WindFuels – Competitive Transportation Fuels From Wind Energy and Waste CO 2
It is essential for the market to help drive the dramatic cut needed in CO2 emissions to prevent a climate disaster in this century. We show that the economics for producing clean liquid hydrocarbons and alcohols from water and waste CO2 on wind farms improved by an order of magnitude between 2002 and 2008, and we present the scientific and technological basis for another factor-of-two improvement in these economics. Wind energy is by far the most competitive renewable energy resource in many regions. The perceived challenge is getting wind energy from good sites to where and when it is needed, both for the transportation sector and for the power grid. Efficient conversion of wind energy and waste CO2 into clean, stable, liquid fuels – also called WindFuels – solves these problems. Annual WindFuels production per land area in good wind regions should exceed biofuels production density in fertile farming areas by a factor of five. First, water and renewable electrical power are fed into an electrolyzer, which produces the required hydrogen. Then waste CO2 (from coal power plants) and the renewable hydrogen are piped into a novel Reverse Water Gas Shift (RWGS) process, which permits practical reduction of CO2 to the CO needed at efficiencies approaching theoretical limits. The renewable CO and H2 are then fed into a novel Renewable Fischer-Tropsch Synthesis (RFTS) process. There they are converted into mostly mid-alcohols (ethanol, propanol, and butanol) and other valuable chemicals. These fuels may then be readily stored and distributed by conventional means – pipelines and tanker trucks. The electrolyzer also produces a huge amount of oxygen, which may be sold if market conditions warrant, or it may be utilized in novel processes to improve the plant efficiency. From basic thermodynamics, one readily calculates that the theoretical maximum chemical efficiency of synthesis of ethanol from H2 and CO2 is 80.1% without utilization of the excess heat released from the Fischer Tropsch Synthesis (FTS) reactor. If this excess heat is also converted in an ideal heat engine, the theoretical combined-cycle efficiency limit is ~90%. One advantage of ethanol may be appreciated by noting that its synthesis from H2+CO2 results in 1.5 molecules of water per carbon atom in the fuel, while the synthesis of alkanes or alkenes results in 2 molecules of waste water per carbon atom in the fuel. Previously, fossil-based FTS efficiencies for environmentally attractive fuels, such as ethanol and propanol, have usually been under 40%. The need for very efficient recycling of the large amounts of H2, CO, and CO2 in the byproducts from mid-alcohols FTS has possibly been the strongest argument against mid-alcohols FTS compared to gasoline, lubricants, and diesel. A novel plant design is presented, validated by many simulations, that permits order-of-magnitude reduction in energy penalties associated with the major separations in a fully recycled mid-alcohols plant compared to previously published designs. The example wind-driven RFTS plant size chosen for illustration and simulated in detail assumes 250 MW mean input electrical power. It achieves 72% FTS-plant higher heating value (HHV) efficiency in production of mid-alcohols and other products from H2 and recovered (waste) CO2, or about 60% net HHV efficiency when including the electrolyzer at expected near-term performance. At least eight separate, substantial innovations in the system design combine to permit this major advance in RFTS efficiency and cost effectiveness. It is worth pointing out that as recycle efficiency and byproduct utilization both approach 100%, which is the primary thrust in the RFTS optimization, catalyst selectivity and activity both become much less important. Catalysts never affect the thermodynamics – only the kinetics of the various pathways. Hence, further development of catalysts will have limited effect on system efficiency in a plant with 95% recycle efficiency, though of course it will influence profitability by changing the product mix.
The microtube strip heat exchanger
The advantages of designing heat exchangers in the laminar-flow regime are discussed from a theoretical standpoint. It is argued that laminar-flow designs have the advantages of reducing thermodynamic and hydrodynamic irreversibilities, and hence increasing system efficiency. More concretely, laminar-flow heat exchangers are free from the turbulence-induced vibration common in conventional heat exchangers, and can thus offer longer life and greater reliability. The problems of manufacturing heat exchangers suited to laminar flow are discussed. A method of manufacture is outlined that allows compact, modular design. Experience with this method of manufacture is described. Experimental results with a prototype heat exchanger bank are presented: these results show good agreement with theory at moderate levels of effectiveness (75--85%), but fall below predicted values at higher levels. It is argued that this discrepancy results from flow maldistribution. The problem of fouling and flow maldistribution are briefly discussed, and some possible applications are mentioned.
Exposure to violence in incarcerated youth from the city of São Paulo.
OBJECTIVE
This study aimed to determine the extent of exposure to community violence among delinquent Brazilian youth in the 12-month period prior to their incarceration and to identify factors associated with this exposure.


METHOD
With an oversampling of girls, a cross-section of youth under 18 years of age from juvenile detention units in the city of São Paulo, Brazil completed a structured interview. Key items related to exposure to violence (witnessed and experienced) were drawn from the Social and Health Assessment questionnaire to cover the 12-month period prior to incarceration.


RESULTS
Participants (n = 325, 89% boys) reported high rates of exposure to violence with largely similar levels for boys and girls. Being threatened with physical harm, being beaten or mugged and/or shot at were the most common forms of violence experienced. After controlling for demographic and family variables, the fact of having peers involved in risk behavior, easy access to guns and previous involvement with the justice system were associated with witnessed violence; whereas having slept on the street was the only variable associated with experienced violence.


CONCLUSION
This group of youth was exposed to high levels of violence and other adverse experiences. Future research should examine the effectiveness of strategies aimed at reducing the exposure to violence of high-risk youth.