DESC:PROCESS OF PRODUCTION OF BIO-CNG FROM BIOGAS

Field of the invention

The present invention relates to a process of production of bio-CNG and more particularly, to a purification of a biogas before conversion into bio-CNG.

Background of the invention

Biogas competes with natural gas as a source of energy. Biogas has few clear advantages as it is renewable and cleaner than natural gas as it does not contain any hydrocarbons other than methane. However, biogas does have lower calorific value. Both the biogas and the natural gas differ significantly as far as the source, composition, temperature and pressure are concerned. The extraction and processing of natural gas is a well-known technique. The typical available natural gas is at flow rates of 200 to 2000 TPD at 50-120 °C and high pressures of 20-100 bar (g).

Biogas generation is well established for varying substrates from liquids to solids. The generated biogas is available at low flow rates of 2-50 TPD at atmospheric temperature and pressure. Further, the biogas and the natural gas have varied compositions with natural gas containing greater than 80% v/v methane, less than 10% v/v CO2, less than 0.5% v/v H2S and remaining of C2 - C5 hydrocarbons and nitrogen. Biogas contains around 50% v/v methane, 40% v/v CO2 and H2S ranging from a few ppm up to 4% v/v depending on substrates used for its bio digestion. Biogas does not contain higher hydrocarbons than methane but it does contain some quantities of Oxygen. Since, biogas does not contain higher order hydrocarbons, to match the higher calorific value of CNG, treated Bio-CNG (also called bio-methane) needs to be purified to about 96% - 99% v/v methane. The release of energy when bio-CNG is combusted allows us to use it as a fuel, it can be used for any heating purpose, such as industrial furnaces. It can also be used in a gas engine to convert the energy in the gas into electricity and heat. Bio-CNG can be compressed, and used to power CNG motor vehicles.

However, biogas contains significant amount of impurities like water, N2, O2, H2S, and CO2 and so forth. Therefore, biogas must be purified prior to the conversion into bio-CNG. Amine Scrubbing is a well-known technique of acid gas removal and is used to treat natural gas, synthesis gas and in petroleum refining. Acid gas removal is the removal of H2S and CO2 from gas streams by using absorption technology and chemical solvents. Sour gas contains H2S, CO2, H2O, hydrocarbons, COS/CS2, solids, mercaptans, siloxanes, NH3, BTEX, and all other unusual impurities require additional steps for their removal.

Commonly used techniques for biogas purification are, pressurized water scrubbing, pressure swing adsorption, chemical (amine, caustic) scrubbing, membrane permeation, temperature swing adsorption, cryogenic approach, physical absorption, and biological filtration.

The above mentioned technologies though advantageous have some drawbacks:

TECHNOLOGY
RANGE OF APPLICATIONS
ADVANTAGES
DISADVANTAGES
Absorption with water.
Non generative Low H2S concentration (0- 300 ppm). Mostly used for CO2 removal High efficiency (>97% CH4), Simultaneous removal of H2S when H2S < 300 ppm, Capacity is adjustable by changing pressure or temperature, Low CH4 losses (<2%), tolerant to impurities Expensive investment and operation, clogging due to bacterial growth, possible foaming, low flexibility toward variation of input gas
Absorption with polyethylene glycol.
Regenerative High efficiency (>97% CH4), Simultaneous removal of organic S components, H2S, NH3, HCN and H2O, Energetic more favorable than water, Regenerative, low CH4 losses Expensive investment and operation, difficult operation, incomplete regeneration when stripping/vacuum (boiling required),
reduced operation when dilution of glycol with water
Chemical absorption with amines.

Regenerative Low to High H2S concentration
(30 to 30,000 ppm) and CO2 removal High efficiency (about 99% CH4), low cost operation, Regenerative, More CO2 dissolved per unit of volume (compared to water), very low CH4 losses (<0.1%), high H2S and high CO2 removal in a single column Expensive investment, heat required for regeneration, corrosion, decomposition and poisoning of the amines by O2 or other chemicals, precipitation of salts, possible foaming
Caustic wash, permanganate.

