L23. Bioenergy Introduction and Wood to Energy

biomass energy Thus far in the course we have discussed conventional bioproducts (wood and paper), followed by emerging bioproducts based on fermentation or thermochemical conversion processes. The last category of products from biorenewable resources is energy. We have three lessons on this topic and we promise it will be fun and you will learn stuff!

A couple of critical things before we start:

The world has a growing need for energy. We use the energy for three main purposes: heating and cooling, generating electricity, and transportation.
Fossil fuels (coal, petroleum, and natural gas) are the primary sources of energy we are currently using to meet this growing energy demand.
Fossil fuels have many advantages, but their extraction and use impacts water quality, air quality, and the global carbon cycle.
Plants capture energy from the sun and convert the energy to biomass. This biomass can be converted into forms of energy that can be used in place of fossil fuels.

While we can replace some of our fossil fuel energy with energy from biomass, the real question is: Is energy from biomass 'better' than energy from fossil fuels?

Our Current Energy System

The world uses about 500 quadrillion BTUs of energy per year. As a point of reference for that number, a clothes dryer requires an input of 25,000 BTUs per hour. So 500 quadrillion BTUs is the equivalent of running 1 million clothes dryers nonstop for 2,283 years (in other words, it's a lot of energy). The important thing here is that the United States, with about 5% of the global population, is consuming about 20% of the 500 quadrillion BTUs.

Ahh, what is a BTU Links to an external site. you ask? A BTU is a standard measure of energy (like an inch is to distance). It is the amount of energy required to raise the temperature of one pound of water one degree Fahrenheit. It is approximated as the heat produced by burning a single wooden match.

Globally fossil fuels currently make up about 82% of this energy (Figure 1), primarily because they are inexpensive and society has done a great job building infrastructure to extract, refine, transport, and convert fossil fuels into useful energy. Nuclear power makes up about 5% of the global energy supply, and renewable sources such as wind, solar and hydroelectric make up about 3%. The remaining 10% of the global energy supply is from biomass, with most of it used for heating and cooking in developing nations. (IPCC 2011 Annex II Links to an external site., page 9 and Table A.II.1)

world enery

Figure 1. Global energy use. (EIA Source Links to an external site.)

Looking more closely at the United States, renewable energy makes up about 11% of our energy supply with about half of it (45%) coming from biomass (Figure 2). Clearly biomass is already playing a vital role in our energy supply.

energy_consumption_by_source_large 2017.jpg

Figure 2. Breakdown of energy use in the United States. (EIA Source Links to an external site.)

As indicated in the introduction, there are three real categories of energy use. First, we require heat and cooling for our homes, office buildings ,and factories. We also need heat for many industrial processes (paper mills, fertilizer production, steel mills, etc.). Second, we use a lot of electricity to power things around the home and office (lights, refrigerators, computers, etc.) and also to power industries that make all the products we use. Finally, we need energy for transportation - to move our cars, trucks, trains, ships, and airplanes full of people, food, or products.

Here is a general breakdown of how we use our fossil fuels:

Coal is used primarily for generating electricity.
Petroleum is used primarily for transportation.
Natural gas is used nearly equally for producing electricity, heating homes and buildings, and in industrial applications.

This is important because when we think about energy from bioresources we need to think about what energy source we are replacing. Using biomass as a source of heat and electricity (referred to as biopower or bioenergy) is far different than converting biomass to a transportation fuel (referred to as biofuel).

What Does "Better" Mean?

apple orange One of the most important themes of this course has been comparing the environmental impacts of a bioproduct to other non-renewable options. We cannot blindly say "It is plant based, therefore it is better." It is way more complicated than that. Is an apple better than an orange? Well, that depends on what you are comparing. Is it vitamin C? Is it fiber? Is it profitability for a farmer? Is it taste? Is it water use? "Better" is only useful in comparison to something else and must be clearly defined.

There are three ways to evaluate "better" as we compare biorenewable resources to fossil fuels.

