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.