Non regenerative 0-300 ppm H2S concentration Elemental S is formed, Low cost, Low removal efficiency for H2S and CO2, Operational difficulties and disposal of saturated solution

PSA/VSA

Regenerative Small scale. Low concentration H2S removal (< 100 ppm) and CO2
removal Highly efficient (95-98% CH4), small quantity of H2S can be removed, low energy use: high pressure, compact technique, also for small capacities, tolerant to impurities Expensive investment and operation, extensive process control needed, CH4 losses when malfunctioning of valves
Membrane technology. H2S and H2O are removed, simple construction, Simple operation, high reliability, small gas flows treated without proportional increase of costs
Gas/gas: removal efficiency: <92% CH4 (1 step) or > 96% CH4, H2O is removed Gas/liquid: Removal efficiency: > 96% CH4, cheap investment and operation, Pure CO2 can be obtained Low membrane selectivity: compromise between purity of CH4 and amount of upgraded biogas, multiple steps required (modular system) to reach high purity, CH4 losses.
Cryogenic separation Low scale 90-98% CH4 can be reached, CO2 and CH4 in high purity, low extra energy cost to reach liquid biomethane (LBM) Expensive investment and operation. CO2 can remain in the CH4
Biological removal 50-20000 kg/day H2S removal
No CO2 removal Removal of H2S and CO2,
enrichment of CH4, no unwanted end products Addition of H2, experimental - not at large scale

Redox (Fe2O3) 0.5-15 ton/day H2S removal
No CO2 removal Elemental Sulphur, High removal efficiency, use of low toxic solution High pressure problems, low quality product, requires specialized supervision

Accordingly, there exists a need to provide a process of production of bio-CNG from a biogas that overcomes the above mentioned drawbacks of the prior art.
Object of the invention

An object of the present invention is to provide bio-CNG which is purified to about 96% - 99% v/v methane.

Summary of the invention

Accordingly, the present invention provides a process of production of bio-CNG from a biogas. In a first step of the process, a feed biogas is pre-conditioned at near atmospheric pressure and temperature to remove moisture droplets and particulate matter from the feed biogas. In a second step, the pre-conditioned biogas is treated with a formulated amine at a predefined pressure and temperature to obtain sweet wet gas and to remove hydrogen sulphide and carbon dioxide. Specifically, the treatment of the biogas with the formulated amine is carried out at pressure from 1 bar to 4 bar (absolute pressure (a)) and at temperature from 40 °C to 130 °C. The formulated amine includes any one of ethanolamine, methyl diethanolamine, triethanolamine, and chemicals such as phosphoric acid and piperazine in water.

In a third step, the sweet gas obtained in the second step is polished at a predefined temperature and a predefined pressure for hydrogen sulphide removal. Specifically, polishing is carried out at pressure from 1 bar to 1.5 bar (a) and at temperature from 40 °C to 50 °C. Specifically, the sweet gas is passed through iron turnings for crude polishing and thereafter through activated carbon for fine polishing to reduce final H2S concentration to less than 10 ppm. In a final step, the water from the polished sweet gas is removed to obtain final bio-CNG. The water in the polished sweet gas is first reduced in a refrigeration unit and thereafter removed from the sweet gas using any one of molecular sieves and glycol dehydration. The glycol dehydration is carried out using any one of triethelyene glycol, diethylene glycol, ethylene glycol and tetraethylene glycol.

Brief description of the drawings

Figure 1 shows of a flowchart of a process of production of bio-CNG from a biogas, in accordance with the present invention; and

Figure 2 shows a process of amine treatment of the biogas used in the process of production of bio-CNG, in accordance with the present invention.

Detailed description of the invention

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiment.

The present invention provides a process of production of bio-CNG from a biogas. The process of the present invention provides multi stage filtration at low differential pressure (DP) for removal of liquid droplets and particulate matter. The process of the present invention is suitable for low pressure end-to-end treatment of the biogas.

The present invention is now illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description.

Referring now to figure 1, a flowchart of a process (100) of production of bio-CNG from a biogas in accordance with the present invention is shown.

In a first step, the process (100) involves pre-conditioning of a feed biogas. Specifically, the pre-conditioning of the feed bio-gas is carried out at near atmospheric pressure and temperature and involves multistage filtration. The feed pre-treatment is required as the biogas obtained from different sources has impurities. Pre-conditioning helps to remove moisture droplets and particulate matter from the feed thereby allowing further processing of the feed. O2 is also removed in the pre-conditioning but only partially as some amount of O2 is required in further processing.

In second step, the process (100) involves treating the pre-conditioned biogas with a formulated amine at a predefined pressure and a predefined temperature to obtain sweet wet gas and to remove hydrogen sulphide and carbon dioxide. In a preferred embodiment, the amine treatment comprises of two packed columns namely an absorber column (10) and a stripper column (20) and accessories thereof (not numbered).

