# Biorefineries and platform chemicals: Biomass as a chemical feedstock

Source page: https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/p1/

[Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 13: Biorefineries and platform chemicals](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/) Part 1 of 5

## Biomass as a chemical feedstock

Before any biorefinery can be judged, we need to know what it is fed. Plant matter is not a green version of crude oil. It is a wet, bulky, seasonal solid made mostly of sugar polymers, with oxygen built into almost every molecule. Those three facts, composition, oxygen and logistics, explain most of what follows about yields, plant sizes and costs.

### 1.1 Plant matter is mostly sugar polymers, locked in a tough composite

Kamm and colleagues estimate that plants build about 170 billion tonnes of biomass a year. Roughly 75% is carbohydrate (mainly cellulose, starch and sucrose), 20% is lignin and only about 5% is everything else: oils, proteins, dyes and flavours. Of the total, people use about 6 billion tonnes, and only 3 to 3.5% of that goes to non-food uses such as chemicals.

The carbohydrates come in very different forms. Starch, the energy store in grain and tubers, is a chain of glucose joined by alpha links. Enzymes in our own gut can cut it, so industry can too, cheaply. Cellulose is also pure glucose, but joined by beta links that let neighbouring chains line up and hydrogen-bond into crystals, rather like strands of a rope pressed into a solid bar. Tolan of Iogen gives cellulose as 35 to 50% of most plant material, with chain lengths of 1,000 to 10,000 glucose units.

Hemicellulose (15 to 25%) is a branched polymer of mostly five-carbon sugars, chiefly xylose and arabinose. It is easy to break: dilute acid at about 120 C dissolves it. Lignin (15 to 30%) is not a sugar at all but a three-dimensional network of phenolic propane units. It glues the fibres together, keeps water out and resists microbes. No natural enzyme splits it cleanly into monomers.

The practical upshot is that grain and sugar cane hand over their sugar easily, while straw, stover and wood guard theirs. That single difference separates first-generation plants, which are commercial, from lignocellulosic ones, which mostly are not.

Data Composition of lignocellulosic feedstocks (mg per g dry solids)

| Feedstock | Cellulose | Xylan | Arabinan | Lignin | Ash | Protein |
|---|---|---|---|---|---|---|
| Wheat straw | 455 | 165 | 25 | 204 | 83 | 64 |
| Barley straw | 406 | 161 | 28 | 168 | 82 | 64 |
| Corn stover | 408 | 128 | 35 | 127 | 60 | 81 |
| Switch grass | 399 | 184 | 38 | 183 | 48 | 54 |
| Maple wood | 500 | 150 | 5 | 276 | 6 | 6 |
| Pine wood | 648 | 33 | 14 | 320 | 0 | 2 |

Xylan and arabinan are the hemicellulose sugars. Iogen found conversion efficiency rises with arabino-xylan content, which is why softwood (pine) needs unacceptably high enzyme doses.
 Source: Kamm (2006), Vol 1 Ch 9 Iogen's demonstration process (Tolan), Table 9.1, pdf pp. 223-225  Common misreading

#### Same glucose, same cost?

Starch and cellulose both break down to glucose, so it is tempting to think they are equally good sugar sources. They are not. Tolan reports that hydrolysing pretreated cellulose needs about 100 times more enzyme than hydrolysing starch, and Dean and colleagues at Genencor give the same factor for cellulase versus amylase protein. The cellulose must also be cooked first to open up the lignin and hemicellulose sheath. The glucose is identical; the cost of getting it out is not.

- Kamm (2006), Vol 1 Ch 1 Biorefinery systems - an overview · pdf pp. 44, 49, 58
- Kamm (2006), Vol 1 Ch 9 Iogen's demonstration process · pdf pp. 223, 233
- Kamm (2006), Vol 1 Ch 3 Development of biorefineries (Dean et al.) · pdf p. 104

### 1.2 Oil lacks oxygen; sugar carries too much, and removing it costs mass

Petroleum is mostly chains of CH2 units with almost no oxygen. Petrochemistry therefore works by adding function: oxygen, nitrogen or chlorine is bolted onto ethylene, propylene or benzene to make useful intermediates. Lichtenthaler, writing in Kamm, puts the contrast neatly. Hydrocarbons are oxygen-free and devoid of functionality; carbohydrates are 'overfunctionalized' with hydroxyl groups. Making industrial chemicals from sugar is mostly the opposite job: taking oxygen out.

