Feedstocks for Compressed Natural Gas Production

 

Biogas is a familiar name in Indian households thanks to various efforts from our government's push for clean fuels across India in the past. Biogas is a gaseous mixture produced by the anaerobic digestion of organic raw materials. Biogas is mainly comprised of Methane, Carbon dioxide, hydrogen sulfide and saturated moisture but not limited to. In the past production of biogas was limited to farm yard manure and utilization for cooking applications in rural parts of India and lately large capacity biogas plants was very popular in industries with high organic loading in their effluents. The large capacity biogas was then mainly used for the generation of heat and power for the captive applications. Moving forward the demand for electricity and electrification of the grid in the interior part of India resulted in the concept of Bio-Power or Alternative energy resources along with solar and wind for the supply of electricity to our household.

Biogas generation is a proven process in the industry and is very close to each one of us as we carry one digester in our belly every day since we were born. If one drives the analogy between the digestion of food by living beings like humans, Animals etc one can say whatever they can use as food either as is or with some preparation like cooking, marinating, fermenting etc but not limited to can be potentially converted to Biogas in a controlled atmosphere.

The material around us generally has some distinct element which has its own significance if biogas generation is desired. These elements can be enlisted as below,

a.      Organic fraction (Carbon)

    1.  Volatile Organic Matter – Generally this fraction can be converted to various reaction products with ease as against balance components.

    2. Non-Volatile Organic Matter – Recalcitrant like organic polymer, lignin etc but not limited to

b.      Inorganic Fraction- All inorganic element collective called Ash.

c.      Moisture – It’s an equilibrium content at a given condition or as per downstream process.

Each and every element/material around us have either of it and the ease of converting it to an end product will drive the conversion process and end product. It could be the table which I am using right now as it is made up of particle board. However, The selection of raw material for the production of compressed biogas has various dimensions as below but not limited to,

1.      Window of availability – Higher is preferred

2.      Access to raw material

3.      Transport requirement

4.      Storage and handling requirements

5.      Cost elements like securing, transport, Processing etc

6.      Yield of product and byproduct

7.      Ease of conversion

8.      Sustainability of supply chain

9.      Environmental impact and opportunity

10.  Benefit to society and their involvement

As a part of the SATAT program, we have identified various raw materials which we as a nation see issues in a way converted to opportunities for energy sustainability of India. Following are some of the raw materials which is in major focus for compressed biogas generation but not limited to,

    A.     Agricultural Waste: This is one of the most readily available with a limited window of availability of 30-60 Days in most cases. These are high on lignin and inorganics need state-of-the-art technology for effective conversion to CBG. The typical yield from this material can be up to 500 Am3/DMT depending upon composition and conversion process. Major agricultural waste is rice straw which is one of the major reason behind Air pollution in the northern part of India which is effectively being converted to Compressed Biogas.

    B.     Cow Dung: Cow dung is another potential feedstock available in abundance which is being potentially converted to Biogas for years altogether and now it can be converted to Compressed biogas at points of bulk generation like large Cow Shelters (Goshals), Diary Farms and cluster etc but not limited to. One can expect biogas yield up to 50 m3/MT depending upon composition and quality and process for conversion.

    C.     Press mud: Press mud is another organized source of feedstock. It is mainly a by-product from Indian sugar mills and is rich in organic which can be effectively converted to Compressed Biogas. Indian sugar production is more than 340 MMT which potentially generates about 1.3 MMT press mud which is presently utilized for bio-composting of Vinasse, Fertilizer ( As is basis), BioFuel (Briquette ) etc but not limited to. Looking at Non-bio-composting based zero liquid discharge trends in the distillery industry possibility of large-scale press mud availability for potential generation of CBG can not be denied. The press mud can produce upto 130 m3/ MT biogas or even higher as claimed by technology providers could be a welcome addition for sugar mills and all entrepreneurs for building CBG business. The press mud window of availability is upto 220 days a year at some places and about 130-150 days on average for mill operation. Press mud is perishable feedstock and without adequate provision for preservation significant loss of yield can be envisaged.