Specifically, figure 2 shows detailed process of amine treatment of the biogas. As shown in figure 2, the biogas enters from a bottom side of the absorber column (10) and is contacted counter currently with a formulated lean amine solution that is fed from a top side. The formulated amine includes any one of ethanolamine (MEA), methy diethanolamine (MDEA), triethanolamine (TEA), and chemicals such as phosphoric acid and piperazine in water. The process operates continuously and the formulated amines can be made up after depletion after a week or so. In the absorber column (10), the lean amine solution absorbs the hydrogen sulphide and carbon dioxide contaminants and the biogas stream (sweet gas) of desired composition comes out from the top of the absorber column (10). This process occurs at low pressure, from 1 bar to 4 bar (absolute pressure (a)), almost at atmospheric pressure unlike other similar processes that take place at high pressure and at temperature from 40 °C to 130 °C. Also, this process helps to remove CO2 and H2S.

Simultaneously, the rich amine solution from the bottom of the absorber column (10) is fed to the stripper column (20) through pumps (30) where the sour gas is stripped out from the top to get lean amine solution from the bottom of the stripper column (20). The lean amine solution obtained from the stripper column (20) is cooled by heating the rich amine solution from the absorber column (10) and further filtered in an anion exchanger bed (40), a carbon bed (50) and two stage filters (not numbered) to remove Heat Stable Amine Salts (HSAS) before circulating back to the top of the absorber column (10). The amine solution thus keeps recirculating and a make-up is provided in the bottoms of the stripper column (20).

The sour gases or tail gases coming out of the stripper column (20) has to be treated before letting off. A knock out drum (60) (hereinafter, “KO drum (60)”) is attached after the absorber column (10) and the stripper column (20) to minimize amine losses. Specifically, to keep the amine entrainment losses to minimum the outgoing sweet gas and sour gas streams from the absorber column (10) and the stripper column (20) are routed through an efficient KO drum (60) with 98% droplet removal efficiency for droplets > 50 Micron. However, it is understood here that depending on size of the plant, either iron turnings or modified claus with just one converter can be used in other alternative embodiments of the present invention. In case of iron turnings, Fe reacts with H2S to form FeSO4 which needs to be dumped in land fillings.

According to the feed available and the application for which it needs to be used, the amine treatment process is tweaked and the methane and CO2 composition is varied by changing the amine recirculation rates and pressure/ temperature conditions.

In third step, the process (100) involves polishing the sweet wet gas at a predefined temperature and a predefined pressure for hydrogen sulphide (H2S) removal. Specifically, the predefined pressure is 1 bar to 1.5 bar (a) and the predefined temperature is 40 °C to 50 °C. The sweet wet gas obtained in the second step is further polished to remove any H2S present therein. Here, the gas is required to contain minimum 70% moisture in a saturated form. If gas does not have sufficient moisture, then water is added to the gas stream.

Specifically, after amine treatment the sweet wet gas is polished further for H2S removal in two stages. In a first stage, the gas is passed through iron turnings for crude polishing (rough polishing to below 100 ppm) where the H2S reacts with iron oxide and FeSO4 is formed that is sent for land fillings. In a second stage, the gas is then passed through activated carbon for fine polishing after which the final H2S concentration reduces to less than 10 ppm. The special grade activated carbon is additionally used, for e.g. Norit Silpure, when the end use of the sweet gas is for fuel cell application to remove any traces of siloxane. However, it is understood here that the fine polishing can be done in a suitable bed consisting of iron oxide impregnated on a carrier of wood chips e.g. Iron sponge from Connelly-GPM or granular iron oxide e.g. Sulfatreat from Schlumberger, amongst others to achieve final desired H2S concentration below 20 ppm in the sweet gas, which is as per Indian Standard IS 16087, 2016, in other alternative embodiments of the present invention.

In fourth step, the process (100) involves removing water from the polished sweet gas obtained in the third step to obtain final bio-CNG. The water in polished sweet gas is first reduced in a refrigeration unit (not numbered) and thereafter removed from the sweet gas using any one of molecular sieves and glycol dehydration. Specifically, water both in droplet and saturated forms is removed from the polished sweet gas. For water removal using molecular sieve, the polished sweet gas obtained in the third step is passed through a refrigeration unit where multistage chilling is done reducing the gas temperate to less than -2 °C followed by molecular sieve section to remove the residual traces of moisture. Alternatively, the polished sweet gas obtained in the third step is passed through glycol dehydration after the refrigeration unit to achieve similar composition. The glycol dehydration is carried out using any one of triethelyene glycol, diethylene glycol, ethylene glycol and tetraethylene glycol in a two-column system.