The DOE screening report (Werpy and Petersen 2004) found exactly this. Because biomass is already highly oxidised, few of its routes need oxidation, while many need hydrogenation or dehydration, that is, adding hydrogen or stripping water.

Removing oxygen costs mass. When yeast turns glucose into ethanol, Kent gives the maximum yield as 0.51 g of ethanol per g of sugar; the rest leaves as carbon dioxide. Dale and Kim note the upside: over 90% of the glucose's energy stays in the ethanol. So sugar-to-fuel conversion is efficient in energy terms but loses half the weight.

Triglycerides (fats and oils) are the exception. Their long fatty chains look like hydrocarbons, so Task 42 calls fatty acid derivatives the easiest biomass components to handle in today's chemical infrastructure. That is why oleochemicals are the oldest and largest bio-based chemical industry, and why Clements counts them as biomass's largest contribution to today's chemical industry.

The Lichtenthaler table below makes a surprising point: by the kilogram, the cheapest sugars cost about the same as basic petrochemicals. The catch is the mass lost on the way to a product.

Data Sugars versus petrochemicals: world output and bulk price, early 2005

| Material | World production (t/yr) | Price (euro/kg) |
|---|---|---|
| Sucrose | 140,000,000 | 0.20 |
| D-Glucose | 30,000,000 | 0.30 |
| D-Xylose | 25,000 | 4.50 |
| D-Sorbitol | 650,000 | 1.80 |
| Citric acid | 1,500,000 | 1.00 |
| L-Lactic acid | 150,000 | 1.75 |
| L-Glutamic acid | 1,500,000 | 1.20 |
| L-Lysine | 740,000 | 2.00 |
| Ethylene | 90,000,000 | 0.40 |
| Propylene | 45,000,000 | 0.35 |
| Terephthalic acid | 12,000,000 | 0.70 |
| Methanol | 25,000,000 | 0.15 |

Only the sucrose figure is a reliable statistic (2004/05 crop); others are producer estimates. Prices are benchmarks for bulk crystalline product.
 Source: Kamm (2006), Vol 2 Ch 1 The key sugars of biomass (Lichtenthaler), Table 1.1, pdf p. 468  Worked example

#### How much sugar makes a tonne of ethylene?

Task 42 says 1 tonne of ethylene needs 1.7 tonnes of ethanol. Kent gives a ceiling of 0.51 t ethanol per t of sugar. So the minimum sugar demand is 1.7 / 0.51 = 3.3 t of glucose-equivalent per tonne of ethylene, before any process losses. Using Lichtenthaler's 2005 price of 0.30 euro/kg for glucose, the sugar alone costs at least 3.3 x 300 = about 1,000 euro per tonne of ethylene, against his ethylene price of 0.40 euro/kg, or 400 euro/t. Cheap per kilogram is not cheap per kilogram of product.

- Kamm (2006), Vol 2 Ch 1 The key sugars of biomass · pdf pp. 467-470
- Werpy & Petersen (2004), Section 7 Pathways and challenges · pdf p. 25
- Kent (2012), Ch 33 Biomass conversion · pdf p. 1260
- Kamm (2006), Vol 1 Ch 2 Biomass refining global impact (Dale & Kim) · pdf p. 82
- Task 42 (2012), Sections 9.2 and 9.7 · pdf pp. 18, 28

### 1.3 Wet, bulky and seasonal: why all biomass is local

Crude oil is a dense liquid that can be piped across continents and stored in tanks. Biomass is a solid with a lot of water. Godshall notes in Kent that bagasse leaves the cane mill at about 50% water; Hettenhaus reports corn stover at harvest is typically 30 to 50% moisture and must dry in the field. Baled stover has a dry density of only 112 to 160 kg per cubic metre.

It also arrives all at once. Hettenhaus gives a harvest window of 20 to 40 days, while a lignocellulosic biorefinery needs 2,000 to 3,000 dry tonnes every day of the year. Delivering a million dry tonnes to one site during harvest would mean about 100 trucks an hour on a 20-hour day. Trucking is charged by distance, 1.00 to 1.50 dollars per km, and bulky loads hit the volume limit before the weight limit: round bales on load-and-go wagons carried 9 dry tonnes against a normal 18.2 t, doubling transport cost per tonne.

