Compared with FTL or DME synthesis, the technology for synthesis of methyl‐, ethyl‐, and propanol‐mixed alcohols (MA) is considerably less commercially advanced, and there is sparse published literature on which to base detailed reactor performance estimates. Catalysts that have been examined in the past can be divided into four categories (Liu et al. 1997): ruthenium‐based catalysts, modified methanol catalysts, modified Fischer–Tropsch catalysts, and molybdenum sulfide–based catalysts. Among these, the MoS2‐based catalysts (originally discovered by researchers at Dow and Union Carbide in the 1980s) have received considerable recent attention due to their high tolerance for sulfur‐contaminated syngas, their water–gas shift activity, and their high activity and selectivity for linear alcohols. Selectivity is an especially important characteristic because if all possible chemical reactions between CO and H2 are allowed to compete without constraints, reactions other than those for synthesis of higher alcohols will thermodynamically out‐compete reactions for synthesis of higher alcohols. In particular, the formation of Fischer–Tropsch hydrocarbons (α‐olefins and n‐paraffins) from CO and H2 is thermodynamically favored over the formation of higher alcohols. For this reason, to maximize performance of alcohol synthesis catalysts, high selectivity is an essential feature.
The simplified reactions scheme adopted for the higher alcohols are as follows:
“WTW” Environmental Impact of Black Liquor Gasification
In addition to energy aspects of the biorefinery systems discussed earlier, we have also examined environmental attributes. Water effluents, air emissions, and solid wastes are all of potential concern. In assessing the impact that biorefinery systems would have on these effluents relative to levels found with Tomlinson power/recovery systems, one may consider changes both in direct effluents and in effluents associated with the displacement of grid electricity generation and conventional petroleum‐based motor fuels. In particular, to effectively estimate the full environmental impacts of biorefineries, the current analysis involves estimating the emissions impacts from resource extraction to end use. This so‐called WTW analysis is a common approach for making meaningful comparisons between different alternative and conventional fuels. This approach is necessary because of the different upstream production and conversion processes, different downstream vehicle/engine types for different fuels and significant differences in fuel properties and combustion characteristics.
Biofuels which are produced from BLG process excel in terms of WTW carbon dioxide emission reduction and energy efficiency. Also, synthetic diesel and DME produced from forest harvest residues over the BLG route both showed among the highest WTW GHG reduction and energy efficiency. The total available black liquor volume in the United States with the conversion efficiency of this process is equivalent to approximately 5 billion gal/year as ethanol. The renewable fuels standard calls for 16 billion gal of cellulosic biofuels by 2022. Therefore, this route can give a significant contribution to meeting this target.
The pulp and paper companies in the United States today are meeting severe competition from low‐cost producers overseas and from alternative solutions in both packaging and printed media. Mill operators and their investors now option to transform mills into biorefineries that use this fuels‐from‐the‐forest process. This transformation completely changes a pulp mill’s competitive position by adding 30–50% of profitable revenue with the typical 25–40% internal rate of return. It also makes needed reinvestment possible by replacing aged recovery boilers with high‐maintenance costs and low performance. The fuel plant investment can also be used to provide additional recovery capacity, allowing for higher pulp production in many cases. Mills producing only 500 T of BLS per day are viable as fuels‐from‐the‐forest biorefineries are using this method. Most mills are significantly larger. Such a biorefinery mill would produce upward of 8 million gal a year of green motor fuel calculated as gasoline equivalents at the minimum capacity size. In a mill investing in second‐generation biofuels technology, jobs are not only preserved but also additional jobs are created, mainly for extraction of biomass from the forest as well as to operate and maintain the biofuels plant. Other economics and social benefits are also significant such as possible tax benefits and air emissions reductions (Bajpai 2014).
Typical capital investment for a biorefinery project that uses fuels from the forest is $200–400 million, depending on plant size and the cost to interconnect to the mill. The BLG industry is vigorously pursuing federal and state grants and loan guarantees to ramp up this technology as soon as possible to large‐scale commercial capacity. As a source of ultra‐clean, renewable motor fuels, the BLG route that transform pulp and paper mills into refineries is standing up to a critical scrutiny as a viable and practical way of producing alternative, renewable energy, while making good use of the land and being gentle to the environment (Bajpai 2014).
Forest biorefinery utilizing gasification in a BLGCC configuration rather than a Tomlinson boiler is predicted to produce significantly fewer pollutant emissions due to the intrinsic characteristics of the BLGCC technology. Syngas cleanup conditioning removes a considerable amount of contaminants and gas‐turbine combustion is more efficient and complete than boiler combustion. There could also be reductions in pollutant emissions and hazardous wastes resulting from cleaner production of chemicals and fuels that are now manufactured using fossil energy resources. In addition, it is generally accepted that production of power, fuels, chemicals, and other products from biomass resources creates a net zero generation of carbon dioxide, as plants are renewable carbon sinks. A key component of the forest biorefinery concepts is sustainable forestry. The forest biorefinery concept utilizes advanced technologies to convert sustainable woody biomass to electricity and other valuable products and would support the sustainable management of forest lands (Farmer 2005). In addition, the forest biorefinery offers a productive value‐added use for renewable resources such as wood thinning and forestry residues and also urban waste (Mabee et al. 2005; Miller et al. 2005).
BLG whether conducted at high or low temperatures is still superior to the current recovery boiler combustion technology (Bajpai 2008, 2012). The thermal efficiency of gasifiers is estimated to be 74% compared to 64% in modern recovery boilers, and the integrated gasification and combined cycle (IGCC) power plant could potentially generate same amount of fuel (Dance 2005; Farmer and Sinquefield 2003). While the electrical production ratio of conventional recovery boiler power plants is 0.025–0.10 MWe/MWt, the IGCC power plant can produce an estimated 0.20–0.22 MWe/MWt (Farmer and Sinquefield 2003; Sricharoenchaikul 2001). This increase in electrical efficiency is significant enough to make pulp and paper mills potential exporters of renewable electric power. Alternatively, pulp mills could become manufacturers of bio‐based products by becoming biorefineries. Additionally, the new technology could potentially save more than 100 trillion Btu of energy consumption annually, and within 25 years of implementation, it could save up to 360 trillion Btu/year of fossil fuel energy (Larson et al. 2003). The new technology also offers the benefits of improved pulp yields if alternative pulping chemistries are included and reductions in solid waste discharges. Also, the process is inherently safer because the gasifier does not contain a bed of char smelt unlike in recovery boilers, which reduces the risk of deadly smelt–water explosions (Argonne National Laboratory 2006; Sricharoenchaikul 2001).
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