The final bio-CNG that is formed after the above process (100) is useful for various purposes as a drop-in replacement for CNG (natural gas) for industrial applications. The bio-CNG is also useful in natural gas engines for combined heat and power (CHP), as an automotive vehicular fuel in CNG vehicles, industrial heating/melting applications replacing crude oil derived fuels such as LPG/propane, diesel, furnace oil, and the like, and in methane fuel.

In accordance with the present invention, if the sweet dehydrated gas is to be used for fuel cell then additional removal of siloxane (silicon compound) to ppb levels is achieved through appropriate activated carbon. In case the sweet dehydrated gas is to be used as a heating fuel/motor vehicle then the sweet gas is compressed up to 250 bar to minimize the volume.

The invention is further illustrated hereinafter by means of examples.
Examples
Example 1:

In this example, a feed biogas containing 3% v/v H2S and 40% v/v CO2 was treated. The H2S and CO2 concentration was reduced to 300 ppm and 400 ppm after the first stage of amine treatment. Chemicals such as piperazine (2%), phosphoric acid (less than 0.1%) and 47% MDEA in DM water were used as the formulated amine. Then the sweet gas was passed through iron turnings where H2S concentration was further reduced to below 100 ppm. Finally, using activated carbon, H2S levels were brought down to below 10 ppm. The water was then reduced in refrigeration unit from 5-7% v/v to below 0.75% v/v. Then either molecular sieve or glycol dehydration was used to remove water to a final desired level of below 5 mg/m3.

Example 2

In this example, a feed biogas containing 1000 ppm H2S and 45% CO2 v/v was treated. The H2S and CO2 concentration was reduced to 80 ppm and 450 ppm after the first stage of amine treatment. Chemicals such as piperazine (4%), phosphoric acid (less than 0.1%) and 45% MDEA in DM water were used as the formulated amine. Then the sweet gas was passed through iron turnings where concentration was further reduced to less than 40 ppm. Finally, using activated carbon, H2S levels were brought down to less than 1 ppm. The water was then reduced in refrigeration unit from 5-7% v/v to below 0.75% v/v. Then either molecular sieve or glycol dehydration was used to remove water to a final level of below 5 mg/m3.

Advantages of the invention

1. The process (100) provides multi stage filtration at low differential pressure (DP) for removal of liquid droplets and particulate matter.
2. The process (100) is suitable for low pressure end-to-end treatment of the biogas.
3. The process (100) facilitates simultaneous removal of CO2 and high quantity (up to 30,000 ppm) of sulphur compounds like H2S, COS, CS2, and like using MDEA and appropriate additives such as MEA, piperazine and phosphoric acid in the absorber column.
4. The process (100) is flexible and desired final purity is achieved by changing the composition/formulation of MDEA and additives.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the scope of the present invention. ,CLAIMS:We claim:

1. A process (100) of production of BIO-CNG from a biogas, the process (100) comprising the steps of:
a) pre-conditioning of a feed biogas at near atmospheric pressure and temperature to remove moisture droplets and particulate matter from the feed biogas;
b) treating the pre-conditioned biogas of step a) with a formulated amine at a predefined pressure and a predefined temperature to obtain sweet wet gas and to remove hydrogen sulphide and carbon dioxide;
c) polishing the sweet wet gas of step b) at a predefined temperature and a predefined pressure for hydrogen sulphide removal; and
d) removing water from the polished sweet gas of step c) to obtain final bio-CNG.

2. The process (100) as claimed in claim 1, wherein treatment of the biogas with the formulated amine is carried out at pressure from 1 bar to 4 bar (a) and at temperature from 40 °C to 130 °C.

3. The process (100) as claimed in claim 3, wherein the formulated amine includes any one of ethanolamine, methy diethanolamine, triethanolamine, and chemicals such as phosphoric acid and piperazine in water.

4. The process (100) as claimed in claim 1, wherein the polishing of the sweet wet gas is carried out at pressure from 1 bar to 1.5 bar (a) and at the temperature from 40 °C to 50 °C.

5. The process (100) as claimed in claim 1, wherein the sweet wet gas is passed through iron turnings for crude polishing and thereafter through activated carbon for fine polishing to reduce final H2S concentration to less than 10 ppm.
6. The process (100) as claimed in claim 1, wherein water in the polished sweet gas is first reduced in a refrigeration unit and thereafter removed from the sweet gas using any one of molecular sieves and glycol dehydration.

7. The process (100) as claimed in claim 6, wherein the glycol dehydration is carried out using any one of triethelyene glycol, diethylene glycol, ethylene glycol and tetraethylene glycol.