Energy Balance: What is the energy required to convert the energy in biomass into a usable energy form? Planting, harvesting, and conversion of biomass require energy inputs. Is this energy requirement more than the energy we get out of the biomass in the form of heat or electricity or transportation?
Environmental: What are the environmental impacts of the biomass to energy process? Remember LCA? We need to look at carbon cycle along with water and air quality issues.
Other: Are we concerned about energy security, energy independence, number of jobs created, economic impact, land use, endangered species, etc.? We will spend less time on these this last set of criteria, but these are also important issues.

The three factors listed above are critically important. But all of them also are tied closely to economics. Energy is a business and businesses need to make money. In addition, consumers like to keep their money and are really good at finding the best deals. They want cheap gas and cheap electricity and sometimes they don't care about the other factors.

Energy Balance

EROI, or Energy Returned over Energy Invested, is a measure of the amount of energy required to get energy the energy we want. A bit confusing?? It may be most helpful to explain EROI with an example. Coal is energy dense with each pound of containing about 10,000 BTUs of energy. However, coal buried in the ground is not very useful to us. First we need to extract the coal from the ground, separate it from the other rock, process it into smaller pieces, and transport it to where we are going to use it. All of these steps require energy. This energy is in the form of fuel needed to run the equipment but also the energy needed to build that equipment, i.e. to make the steel used to build the equipment. We also need to build the power plant that will burn the coal to produce steam which generates electricity.

If all this is calculated, the energy inputs to get that pound of coal are about 830 BTUs. This means that for every pound of coal (10,000 BTU of energy) we need to put in 830 BTUs of energy. The math is: 10,000 BTUs output divided by 830 BTUs input gives an EROI of 12. As might be expected, EROI values are hard to calculate and can vary considerably. For instance, coal extraction from deep underground will require different energy inputs than from surface mined coal. Also, there are several types of coal, each with their own energy content. More importantly, EROI values for fossil fuels have decreased over time as the fossil fuels require more energy to extract. Remember in an early lesson we talked about how much more energy is required to extract the coal now than in the past because we already extracted the "easy stuff". Bottom line is EROI is important, hard to calculate, and changing as technologies change and as the fuels get harder to extract.

We will be discussing EROI of biomass later in this lesson and in the subsequent two lessons. Just remember, nothing is free. Energy is required to make energy.

Environmental Impacts

Environmental Impacts: Greenhouse Gas Emissions

The second category of "better" we need to discuss is environment impacts. There are many environmental issues that could be discussed related to water quality and air quality (remember Lesson 5?). These impacts are all important, but the most relevant for the energy discussion is GHG emissions. This is because the use of fossil fuels for energy results in stored carbon in fossil fuels being released into the atmosphere causing climate change. It is only a good idea to replace fossil fuels with biorenewable resources if this substitution results in less GHG emissions per unit of energy produced.

Evaluating GHG emissions for coal to electricity is similar to the EROI discussion. Once we know the energy inputs for coal from cradle to grave, and the type of energy used (e.g. petroleum, coal, solar power, hydroelectric), we can calculate GHG emissions. With effort, we can track all of the energy in every step of the process from cradle-to-grave of producing electricity from coal or electricity from wood. Pretty cool yet super complicated.

Environmental Impacts: Beyond GHG Emissions

The production and use of energy also has other environmental impacts. A quick Google search will give you many scientific studies on the impacts of coal mining on water quality. Similar searches on the burning of coal will discuss air quality issues related to mercury emissions or the release of small particles that can impact human health. Petroleum extraction and burning will give similar dire results. This should be no surprise as everything we do impacts the environment. The key is quantifying these impacts which we know is done through an LCA. When we quantify these impacts there are winners and losers.

As we look at biomass to energy we will discover some of these other impacts - but always in comparison to the fossil fuel we are trying to replace.

Other Factors

As we learned, the United States uses a lot of energy. We have our own supply of coal and natural gas but are importing about 20% of our petroleum from other countries (EIA¹ Links to an external site., EIA1² Links to an external site.). Many suggest that this reliance on other countries for petroleum is a national security threat. Some also see this as an economic issue because we are sending US dollars to other countries to buy energy rather than keeping the money in the US. Both issues are worth discussing, but not in this class.