    D.     MSW: Over 60 MMT of generation and lack of processing & treatment facility to the scale of generation makes MSW is one of the major issue to tackle in India. Most of the MSW generated in India can be potentially converted to biogas. The segregation of the the MSW fraction is the ley hurdle in the process which need to be addressed to open up the CBG potential. The segregated MSW can produce upto 130 m3/MT biogas depending on level of segregation and composition of the MSW.

    E.      Food waste: India is known for diversity in food across the geography of the country. Sad part of the story is that we also waste significant amount of food via organized value chain of hotels and restaurants. Some of the cities in India are putting their best efforts in collection of food waste and treating them in dedicated facility for better utilization of its potential.

    F.      Municipal Sewage treatment plants: Indian STP are dominated by aerobic treatment technologies which generate lot of sludge for disposal which can be treated for production of biogas. Also all upcoming STP shall be designed to produce higher sludge for recovery of energy in the form of CBG. The implementation of Anaerobic treatment technologies for direct generation of biogas from STP and then converting it to CBG also can be thought of moving forward.

    G.     Industrial Waste water: All industrial wastewater with higher organic loading and significant biogas generation potential can be thought of to produce CBG as value added co-product rather as source of Heat and Power wherever possible.

  The future of the compressed biogas project lies in access to lower cost Feedstocks that are readily available in the vicinity of the plant for a larger window of availability during the year.

Identification and Evaluation of best suitable feedstock for Compressed Biogas production

My childhood memories cannot be complete without one of our family friends who loves to ask us tricky questions to us and I personally always fall into his trap. One of the questions is “Which one is more? One kilogram of cotton or One kilogram of iron.” The beauty of the question is that it looks very simple and tempting but one needs to have adequate knowledge about the subject otherwise it will trick you for sure. Over the years in the industry, such questions rather dilemmas never lost their relevance irrespective of innovation in ways of doing things in the recent past.

The compressed biogas (CBG) industry is not alien to such questions. One such question which we would like to elaborate on today is the selection of feedstock quantity. Most of us must have come across many aspirants in the market who want to set up a plant for ABC Ton of waste to CBG plant and they appear to be firm in the ton of waste processing irrespective of its specification and its overall impact on final products. It is not about the capability of the project developer but the area where we need to put our best effort to ensure we select the best mix of raw materials for a sustainable CBG project in a longer duration. For doing so one needs to understand the basic of feedstock and how it get translated to final products mainly CBG. We would be using some of the typical composition of the feedstock with an average yield of gas and then extend the data to some common baseline to evaluate all the feedstock on the same level field.

The feedstocks in consideration are as below and their typical specification is as mentioned in the table. Please don’t go by the values they could be here and there but the trend needs to be captured.

Sr. No	Feedstock	Typical TS	Typical TVS	Avg. Biogas Yield on as is basis (Sm3/MT)
		% w/w	% w/w
1	OFMSW	25-30	65-75	80-100
2	Napier Grass	25-30	75-80	120-180
3	Poultry Litter	25-28	70-75	80-100
4	Press Mud	25-30	70-75	80-120
5	Segregated Food Waste	25-30	85-90	110-130
6	Paddy Straw	90-92	70-75	350-400
7	Cow Dung / Farm manure	15-20	55-65	40-50

The yield per MT of as is feedstock varies from 40 to 400 sm3/MT which is the very high range and could be confusing for some for comparison, evaluation and arriving at the right blend for the particular plant under a given set of conditions. In business what works there might not work here so the team needs to be very vigilant when referring the set of feedstock from the particular location/plant in content to some other plant.