Storage is a trade-off too. Dry bales lose over 10% of their mass and burn easily; wet piles above 60% moisture lose under 5% and cannot catch fire, but water then rides along to the plant.

Dale and Kim draw the conclusion: because biomass is bulky, it pays to refine it near where it grows. Clements and Van Dyne quantify the limit. For a combined ethanol and furfural plant the best size was 4,360 tonnes of feedstock a day; below about 750 t/d capital and operating costs were too high, and above about 4,400 t/d gathering enough feedstock cost too much.

Data Storing crop residue: dry bales versus wet bulk piles

| Property | Dry (bales) | Wet storage |
|---|---|---|
| Dry density | 7-10 lb/ft3 (112-160 kg/m3) | 12-14 lb/ft3 (up to 200 kg/m3 in 40 m piles) |
| Storage area needed | 10x | 1x |
| Storage loss | > 10% | < 5% |
| Fire hazard | High | None |
| Weather risk | Rain | Extreme cold |
| Typical scale | Small, mostly on farms | Large, bagasse for pulp mills |

One million dry tonnes in square bales needs about 500 acres; wet piles need about 50 acres.
 Source: Kamm (2006), Vol 1 Ch 14 Biomass commercialization and residue collection (Hettenhaus), Table 14.11, pdf p. 362  In the data

#### What a dollar of feedstock does to fuel cost

NREL's base case assumes corn stover delivered at 33 dollars per dry tonne. Hettenhaus reports that a 5 dollar per tonne change moves ethanol cost by 1 to 1.5 US cents per litre, depending on yield. His farm budget shows where the 33 dollars goes: custom baling about 16 dollars per tonne and hauling within 50 km about 9.9 dollars, leaving the farmer a margin of only 41 to 54 dollars per hectare, around the minimum of about 50 dollars per hectare most often cited as needed to interest growers.

- Kamm (2006), Vol 1 Ch 14 Biomass commercialization and agriculture residue collection · pdf pp. 353-354, 361-366
- Kamm (2006), Vol 1 Ch 2 Biomass refining global impact · pdf pp. 88-89
- Kamm (2006), Vol 1 Ch 5 The lignocellulosic biorefinery (Clements & Van Dyne) · pdf pp. 152-154
- Kent (2012), Ch 35 Sugar and other sweeteners · pdf p. 1412

### See it in the table
[H₂O Water](https://www.materialinsights.org/commodities/water/)[Oil Crude oil](https://www.materialinsights.org/energy/crude_oil/)[Su Sugar](https://www.materialinsights.org/commodities/sugar/)[Cl2 Chlorine](https://www.materialinsights.org/chemicals/chlorine/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[CO2 Carbon dioxide](https://www.materialinsights.org/chemicals/carbon_dioxide/)[C2H4 Ethylene](https://www.materialinsights.org/chemicals/ethylene/)[C3H6 Propylene](https://www.materialinsights.org/chemicals/propylene/)[Bz Benzene](https://www.materialinsights.org/chemicals/benzene/)[Dye Dyes](https://www.materialinsights.org/chemicals/dyes/)

### Check your understanding

Why does Werpy and Petersen's screening find that many sugar-to-chemical routes need hydrogenation or dehydration?
 Carbohydrates carry a hydroxyl on almost every carbon. Turning them into typical industrial intermediates means stripping oxygen, by adding hydrogen or removing water.

Glucose costs about the same per kilogram as ethylene, yet sugar-based ethylene is expensive. Why?
 At 0.51 t ethanol per t sugar and 1.7 t ethanol per t ethylene, at least 3.3 t of sugar go into each tonne of ethylene.

Clements and Van Dyne found an ethanol and furfural plant became less profitable above about 4,400 t/day. What drove this?
 Economies of scale in the plant are eventually outweighed by the rising cost of collecting and hauling bulky biomass from further away.

[← Module contents](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/)[Mark read · Part 2: The biorefinery concept and its types →](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/p2/)

## Other sections

- [Module overview](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/index.md)
- [The biorefinery concept and its types](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/p2/index.md)
- [Conversion routes: how the plants work](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/p3/index.md)
- [Platform chemicals: the DOE top twelve and their families](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/p4/index.md)
- [Economics and real cases](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/p5/index.md)
- [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/biorefineries-platform-chemicals/review/index.md)