Energy decisions can also be evaluated on land use issues. Should forests be harvested for energy or for lumber? Should cropland be used to grow food or to produce corn for ethanol?

What about local jobs? Corn and ethanol production is critical to the local economies of Iowa and other Midwest states. Replacing coal with wood energy is an issue because jobs would be lost in the coal mines but replaced in some other geographic area where trees are grown.

One of the most critical pieces of the energy discussion is economics. Historically, fossil fuels have been the cheapest source of energy. If energy decisions are based only on price per unit of energy (for instance $/gallon of gasoline), and none of other factors considered, fossil fuels will win. As we said earlier, we have created a very efficient system and infrastructure for the extraction, processing, and distribution of fossil fuels.

The key to this course is understanding that biomass to energy systems must be economically viable OR policies need to be put in place to put a value on things like GHG emissions, water quality issues, air quality issues, land use concerns, etc. We will be discussing some of these policy options as we continue this discussion.

Wood to Electricity

For our first real example let's look at using wood to produce electricity. Other biomass sources can be converted to electricity but wood to electricity is the most common.

The concept of wood to electricity is relatively simple. Electricity is generated from machines called generators (good naming). The generator needs to spin to generate the electricity. The generator is connected to an engine or to a turbine to make it spin. To visualize this, picture a wind turbine. As wind blows, the blades on the wind turbine spin and turn the generator. The most common turbine for generating electricity is a steam turbine. Steam turbines use steam to turn the turbine blades. In these systems, coal, natural gas, nuclear fuel, or biomass produces heat that boils water to make the steam to drive the turbine.

As you can imagine, these systems produce a lot of heat. To make these electric power plants more energy efficient, the waste heat is captured and used for space heating or for other processes. For instance, most paper mills have a electric generation power plant that uses waste wood and lignin (black liquor) to produce electricity. The waste heat from the power plant is captured and used in the pulping and paper making process.

CHP These systems that produce electricity (power) and capture and use the heat are called CHP systems (Combined Heat and Power). The process is also referred to as co-generation.

So, is using wood better than coal or natural gas for producing electricity? Is wood better than solar power or wind power?

As you may have anticipated, the answer is not simple. Lets look at some of the criteria.

Energy Balance

Let's first think about EROI - the energy balance. Wood and any other biomass can be burned and used to produce steam. Wood has an energy density that is just a bit less than coal, about 7500 BTU/lb (this is the energy we get out). We also have to consider the harvesting, transporting, and processing of the wood to prepare it for burning in the power plant. Wood used in an electric power generation plant needs to be made into small pellets and dried. This requires additional energy inputs. In most cases the wood pellets are burned with coal in a traditional coal plant which means they have to be transported to the coal plant.

This EROI calculation is already complicated, but what if we add other variables such as: Is the biomass a waste product from a paper mill or a saw mill? Is it forest residual? Is it from short rotation woody biomass? Is it recycled newspaper? The EROI answer is different depending on the source of the biomass. Each of these biomass sources requires an analysis of cradle-to-grave of EROI.

A study in 2016 of electricity production in the UK from a mix of energy sources put all of these EROI factors together and found that biomass to electricity had an EROI_el of 1.1 whereas the EROI_el for coal was 3.6. Hyrdoelectric was the winner with an EROI_el of 58 (Figure 3).

EROI EL Raugei

Figure 3. EROI_el for various fuels from a study in the UK ( Raugei and Leccisi Links to an external site., 2016). Note these EROI values include the generation of electricity.

Note the difference in this EROI_el for coal compared to what was listed in the example above (EROI ~12). The difference is that EROI of 12 is for just the production of coal. The EROI_el includes the conversion of the coal to electricity which is not a very efficient process.

Greenhouse Gasses

EROI is not the only factor. Just because the EROI is low does not mean it is bad. Remember, there are many other things to consider. The most important consideration currently is greenhouse gas emissions. Does using wood for energy result in lower greenhouse gas emissions than using coal for energy?

As you all know, plants are carbon neutral meaning that the carbon captured by plants through photosynthesis is eventually returned to the atmosphere. However, there are always additional energy inputs and related GHG emissions for things like planting, harvesting, processing, transporting, etc. that have to be considered. So biomass to electricity is very close to zero GHG emissions but not quite carbon neutral (as can be seen from Figure 4).