When one say a particular feedstock has X yield and some other has Y then what do they actually mean? Let's try to elaborate this further. One can notice that above mentioned feedstock has a varying levels of TS and Moisture content. As biogas generation process only generates biogas which is methane, Carbon dioxide, Hydrogen sulfide etc with saturated moisture by equilibrium. To make it simple the volatile / digestible portion of organic only get converted to biogas and average efficiency of its conversion is typically 55-65% based on the type of reactor assuming stable operating conditions. Generally, total volatile solids fraction is considered to be digestible and the rest is non-digestible portion. Non-digestible is mostly Ash part of it and lignin like hard to digest components. For this evaluation lets consider TVS as a basis then all the feedstock on dry basis looks like as mentioned in the below table.

Sr. No	Feedstock	Typical TS	Typical TVS	TVS
		% w/w	% w/w	Kg/DMT
1	OFMSW	30	70	700.0
2	Napier Grass	25	80	800.0
3	Poultry Litter	28	75	750.0
4	Press Mud	30	72	720.0
5	Segregated Food Waste	30	90	900.0
6	Paddy Straw	90	75	750.0
7	Cow Dung / Farm manure	20	65	650.0

  The average volatile solid content of any feedstock is about 750 kg per Dry MT and considering 55 to 65% avg degradation efficiency of TVS the average biogas generation could be 350 sm3/ DMT. If this is to be represented as CBG potential it will look somewhat as below,

Sr. No	Feedstock	Typical TS	Typical TVS	TVS	CBG yield
		% w/w	% w/w	Kg/DMT	kg/DMT
1	OFMSW	30	70	700.0	113.3
2	Napier Grass	25	80	800.0	240
3	Poultry Litter	28	75	750.0	128.6
4	Press Mud	30	72	720.0	113.3
5	Segregated Food Waste	30	90	900.0	146.7
6	Paddy Straw	90	75	750.0	155.6
7	Cow Dung / Farm manure	20	65	650.0	90.0

This is only one dimension of yield and as a common logic goes one should use the feedstock with the highest yield and try to avoid the one with the lowest yield, right. But wait let’s try to superimpose the economical viewpoint on this table and see how much each kg of CBG produced cost us for sourcing the feedstock. We have assumed the cost of feedstock as landed cost inside the plant and if some other components which could be site specific will impact either of the value accordingly.

Sr. No	Feedstock	Typical TS	Biogas Yield	CBG Yield	Feedstock Cost	CBG Loading
		% w/w	m3/MT as is	kg/ MT as is	Rs/ MT As is	Rs / Kg CBG
1	OFMSW	30	85	34	100	2.9
2	Napier Grass	25	150	60	1000	16.7
3	Poultry Litter	28	90	36	400	11.1
4	Press Mud	30	85	34	300	8.8
5	Segregated Food Waste	30	110	44	100	2.3
6	Paddy Straw	90	350	140	3000	21.4
7	Cow Dung / Farm manure	20	45	18	500	28

There could be cases of higher yield at higher costs, Lower yield at a higher cost, and Moderate yield at average cost of feedstock. This matrix will be project-specific and values will change from project to project but the trend and impact of it need to be evaluated in accordance with the specific projects so that the optimum feedstock mix can be arrived at. The feedstock is a very important aspect of any CBG project as it is on of the highest recurring cost of operation and can be as high as 35 to 40% of the overall operating expenses of the plant. Not only the cost of sourcing the feedstock mix will drive storage and handling requirement, Processing technology and utilities consume mainly power for conversion to CBG. Please note power and feedstock togather controls upto 70% of overall plant operating cost and through full evaluation of it through a systematic evaluation template and method for each plant.

At actuality how much amount could be spared for sourcing feedstock will depend on price of final product realized at the particular site along with the encashment of byproducts. However, looking at the present scenario I will personally be very cautious whenever the feedstock loading on CBG goes beyond INR 15 per kg CBG in the preliminary evaluation and mark red for further detailing.

Please don’t go by the values as these will change its relevance case to case and level of detailing but the overall trend shall not be missed.

The article https://www.linkedin.com/pulse/feedstocks-compressed-natural-gas-production-bharat-kadu/ could be best pre-read for this article.