WO2013034947A1

WO2013034947A1 - Upgrading of biogas to marketable purified methane exploiting microalgae farming

Info

Publication number: WO2013034947A1
Application number: PCT/IB2011/002099
Authority: WO
Inventors: Krisada Kampanatsanyakorn; Suradit Holasut; Piriyathep KACHANADUL
Original assignee: Cellennium (Thailand) Company Limited
Priority date: 2011-09-08
Filing date: 2011-09-08
Publication date: 2013-03-14

Abstract

Raw biogas from organic waste anaerobic digestion is purified to marketable bio- methane by scrubbing raw biogas in an absorber using a sodium carbonate rich growth liquor of microalgae of an algae farming plant for absorbing carbon dioxide and other gaseous substances contained in the biogas and reacting carbon dioxide with the sodium carbonate to form sodium bicarbonate in the liquor; returning a bicarbonate enriched liquor leaving the absorber to the algae farming plant as bicarbonate feed to the growing algae for having the carbon dioxide metabolized and regenerating sodium carbonate in the liquor, which is thence reusable for scrubbing fresh raw biogas in said absorber. An outstandingly effective synergy is implemented in exploiting an on-going micro algae farming plant for regenerating the carbonate in the growth liquor while feeding on the bicarbonate for their growth, substantially avoiding any emission of carbon dioxide.

Description

UPGRADING OF BIOGAS TO MARKETABLE PURIFIED METHANE EXPLOITING MICRO ALGAE FARMING

BACKGROUND

Technical Field

The present disclosure relates in general to methods for upgrading biogas generated in anaerobic digesters of organic wastes to marketable purified methane by separating the large amount of carbon dioxide and traces of hydrogen sulfide and ammonia normally present in biogas, and more in particular methods that do not release carbon dioxide in the environment.

Review of Prior Art

Anaerobic digestion of organic waste (manure, organic fraction of household and industrial waste and crops destined to power generation) is becoming a vital technology, far more relevant than power generation from solar and wind energy. The first benefit is the elimination of undesirable noxious wastes but the vital objective is the production of biomethane, necessary to substitute depleted resources of natural gas, and organic fertilizers, necessary to substitute costly chemical fertilizers, which in the long term are detrimental to field productivity if used extensively.

Biogas generally consists of methane (approx. 65% in volume), carbon dioxide (approx. 35% in volume) and traces of hydrogen sulfide (<2%) and ammonia (<1%). The high content of carbon dioxide and the presence of hydrogen sulfide and ammonia make it unsuitable to be used in place of natural gas in gas distribution networks neither to be compressed in cylinders, for transportation where concentrations higher than 90 to 97% are generally required. Absence of hydrogen sulfide and ammonia is a must to avoid corrosion in compressors, gas storage tanks, pipes and engines.

Anaerobic digestion produces biogas and a liquid/solid residue. Often, in rural territories, the raw biogas is straightly burned in a combustion engine or in a turbine connected to an alternator for producing electric energy. However, such a direct in situ production of electricity using biogas has an intrinsic low efficiency because of the large amount of carbon dioxide contained in the biogas and because a great part or all the heat generated is normally wasted. Purification of biogas to obtain pure methane would produce marketable biomethane of commercial value well in excess of the value of the electricity that can be generated by burning the raw biogas.

Biomethane, herein intended as generated by ongoing biological processes, can be produced by upgrading biogas. Several technologies for upgrading biogas to biomethane, typically containing 95-97% methane and 1-3% carbon dioxide, have been developed and are extensively used in many countries. Generally these techniques have a significant impact on the production cost of biomethane.

Water scrubbing

The water scrubber method (absorption with water) is the most common, simplest and less expensive upgrading technique. This method is highly efficient (>97%), it is simple to operate and together with carbon dioxide also noxious hydrogen sulfide and ammonia, commonly present in biogas, are removed. The technique is based on the fact that carbon dioxide has a higher solubility in water then methane and therefore can be dissolved to a higher extent than methane, particularly at lower temperature as shown in the diagram of Fig. 1 reproduced from "GAS ENCYCLOPEDIA".