The reported units in Figure 4 are Carbon Dioxide Equivalents (written CO_2eq) per unit of electricity produced. CO_2eq rerpresent how much global warming impact there is and combines all the different greenhouse gasses into one unit. Electrical energy is typically measured in kilowatt hours (kWh) so the units on the graph are grams CO_2eq/kWh. In this graph, the bigger the number the worse it is. Focus on the dark blue line which is the middle value reported in the literature. To "win" the GHG emission game you need to be closest to the zero line. You can see how low the renewable energy options are compared to fossil fuels.

Figure 4. Survey of Life Cycle GHG emissions for electricity (NREL, 2013 Links to an external site., IPCC 2011 Annex II Links to an external site.)

The common thought is that using biomass to generate electricity is a clear winner over coal and other fossil fuels.

These positive results for wood to electricity reflect the understanding that wood (biomass) to energy is carbon neutral and a renewable source of electricity. Both Europe, and most recently the US EPA, have outlined "Wood to Energy" as carbon neutral in their energy policies. In fact, Europe's renewable energy policies have led to many of their coal plants converting to wood pellets - much of it supplied by wood pellets produced in the United States.

Some, however, disagree with wood to energy being a good thing for atmospheric carbon. This is because of the issue of timing for carbon neutral. If the goal is reducing atmospheric carbon TODAY (or in the next 20 years), harvesting and burning whole trees instead of coal will not help. Think about it this way: Coal sequestered carbon millions of years ago and trees sequestered carbon over the last 20, 50, or 100 years. But when either one of those carbon sources are burned TODAY, they release carbon into the atmosphere, and the atmosphere sees no difference. Carbon dioxide concentrations in the atmosphere still increase. These studies (Sterman et al. Environmental Research Letters, Links to an external site. Jan 2018) suggest that over time there may be some benefit to wood to energy, but if we want to reduce atmospheric carbon in the next 15-20 years (or maybe even 100 years), wood may not be a good replacement for coal. Their research and others suggest a quicker way to reduce atmospheric carbon would be to keep the trees growing and use more wind and solar power.

The counter argument is that once the cycle of growing, harvesting, and burning trees for energy is started, the energy production from wood would be near carbon neutral over time, and with a wood to energy system in place the atmosphere would see no additional carbon from coal.

It is important to read the fine print with any analysis of carbon neutrality and carbon emissions with wood to energy systems. Clear cutting older forests, wood waste, forest residues, thinning of trees from sustainably managed forests, dedicated wood crops, or plantation-grown trees will all have slightly different life cycle GHG emissions. There are also issues regarding the efficiency of burning wood and the storage of carbon in the soil.

Other Considerations

By far the most important and debated Wood to Energy topic is atmospheric carbon with other factors considered secondarily. These include:

Mining of coal causes many air and water quality problems, but if you remember the discussion on forest plantations, there are issues here also. Both can result in water quality issues.
Burning of coal or wood emits carbon dioxide along with other toxic gasses that need to be controlled.
Having markets for the wood products, including energy markets, ensures that landowners will continue to grow trees. This is indeed a benefit as we have seen sustainably grown forests offer many environmental benefits.
Job creation is a local issue. If wood is used, it creates jobs in the forest industry but reduce jobs in the coal industry. Jobs gained in one industry would displace jobs in another industry.
Regarding energy security, nearly 100% of our electricity is generated from coal or natural gas. We are not dependent on any other country for this energy resource, so it is not a very relevant consideration here.

Summary

So is wood to electricity better than coal to electricity? The answer is typically yes, but with qualifications and not in all categories. Are we talking about energy balance? GHG emissions? Carbon neutrality? Preservation of trees and forestland? What if you compare wood to natural gas or hydroelectric? All good discussions and comparisons.

If the focus is on GHG emissions or atmospheric carbon concentrations, then the source of the wood, the management of the forests, and the timeline for carbon neutrality are additional considerations.

Oh, if life were easy :)