Chemistry: molecular biology and microbiology – Micro-organism – tissue cell culture or enzyme using process... – Preparing element or inorganic compound except carbon dioxide
Reexamination Certificate
1998-08-07
2002-01-29
Marx, Irene (Department: 1651)
Chemistry: molecular biology and microbiology
Micro-organism, tissue cell culture or enzyme using process...
Preparing element or inorganic compound except carbon dioxide
C435S167000, C210S203000, C210S920000
Reexamination Certificate
active
06342378
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improved two phase anaerobic digestion having separated hydrolysis and biogasification reactors which convert biomass to desired methane product gas with high efficiency.
BACKGROUND OF THE INVENTION
Anaerobic digestion has been known to stabilize sludge and other predominantly organic materials, and usable product gas, of varying composition, has been obtained from such anaerobic digestion processes. The organic feed mixture which provides the substrate for anaerobic biodegradation can comprise a wide variety of organic carbon sources, ranging from raw sewage sludge to municipal refuse, or biomass material such as plants and crop wastes. The process of anaerobic digestion degrades any of these organic carbonaceous materials, under appropriate operating conditions, to product gas which contains the desirable methane gas.
Anaerobic digestion uses a consortium of natural bacteria to degrade and then convert an organic substrate into a mixture of carbon dioxide and methane. The existing anaerobic digestion systems for organic substrate digestion can be separated into two major types, one phase systems and two phase systems. Existing one phase systems include the batch digester, completely mixed digester and the plug flow digester. These one phase systems, in which the organic substrate and the microorganisms are housed together are easy to operate and of low cost. Completely mixed digesters and plug flow digesters require continuous handling of feedstock and do not operate in batch mode. Further, the biogas produced in one phase systems consists primarily of carbon dioxide in the early stages of digestion. The high carbon dioxide content of the biogas is attributable to the slow growth of the methanogenic microorganisms and their inhibition by high concentrations of volatile fatty acids (VFAs). In order to reduce the inhibition of the microorganisms by the VFAs, the two phase digester has been introduced.
Separated two phase anaerobic digestion systems have been found to enhance the conversion efficiency, such as described in Pohland and Ghosh,
Biotechnol. and Bio-eng. Symp
. No. 2, 85-106 (1971), John Wiley and Sons, Inc. and by the same authors in
Environmental Letters
, 1: 255-266 (1971). A typical two phase anaerobic digester system comprises an acid phase digester and a biogasification reactor. The acid phase digester is usually designed as a solid-bed batch reactor where solid waste is housed and leached soluble compounds are collected. In the acid first phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous materials to carbonaceous materials of lower molecular weight, primarily volatile fatty acids. The liquid and solid effluent from the acid phase is conveyed to a biogasification second phase, where methanogenic organisms convert the volatile fatty acids to product gas that is composed primarily of methane and carbon dioxide. Product gas is removed from the biogasification reactor and processed, or scrubbed, to separate the methane component that is drawn off as pipeline gas.
Two phase anaerobic digestion has been carried out in a single reactor as taught, for example, by U.S. Pat. No. 4,735,724 which teaches a non-mixed vertical tower anaerobic digester and anaerobic digestion process which provides passive concentration of biodegradable feed solids and microorganisms in an upper portion of a continuous digester volume and effluent withdrawal from the middle to the bottom portion of the digester, resulting in increased solids retention times, reduced hydraulic retention times and enhanced bioconversion efficiency.
U.S. Pat. No. 4,022,665 discloses certain specific operating conditions for a two phase anaerobic digestion process, such as feed rates and detention times, which promote efficient conversion of organic materials. Additionally, the '665 patent discloses two separated biogasification reactors, a biogasification reactor I operated in series with a biogasification reactor II. The biogasification reactor II receives effluent fluid and/or effluent gas from biogasification reactor I. A somewhat similar process is disclosed in U.S. Pat. No. 4,696,746 which teaches a process for two phase anaerobic digestion with two discrete biogasification reactors operated in parallel.
U.S. Pat. No. 3,383,309 teaches that the rate and efficiency of the anaerobic digestion process, particularly in the methane forming phase, are increased when hydrogen gas is introduced into the digester sludge. According to the '309 patent, hydrogen gas is introduced into both the acid forming and the methane forming phases, to increase the availability of energy rich “hyper-sludge.” All improvements disclosed in U.S. Pat. Nos. 4,022,665, 4,696,746 3 and 383,309 can be adapted for use according to the improved process of the present invention and the teachings of that patent are incorporated herein by reference.
French Patent No. 78 34240 describes an apparatus for biogasification which is known in the art as an upflow sludge blanket reactor. This apparatus utilizes a two-stage digestion apparatus. The apparatus is designed for and uses continuous recirculation between the reactors of the two stages. Continuous recirculation requires a relatively complex apparatus including filters, pumps and manifolded inlets to disperse the recirculated liquid stream and to avoid its “short circuiting directly to the outlet of the reactor into which it was just circulated. Additionally, the continuous recirculation requires two pumps that must operate continuously. In contrast, the present invention utilizes intermittent recirculation.
The sequential batch anaerobic composting (SEBAC) reactor is a relatively new digestion system. See, Chynoweth et al.,
Appl. Biochem. Biotech
. 28: 421-32 (1991). The SEBAC system consists of three reactors. Each reactor operates as a single phase batch digester. The three reactors are interconnected and operated on a different digestion schedule, the first being newly started, the second running in the middle of a digestion and the third running toward the end of a digestion. When new feedstock is loaded into the first reactor, the liquid from the third reactor is transferred to the first reactor to inoculate the feedstock and speed-up the digestion process.
A broad range of organic substrates are appropriate feedstocks for biogasification reactors. An exemplary feedstock is agricultural waste. Agricultural waste consists mostly of carbonaceous organic materials and it presents a particularly attractive renewable source of raw material for the generation of methane. The use of agricultural waste for this purpose serves a dual purpose, it produces a useful product and reduces the volume of agricultural waste which must be disposed of Many different types of agricultural waste can be digested utilizing a two phase anaerobic digestion scheme. The waste from the production of rice provides a salient example.
In California, for examples large quantities of rice straw are produced each year as by-products of rice production. In the Sacramento Valley alone, 1,452,000 tons of rice straw were produced in the crop year of 1994-1995 (CARB-CDFA, Progress report on the phase down of rice straw burning in the Sacramento Valley Air Basin, Report To The Legislature, California Air Resources Board and California Department of Food and Agriculture (1995)). Due to lack of feasible conversion technologies, however, utilization of these materials for energy production has not become practical for the agricultural sectors.
Current methods for disposal of these agricultural residue materials have caused widespread public concerns with regard to their environmental impact. In the case of rice and wheat straw disposal, for example, open field burning is considered as a practice causing serious air pollution problems, because of the emissions of smoke and other air pollutants, such as gases, particles and aerosols.
Current California legislation (the Connelly-Areias-Chandler Rice St
Zhang Ruihong
Zhang Zhiqin
Marx Irene
The Regents of the University of California
Townsend and Townsend / and Crew LLP
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