The carbon dioxide remains in the washing water and is not recovered. Due to the low solubility of carbon dioxide in water, large quantities of water are required unless absorption is conducted under pressure. A high pressure allows absorbing a quantity of carbon dioxide in a greatly reduced volume of water and also allows recovering of the carbon dioxide downstream by reducing the pressure. However, operating at high pressure makes the system much more expensive to construct, as compressors, high pressure pumps and expensive pressurized absorbing tower are needed, and costs of operation are significantly increased. Moreover when high pressures (up to 150 Bar) are used, more methane dissolves in water and undesirable losses of methane increase proportionally with the pressure.

When carbon dioxide dissolves in water, the pH of the water proportionately decreases well below 6 and this in turn decreases solubility of carbon dioxide. However, if the water contains a chemical compound capable to absorb carbon dioxide without decreasing the pH below 7, then pressure would not be required unless required for regenerating the liquor and/or for recovering the carbon dioxide instead of releasing it to the atmosphere.

Chemical scrubbing with alkaline solutions or ammines

According to this technique, carbon dioxide is not only absorbed in the liquor, but also reacts chemically with the alkali (NaOH, KOH and Ca(OH)₂) or with amines present in the absorbing liquor. The concentration of compounded carbon dioxide in the liquid phase that can be reached is much higher than the concentration with water scrubbing and this reduces the required volume of circulated liquid and the dimensions of the scrubbing tower.

Because the chemical reaction is strongly selective, methane losses may be reduced below 0.1%.

When caustic soda, caustic potash or calcium hydroxides are used, the solution cannot be regenerated and the carbon dioxide is not recovered. Differently, using amines as carbon dioxide sequestering compound, the solution in which carbon dioxide chemically react is regenerated by heating the carbon dioxide laden solution, downstream of the absorption tower. Hydrogen sulfide and ammonia are generally removed before absorption in the amine scrubber because otherwise regeneration of the amine solution would require much higher temperatures. This technique implies a relatively large investment, is economical to operate but requires availability of low cost heat for the regeneration. There is still need for an upgrading process of biogas of relatively low investment and low energy consumption. Moreover, it is desirable that the process minimize or avoid emission of methane, because methane has a greenhouse effect 23 times greater than that of carbon dioxide.

SUMMARY OF THE INVENTION

A simple and cost effective method of upgrading the biogas coupled with an associated production of microalgae has been devised. The undesirable impurities of the biogas, namely: carbon dioxide, hydrogen sulfide and ammonia, are efficiently removed from the biogas at atmospheric pressure and room temperature by washing them out with an aqueous solution of sodium carbonate-bicarbonate. The solution obtained, laden with the so chemically compounded carbon dioxide, is used as growth liquor of microalgae.

Microalgae, the fastest growing vegetable, can then be used as animal feed or human food, for the production of biofuel or digested for producing fresh biogas and fertilizers.

The presence of sodium carbonate in the aqueous solution used for scrubbing the biogas increases the alkalinity of the water and therefore the solubility in it of the carbon dioxide component of the biogas and reacts with the absorbed carbon dioxide forming bicarbonate according to the reaction:

Na₂C0₃ + C0₂ + H₂0→ 2 NaHC0₃ (1)

The scrubbing solution leaving the absorption tower or equivalent reactor, thus enriched in bicarbonate by the carbon dioxide chemical sequestering mechanism, may be returned to the hydraulic circuit of microalgae laden growth liquor of a algae farming plant. The algae feed on the bicarbonate metabolizing the carbon dioxide and regenerating the carbonate.

According to the applicant's method, an outstandingly effective synergy is implemented in exploiting an on-going algae farming plant for regenerating the carbonate in the growth liquor while feeding on the bicarbonate for their growth, thus substantially avoiding any emission of carbon dioxide.

The carbonate containing growth liquor, spilled out of the circuit of the algae farming plant, is freed from the algae suspended therein by passing it through a solid/liquid separator, and introduced in the absorption tower or equivalent reactor for continuing scrubbing of fresh biogas and so forth.

Alternatively or concurrently, if so desired or for dealing with temporary unbalances between the rate of generation of bicarbonate in the scrubbing solution and the rate of consumption of it by the growing algae, pure carbon dioxide may be recovered by thermal decomposition of the portion of bicarbonate that has not been consumed by the algae contained in the separated liquor being conveyed to the absorption tower or equivalent reactor, or in a purposely diverted stream of the solution leaving the algae/liquor separator.

