can Support Environmental, Economic and Social Goals

The US Congress passed the Renewable Fuels Standard (RFS) seven years ago. Since then, biofuels have gone from darling to scapegoat for many environmentalists, policy makers, and the general public. The reasons for this shift are complex and include concerns about environmental degradation, uncertainties about impact on food security, new access to fossil fuels, and overly optimistic timetables. As a result, many people have written off biofuels. However, numerous studies indicate that biofuels, if managed sustainably, can help solve pressing environmental, social and economic problems (Figure 1). The scientific and policy communities should take a closer look by reviewing the key assumptions underlying opposition to biofuels and carefully consider the probable alternatives. Liquid fuels based on fossil raw materials are likely to come at increasing environmental cost. Sustainable futures require energy conservation, increased efficiency, and alternatives to fossil fuels, including biofuels.

Determination of thermal properties of composting bulking materials

Thermal properties of compost bulking materials affect temperature and biodegradation during the composting process. Well determined thermal properties of compost feedstocks will therefore contribute to practical thermodynamic approaches. Thermal conductivity, thermal diffusivity, and volumetric heat capacity of 12 compost bulking materials were determined in this study. Thermal properties were determined at varying bulk densities (1, 1.3, 1.7, 2.5, and 5 times uncompacted bulk density), particle sizes (ground and bulk), and water contents (0, 20, 50, 80% of water holding capacity and saturated condition). For the water content at 80% of water holding capacity, saw dust, soil compost blend, beef manure, and turkey litter showed the highest thermal conductivity (K) and volumetric heat capacity (C) (K: 0.12-0.81 W/m degrees C and C: 1.36-4.08 MJ/m(3) degrees C). Silage showed medium values at the same water content (K: 0.09-0.47 W/m degrees C and C: 0.93-3.09 MJ/m(3) degrees C). Wheat straw, oat straw, soybean straw, cornstalks, alfalfa hay, and wood shavings produced the lowest K and C values (K: 0.03-0.30 W/m degrees C and C: 0.26-3.45 MJ/m(3) degrees C). Thermal conductivity and volumetric heat capacity showed a linear relationship with moisture content and bulk density, while thermal diffusivity showed a nonlinear relationship. Since the water, air, and solid materials have their own specific thermal property values, thermal properties of compost bulking materials vary with the rate of those three components by changing water content, bulk density, and particle size. The degree of saturation was used to represent the interaction between volumes of water, air, and solids under the various combinations of moisture content, bulk density, and particle size. The first order regression models developed in this paper represent the relationship between degree of saturation and volumetric heat capacity (r=0.95-0.99) and thermal conductivity (r=0.84-0.99) well. Improved knowledge of the thermal properties of compost bulking materials can contribute to improved thermodynamic modeling and heat management of composting processes.

Compost mineralization in soil as a function of composting process conditions

Compost has been shown to have a range of positive impacts on soil quality and can provide an important source of nutrients for plants. While these benefits have been documented for many finished composts, there is presently little understanding of the impact of composting process conditions and the extent of compost decomposition on soil C and N mineralization after compost incorporation. This study evaluated the impact of composting process conditions and the extent of compost decomposition on soil C and N mineralization after compost incorporation. Dried, ground composts were blended with equal parts of quartz sand and soil and incubated aerobically for 28 d at 30 °C. Cumulative respired CO 2‐C and net mineralized N were quantified. Results indicate that (1) organic substrates that did not degrade due to sub-optimal conditions during the composting process can readily mineralize after incorporation in soil; (2) C and N cycling dynamics in soil after compost incorporation can be affected by compost feedstock, processing conditions, and time; and (3) denitrification after compost incorporation in soil can limit N availability from compost.

Energy Use and Greenhouse Gas Emissions from Crop Production Using the Farm Energy Analysis Tool

Using the Farm Energy Analysis Tool (FEAT), we compare energy use and greenhouse gas (GHG) emissions from the cultivation of different crops, highlight the role of sustainable management practices, and discuss the impact of soil nitrous oxide (N2O) emissions and the uncertainty associated with denitrification estimates in the northeastern United States. FEAT is a transparent, open-source model that allows users to choose parameter estimates from an evolving database. The results show that nitrogen fertilizer and N2O emissions accounted for the majority of differences between crop energy use and GHG emissions, respectively. Integrating sustainable practices such as no tillage and a legume cover crop reduced energy use and GHG emissions from corn production by 37% and 42%, respectively. Our comparisons of diverse crops and management practices illustrate important trade-offs and can inform decisions about agriculture. We also compared methods of estimating N2O emissions and suggest additional research on this potent GHG.

Enzymatic hydrolysis of cellulose coupled with electricity generation in a microbial fuel cell

Electricity can be directly generated by bacteria in microbial fuel cells (MFCs) from a variety of biodegradable substrates, including cellulose. Particulate materials have not been extensively examined for power generation in MFCs, but in general power densities are lower than those produced with soluble substrates under similar conditions likely as a result of slow hydrolysis rates of the particles. Cellulases are used to achieve rapid conversion of cellulose to sugar for ethanol production, but these enzymes have not been previously tested for their effectiveness in MFCs. It was not known if cellulases would remain active in an MFC in the presence of exoelectrogenic bacteria or if enzymes might hinder power production by adversely affecting the bacteria. Electricity generation from cellulose was therefore examined in two‐chamber MFCs in the presence and absence of cellulases. The maximum power density with enzymes and cellulose was 100 ± 7 mW/m2 (0.6 ± 0.04 W/m3), compared to only 12 ± 0.6 mW/m2 (0.06 ± 0.003 W/m3) in the absence of the enzymes. This power density was comparable to that achieved in the same system using glucose (102 ± 7 mW/m2, 0.56 ± 0.038 W/m3) suggesting that the enzyme successfully hydrolyzed cellulose and did not otherwise inhibit electricity production by the bacteria. The addition of the enzyme doubled the Coulombic efficiency (CE) to CE = 51% and increased COD removal to 73%, likely as a result of rapid hydrolysis of cellulose in the reactor and biodegradation of the enzyme. These results demonstrate that cellulases do not adversely affect exoelectrogenic bacteria that produce power in an MFC, and that the use of these enzymes can increase power densities and reactor performance. Biotechnol. Bioeng.