Above 70°C, the sodium bicarbonate gradually decomposes into sodium carbonate, water and carbon dioxide, according to the following reaction:

2 NaHC0₃→ Na₂C0₃ + H₂0 + C0₂ (2)

Conversion would be extremely fast at boiling point of the solution.

The invention is defined in the annexed claims, the content of which is intended to constitute part of this description and is herein incorporated by express reference.

The various aspects and advantages of the invention will become even more evident through the ensuing description of significant embodiments and by referring to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a basic diagram of an exemplary plant according to a first embodiment of this invention; FIG. 2 is a basic diagram of an exemplary plant according to an alternative embodiment of this invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figure 1 is a schematic view of a plant adapted to practice this invention, according to a first embodiment. The common symbology used in the drawings makes the schemes immediately legible by the expert technician.

On the left side of the drawing, is depicted an anaerobic digester of organic waste D, such as: manure, organic fraction of household and industrial wastes, crops destined to power generation, provided with an outlet pipe Dl of biogas rising to the top of the sealed digester.

On the right side of the drawing is depicted a typical photo-bio reactor Al, A2, A3, A4, adapted for growing so called micro-algae (typically of the Spirulina, Chlorella, or similar straints) in batch mode, in semi batch-continuous mode or most preferably in continuous mode.

The partial and simplified symbolic representation of the algae farming photo-bio- reactor shows the main elements that may compose it; namely: an array of transparent pipes Al of circulation of algae laden growth liquor lifted along each transparent pipe Al by a buoyant air stream that may be injected by an air blower A5, or gaseous mixture of air with an automatically controlled percentage or carbon dioxide fed to the growing algae to lower excessively high alkalinity; a settler tank A2, through which the continuously circulating suspension of growing algae in the liquor eventually passes with a residence time adapted to permit to a part of the algae suspended in the liquor to settle into a conical trough bottom A2b, in case of a cylindrical vessel or into several closely spaced conical or inverted square-base pyramidal troughs A2b, in case of an extended parallelepiped vessel A2.

In any case, the vessel A2 connects with all the spaced loops of the array of transparent circulation pipes Al.

At the vertex of every inverted pyramidal or conical trough A2b, release valve allows to bleed out of the system a dense wet biomass crop of algae.

To the algae farming bioreactor is also normally associated a tank A3 adapted to contain a reserve volume of growth liquor solution, that may be freshly prepared from time to time to replenish it, which is automatically added into the photo-bio-reactor system whenever needed to make up the liquor discharged with the wet algae crop and maintain the liquid level in the settling vessel A2.

A photo-bio-reactor particularly adapted to the requirements of implementation of the present invention is extensively described in prior international patent application PCT/IB2011/000181, of the same Applicant, filed on 24 February 2011.

According to the method of the present disclosure, the growth liquor that may be separated from the algae crop using a centrifuge for similar solid/liquid separator, instead of being returned as customary into the photo-bio-reactor circuit or into the liquor reserve tank A3, is conveyed as carbon dioxide absorbing aqueous solution to a gravity feed packed column absorber 1 adapted to scrub the raw biogas coming from the digester which is injected at the bottom of the packed column through which the separated growth liquor, freed of the largest part of the algae suspended in it, is made to percolate by gravity in counter current to the biogas before being returned to the reserve tank A3 of the photo-bio reactor plant and thence to the circulating algae laden growth liquor in the photo-bio-reactor circuit.

The dark environment of the piping and of the packed column absorber blocks any biologic activity of the residual algae that may be present in the separated growth liquor as exiting the separator A4. However, it may be even possible to subject the separated liquor to an appropriate bio-destructive treatment before conveying it to the column 1, as an additional safeguard measure for preventing algae to stick onto surfaces of the packed column and consequent possible clogging.

The post-separation bio-destructive treatment may be the same sonification treatment conducted with ultrasonic waves of wavelength adapted to break the cellular membrane of the algae that is commonly used for extracting oily substance from the algae crop to produce bio-fuel. Alternatively, or in association with it, microwaves (RF energy) may even be used.

The carbonate containing growth liquid, spilled out of the hydraulic circuit of the algae farming photo-bio-reactor, freed from the majority of algae suspended therein, readily absorbs the carbon dioxide and the trace amounts of other gaseous substances, typically hydrogen sulfide and ammonia together with a minor amount of methane that has a far lower solubility in the saline water solution that carbon dioxide, converting the absorbed carbon dioxide into the carbonate according to the reaction

Na₂C0₃ + C0₂ + H₂0→ 2 NaHC0₃ (1)

The relatively high pH of the growth liquor favor absorption of carbon dioxide and is coupled to the fact that the absorbed carbon dioxide reacts with the sodium carbonate contained in the liquor forming bicarbonate makes the rising gas stream gradually shed its carbon dioxide content while moving up the packed column 1.

The Chlorella strain is most happy at a pH of 8.5 and its growth rate practically drops to nil at pH 7. Keeping the pH between 8 and 9 is an optimal choice, though 7.5 to 9.5 may still be an acceptable range of pH control that avoids biological damage or death to Chlorella.

Using the Spirulina strain, the situation would even be less critical because Spirulina fosters with pH between 9 and 10.

Considering that with only sodium carbonate in the saline growth liquor the pH would reach 10.4 and with only sodium bicarbonate the pH would drop 7.5, with Chlorella it is highly advisable to operate between pH 8.5 and 9. Correction of the pH may even be done by adding to the growth liquor other compounds that may be metabolized as nutrients by the growing algae like for example: phosphoric acid, sodium or potassiun fosfate, sodium or potassium bisulfate, ammonia, etc.

The scrub liquor stream exiting the packed column 1, with C0₂ dissolved in it and rich of bicarbonate, may have a pH between 7 and 7.5, and may be returned to the reserve tank A3 of photo-bio reactor system.

Generally, the novel method of the applicants may be regarded as a way of transferring all carbon dioxide stripped from the biogas to the algae both as carbon dioxide dissolved in the growth liquor and as sodium bicarbonate.

At the top of the packed tower absorber 1, purified bio-methane gas with a relatively low residual carbon dioxide content, meeting the required specifications of a carbon dioxide content of less than 8% by volume, of marketable methane, is dried by passing it through a de-moisturizing column 2 or 2bis, packed with any appropriate known drying agent, before being conveyed a compression and bottling station 4 or injected into a public gas distribution network.

The biogas generated in the digester and the gas leaving the scrubbing column are normally saturated with water vapor. Moisture content in the gas increases with temperature and at a temperature of about 35°C, the water content is around 5% by volume.

For removing such moisture content from the scrubbed methane gas, it is common to pass the moisture containing gas in a column packed with drying agents such as for example, zeolites, silica gel, aluminum oxide or magnesium oxide. The drying stage usually employs two similar columns in order to permit switching the moist gas stream through one of the two while the other is being regenerated by drying the packing.

Optionally, the solution of carbon dioxide in the growth liquor aqueous solution may be enhanced by the presence of carbon-hydrase enzymes that catalyze the reaction: C0₂ + H₂0 - H⁺ + HC0₃

thus incrementing the rate of transfer of carbon dioxide to the running microalgae farming photo-bio reactor.

An alternative embodiment is depicted in FIG. 2.

As a source of gaseous carbon dioxide to be ejected in a stream or forced air normally distributed into the circulating microalgae laden growth liquor at a low point of each transparent tube loop Al for gas-lifting the suspension in order to keep it in continuous circulation through the transparent tubes, the bicarbonate that forms in the growth liquor while passing through the absorber, because of the reaction of carbon dioxide with the sodium carbonate contained in the solution taking place in the packed column absorber 1 , and the portion of which that is non consumed by the algae, may be partly decomposed by heating part of the bicarbonate containing liquor leaving the separator A4.

The liquor may be heated to 70°C or higher, normally up to 90-100°C, in a dedicated heater 5, from the top of which substantially pure carbon dioxide is recovered to be stored or used as feed stream into the photo-bio-reactor circuit.

The presence of such a decomposition stage of bicarbonate in the liquor that occasionally may even become intolerably excessive because of a temporary inability or severely reduced ability of the growing algae to consume all the bicarbonate being continuously formed afresh, offers a most convenient instrument of maintaining a correct concentration of bicarbonate in the growth liquor and an undiminished ability of the carbonate containing liquor in effectively stripping carbon dioxide from the methane.

Moreover, heat is recovered from the cooling down liquor leaving the heated reactor in a heat exchanger 6, usefully warming up the incoming separated liquor from the algae/growth liquor separator A4 that is thence introduced at the top of the packed column 1.

Claims

1. A process for upgrading biogas to marketable purified biomethane comprising the steps of

a) scrubbing raw biogas in an absorber using a sodium carbonate rich growth liquor of microalgae of an algae farming plant for absorbing carbon dioxide and other gaseous substances contained in the biogas and reacting carbon dioxide with the sodium carbonate to form sodium bicarbonate in the liquor; b) returning a bicarbonate enriched liquor leaving the absorber to the algae farming plant as bicarbonate feed to the growing algae for metabolizing the carbon dioxide and regenerating the sodium carbonate in the liquor reusable for scrubbing fresh raw biogas in said absorber.

2. The process of claim 1, wherein said algae farming plant comprises a photo-bio reactor with transparent pipes of circulation of algae laden growth liquor.

3. The process of claim 1, further comprising the step of separating the algae from the growth liquor before using it for scrubbing the biogas.

4. The process of claim 3, wherein at least a portion of the carbon dioxide stripped from the biogas is recovered by heating up to 70°C or higher a diverted portion of unconsumed bicarbonate containing liquor separated from the algae is conveyed to a flow through heated reactor for thermally decomposing bicarbonate to carbonate and evolving collectable gaseous carbon dioxide, before cooling it and conveying it to the absorber.

5. The process of claim 3, further comprising the step of subjecting the liquor separated from the algae to a bio-destructive treatment.

6. The process of claim 4, characterized in that the carbon dioxide collected from the heated reactor is mixed with air and the gas mixture is introduced into transparent tubes of circulation of algae laden growth liquor for gas-lifting it up the pipes.

7. The process of claim 4, wherein heat is recovered from the cooling down the liquor leaving said heated reactor by exchanging it with the liquor conveyed to it.

8. The process of claim 1, wherein the scrubbed bio-methane exiting said absorber is de-moisturized by passing it through a packed column containing a drying agent.

9. A biogas upgrading plant comprising:

a) a source (D, Dl) of biogas, composed of methane and carbon dioxide and minor amounts of other gaseous substances, from anaerobic digestion of organic waste;

b) a photo-bio reactor (Al, A2, A3, A4), adapted for growing micro-algae in either batch, semi batch-continuous or continuous mode, comprising an array of transparent pipes (Al) of circulation of algae laden growth liquor lifted along each transparent pipe by buoyant air or gaseous mixture of air and carbon dioxide bubbles, a settler tank (A2), through which the continuously circulating suspension of growing algae in the liquor transits with a residence time adapted to permit to part of the algae suspended in the liquor to settle into a bottom trough (A2b) with release valve for discharging a dense wet crop of algae;

c) a tank (A3) adapted to contain a reserve volume of growth liquor solution to make up for the liquor discharged with the wet algae crop and maintain a constant liquid level in the settling vessel;

d) an algae/liquor separator (A4) for said discharged wet algae crop;

e) a packed scrubber (1) of said biogas introduced at the bottom of the scrubber, with growth liquor exiting said separator (A4) and introduced at the top of a packed section of the scrubber; ί) a de-moisturizer (2, 2bis) of bio-methane exiting said scrubber (1), stripped of a major part of said carbon dioxide and other gaseous substances;

g) a compressor 3 of the de-moisturized bio-methane exiting said de-moisturizer (2, 2bis) for bottling or delivering it to a gas distribution network.

10. The biogas upgrading plant of claim 10, further comprising:

i. a heated reactor (5) of a diverted stream of said separated liquor exiting said separator (A4) for thermally decomposing sodium bicarbonate contained therein to sodium carbonate and recoverable carbon dioxide before conveying cooled down liquor to the top of a packed section of the scrubber;

i. a heat exchanger (6) for cooling down the liquor exiting the heated reactor (5) and warming up incoming separated liquor from the algae/growth liquor separator (A4).